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 LIBRARY 
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PLATE I. 
 
 III 
 
 g. 1 Fig. 2 Fig. 3 
 
 Fig. 4 
 
A 
 
 COURSE 
 
 IN 
 
 EXPERIMENTAL PSYCHOLOGY 
 
 BY 
 
 EDMUND C. SANFORD, PH.D. 
 
 ASSISTANT PROFESSOR OF PSYCHOLOGY, 
 CLARK UNIVERSITY. 
 
 PART I: SENSATION AND PERCEPTION. 
 
 BOSTON, U.S.A.: 
 
 D. C. HEATH & CO., PUBLISHERS. 
 1897. 
 
53 
 
 BIOLOGY 
 
 LIBRARY 
 
 G 
 
 COPYRIGHT, 1894, 
 BY EDMUND C. SANFOBD. 
 
 TYPOGRAPHY BY C. J. PETEBS & SON, BOSTON. 
 
 PBESSWOBK BY S. J. PABKHILL & Co., BOSTON. 
 
PBEFATORY NOTE 
 
 TO 
 EDITION OF ADVANCED SHEETS. 
 
 THE portion of the course which follows will be found to 
 treat of the senses only, and indeed not fully of them, for 
 it still lacks a chapter upon some of the most interesting 
 experiments in vision. The author's excuse for allowing 
 the publication, even in this modest form, of so incomplete 
 a work, must be the very extraordinary condition of experi- 
 mental psychology at this time. Many laboratories have 
 been opened, and many teachers of psychology are anxious 
 to give their students the benefit of demonstrations and 
 practice work, and yet there is absolutely no laboratory 
 handbook of the subject to be had. At such a time half a 
 loaf may be better than 110 bread at least, so a number of 
 the author's professional friends have seemed to believe; 
 and, since the completion of the whole must be still further 
 delayed, he offers this half loaf. 
 
 The course as planned consists of two parts : PART I on 
 sensation and perception ; and PART II on more complex 
 mental phenomena. 
 
 PART I needs three chapters more to complete it : Chap- 
 ter VII, on the Visual Perception of Extent, Distance, 
 Direction, and Motion; Chapter VIII, On the Psycho- 
 physic Methods and Weber's Law; and Chapter IX, On 
 Apparatus for the Study of the Senses. PART II will con- 
 tain chapters on the following topics : Eeflex and Voluntary 
 
 iii 
 
iy PREFATORY NOTE. 
 
 Movement, The Time Relations of Mental Phenomena, 
 Association, Memory, Attention, and Emotion, so far as 
 these subjects can be approached with experiments of mod- 
 erate difficulty, together with a chapter on the apparatus 
 necessary for such experiments. 
 
 E. C. S. 
 WORCESTER, July^ 1894. 
 
LABORATORY COURSE IN PSYCHOLOGY. 
 
 CHAPTER I. 
 The Dermal Senses. 
 
 THE sense organs of the skin give us besides pain, tick- 
 ling, shudder, and the like, the more special sensations of 
 contact, heat, cold, and pressure. All these may be received 
 passively when our members are at rest, or actively when 
 our members are in motion, in which case special sensations 
 of motion are blended with those just mentioned. We also 
 assign to each sensation a more or less exact location. To 
 examine some of these skin sensations is the purpose of this 
 chapter. 1 
 
 SENSATIONS OF CONTACT. 
 
 1. The Location of Touches. Touch yourself in several 
 places with the same object, and analyze out, as far as you 
 can, the particular quality of the sensation by which you 
 recognize the place touched. This quality of a sensation is 
 known as its " Local Sign." 
 
 Lotze, 2 A, 328 ff., 405 ff. ; J5, 39 ff. Stumpf. 
 
 1 As a general term for perceptions of touch in the widest sense, Max Dessoir 
 (p. 242) suggests Haptics as an analogue of Optics and Acoustics. This he further 
 divides into Contact-sense (including a, pure contact, and 6, pressure) and Psela- 
 phesia, from i//T)Aa<J>T7<n?, touching, handling (including a, active touch, and 6, 
 "muscle sense"). 
 
 2 For full titles of books and articles referred to, see the bibliography at the 
 end of the chapter. When several articles from one author are given, they have 
 been lettered A, B t C, etc., and the references marked accordingly. 
 
 i 
 
2 LABORATORY COURSE IN PSYCHOLOGY. 
 
 2. Location of Touches. Cause the subject to close his 
 eyes ; touch him on the fore-arm with a pencil point ; and 
 require him to touch the same point with another pencil 
 immediately afterward. Estimate the error in millimetres 
 and average the results for a number of trials, noting the 
 direction of error, if it is constant. The subject must be 
 allowed to correct his placing of the pencil if not satisfied 
 with it on first contact. 
 
 3. Aristotle's Experiment. Cross the middle finger over 
 I x the first in such a way as to bring the 
 
 \ tip of the middle finger on the thumb 
 
 / \ side of the first finger. Insert between 
 
 the two a pea or other small object. A 
 more or less distinct sensation of two 
 objects will result, especially when the 
 fingers are moved. Some experimenters 
 may find the illusion more marked when 
 the pea is rolled about on the surface of 
 the table with the crossed fingers, or 
 when the third and little fingers are used 
 instead of the first and middle fingers. 
 Aristotle, Hoppe, James, II., 86-87. 
 
 4. Eccentric Projection of Touches. Close the eyes, and 
 tap with the tip of a cane on the floor, or, better still, on 
 the walls and floor near a corner of the room. Notice that 
 the origin of the sensations seems to be the tip of the cane 
 and not the fingers or the arm. Attention to these parts, 
 however, will show the true place of origin. If the cane is 
 held rigidly at the lower end, there is little or no tendency 
 to shift the sensations from the fingers and arm, unless the 
 cane is limber. The eccentric projection of touches is only 
 a special case of their location, and follows the same general 
 laws. See also Ex. 41. 
 
 Weber, 483 f.; James, II., 31-43, 195-197; Dessoir, 219-232. 
 
TEE DERMAL SENSES. 3 
 
 5. Judgments of Motion on the Skin. a. Let the sub- 
 ject close his eyes. Rest a pencil point or the head of a pin 
 gently on his fore-arm and move it slowly and evenly up or 
 down the arm. Bequire him to indicate his earliest judg- 
 ment of the direction. If the experiment is carefully made, 
 the fact of motion will be perceived before its direction. 
 
 b. Try a number of times, estimating the distances trav- 
 ersed in millimetres and averaging for the two directions 
 separately. It will probably be found that the downward 
 distances have been greater than the upward. 
 
 c. Starting from a fixed point on the fore-arm, move the 
 pencil in irregular order up, down, right, or left, and require 
 the subject to announce the direction of motion as before. 
 Compare the results found with those found in Ex. 7. 
 
 Hall and Donaldson. 
 
 6. Feelings of Double Contact, a. If two parts of the 
 body of like temperature are brought in contact, the two 
 sensations do not blend, but the part that moves feels the 
 one that does not ; i. e., the sensations received by the mov- 
 ing part generally get more attention and are externalized. 
 Try with the tips of the thumbs or fingers in contact. This 
 general rule, however, has exceptions. Feel of the palm of 
 the right hand first with the ball of the left thumb (which 
 gives results in accord with the rule), then with the 
 knuckle of the same thumb sharply bent. Light tapping 
 of the forehead with the finger we feel in the forehead more 
 markedly than in the finger, though usually with the hand 
 on the forehead we feel the forehead. 
 
 b. If the parts are not of like temperature that which 
 varies most from the normal bodily temperature will be felt 
 by the other. Warm the right hand by holding it closed 
 for a minute or two and then apply it to the forehead. The 
 higher temperature will be perceived by the forehead, while 
 
4 LABORATORY COURSE IN PSYCHOLOGY. 
 
 at the same time the hand as the more expert touch organ 
 will perceive the form of the forehead. Cool the right hand 
 by holding it a few minutes in cold water, dry it and apply 
 it to the back of the left hand. The right hand may seem 
 to be feeling of a cold left hand. In this case of course 
 both the temperature and form feelings are credited to the 
 right hand. If the temperature is not very different the 
 direction of attention may dictate which shall be felt by 
 the other. 
 
 Weber, 556-559 ; Dessoir, 229. 
 
 7. Weber's Sensory Circles, a. Find the least distance 
 apart at which the points of the aesthesiometric compasses l 
 can be recognized as two when applied to the skin of the 
 fore-arm. Try also the upper arm, the back of the hand, 
 the forehead, the finger-tip, and the tip of the tongue. Be 
 very careful to put both points on the skin at the same time 
 and to bear on equally with both. Cf. Weber's measure- 
 ments as given in the text-books ; also Goldscheider's 
 (quoted by Ladd, p. 411). 
 
 b. Compare the distance between the points just recog- 
 nizable as two when applied lengthwise of the arm with 
 that found when they are applied crosswise. Compare the 
 results found in a and b with those found in Ex. 5, but 
 remember that this compass experiment requires the dis- 
 crimination of the points. 
 
 c. Give the points a slightly less separation than that 
 found for the fore-arm crosswise, and beginning at the 
 elbow draw the points downward side by side along the 
 arm. They will at first appear as one, later as two, after 
 which they will appear to separate as they descend. Some- 
 thing similar will be' found on drawing the points from side 
 
 1 For the apparatus needed in this and later experiments, see the list and 
 descriptions in the chapter on apparatus below. 
 
THE DERMAL SENSES. 5 
 
 to side across the face so that one shall go above, the other 
 below the mouth. 
 
 d. Make the skin anaesthetic with an ether spray and test 
 the discriminative sensibility as before. 
 
 Weber, 524-530, 536-541; Goldscheider, B, 70 ff., 84 ff. 
 
 8. Filled Space is relatively under-estimated on the 
 skin. Set up in a small wooden rod a row of five pins 
 separated by intervals of half an inch, and in another two 
 pins two inches apart. Apply to the arm like the com- 
 passes above. The space occupied by the five pins will 
 seem less than that between the two. A still simpler 
 way given by James is as follows: Cut one end of a 
 visiting card into a series of notches, and the other into 
 one long notch so as to leave two points as far apart as 
 the outer points at the other end, but separated by an 
 empty interval. Apply to the skin as before. This illu- 
 sion, though very clear for some experimenters, does not 
 seem equally so for all, and some have difficulty with 
 it. 
 
 James, II., 141, footnote. 
 
 9. Active Touch is far more discriminating than mere 
 contact. Compare the sensations received from simply 
 resting the tip of the finger on a rough covered book with 
 those received when the finger is moved and the surface 
 felt of ." 
 
 10. The Time Discriminations of the sense of contact are 
 very delicate. Strike a turiing-f ork ; touch it lightly, and 
 after about a second remove the finger so as not to stop the 
 fork. The taps of the fork on the skin do not blend into a 
 smooth sensation even when the vibrations are several hun- 
 dred a second. One may assure himself that the touching 
 does not much alter the rate of the fork by using another 
 that beats with the first. If the touching is carefully done, 
 
6 LABORATORY COURSE IN PSYCHOLOGY. 
 
 the rate of the beats will not be noticeably altered. (On 
 beating forks see Chap. IV.) The roughness may also be 
 felt but not so strongly, by setting the stem of the fork 
 upon the skin. The roughness of the pulses of air from 
 large tuning-forks can also be felt when the hand is brought 
 near, but not into actual contact with them. 
 Wittich, 335 ff.; Schwaner; Sergi. 
 
 11. After-images of Touch. Touch the skin of the wrist 
 lightly with the point of a needle, and notice that beside 
 the original sensation, there is, after a more or less free 
 interval, a second pulse of sensation. The interval is 
 brief, a second or under, and the sensation appears to come 
 from within. In quality it is like the first, but without the 
 pressure component. The prick of the needle point is not 
 essential ; the second sensation can be observed when the 
 head of a pin is applied. Too hard touches must be avoided 
 in testing for these images, as they give rise to a continuous 
 after-image that fills the interval. The second image is 
 apparently due to a double conduction in the spinal cord, 
 and is therefore different from the after-images of the other 
 senses. A portion of the original excitation is conveyed in 
 the posterior columns of the cord to the cortex. Another 
 portion goes by a slower path through the central gray 
 matter of the cord. Of. Ex. 32. 
 
 Goldscheider, H, 168 f. 
 
 12. An Interesting Illusion of Length, based on the time 
 during which a touch sensation continues, may be observed 
 as follows : Require the subject to close his eyes. Take a 
 piece of coarse thread a couple of feet long and make a knot 
 in the middle of it. Place the knot between the thumb and 
 forefinger of the subject, asking him to press it gently. 
 Then draw the thread slowly through between his thumb 
 and finger and ask him to estimate its length. Eepeat the 
 
THE DERMAL SENSES. . 7 
 
 process, this time drawing it rapidly. The drawing must 
 not be too slow in the first case nor too fast in the second, 
 or the nature of the illusion may be suggested to the sub- 
 ject and more or less completely corrected. 
 
 Loeb, 121-122. 
 
 For Minimal Contact in relation to Pressure, see Ex. 22 ; 
 in relation to Tickle, see Ex. 31. 
 
 SENSATIONS OF TEMPERATURE. 
 
 13. Hot and Cold Spots, a. Move one of the pointed 
 brass rods, or even a cool lead-pencil, slowly and lightly 
 over the skin of the back of the hand. At certain points 
 distinct sensations of cold will flash out, while at others no 
 temperature sensation will be perceived, or, at most, only 
 faint and diffuse ones. Heat one of the rods slightly in 
 the gas flame and repeat the experiment. More care will 
 be required in locating the hot spots than the cold spots, 
 for their sensations seem less distinct. 
 
 b. On some convenient portion of the skin mark off the 
 corners of a square 2 cm. on the side. Go over this square 
 carefully both lengthwise and crosswise for both heat and 
 cold, drawing the point along lines 1mm. apart, and note on 
 a corresponding square of millimetre paper the hot and cold 
 spots found, hot spots with red ink, cold with black. This 
 time the points should be heated or cooled considerably by 
 placing them in vessels of hot or cold water, and should be 
 kept at an approximately constant temperature by frequent 
 change, one being left in the water while the other is in use. 
 Break the experiment into a number of sittings so as to 
 avoid fatiguing the spots, for they are very easily fatigued. 
 A map made in this way cannot hope to represent all the 
 spots, but it will suffice to show the permanence of some of 
 them and possibly to show a little their general arrange- 
 ment. When the map has been made, select a responsive 
 
8 LABORATORY COURSE IN PSYCHOLOGY. 
 
 and isolated cold spot, and try it with a warm point. Try 
 a similar hot spot with a cold point. 
 
 c. Notice the very distinct persistence of the sensations 
 after the point has been removed, that is, the temperature 
 after-images. 
 
 An interesting question suggested by this punctual loca- 
 tion of temperature sensations is this, namely : How does it 
 come about that we ordinarily conceive such sensations as 
 continuous over considerable areas. 
 
 Blix; Goldscheider, A, B, E ; Donaldson. 
 
 14. Mechanical and Chemical Stimulation of the Temper- 
 ature Spots. 1 The temperature spots respond with their 
 characteristic sensations to mechanical and chemical stimu- 
 lation (and some observers find also, to electrical stimula- 
 tion), and do not give pain when punctured. 
 
 a. Choose a very certainly located cold spot and tap it 
 gently with a fine wooden point (not too soon after locating 
 it, if it has been fatigued in locating) ; or better, have an 
 assistant tap it. Thrust a needle into a well-located cold 
 point. Try both for comparison on an adjacent portion of 
 the skin. 
 
 b. Choose a convenient area, say, on the back of the hand 
 or the temple, and rub the skin lightly with a menthol 
 pencil. After a little the sensation of cold will appear. 
 Groldscheider's tests with a thermometer applied to the skin 
 s how that the sensation is not due to an actual cooling of it. 
 The menthol makes the nerves of cold at first hyperses- 
 thetic (so that they respond with their specific sensation to 
 
 1 Such experiments as these illustrate the Law of the Specific Energy of 
 Nerves, which may be stated somewhat as follows: Every stimulus that can 
 excite a sensory nerve at all, causes such sensations as follow the stimulation of 
 that nerve in its customary way and only such. As regards the interpretation to 
 be put on the phenomena thus generalized there is dispute. Goldscheider /; 
 Wundt, 3te Aufl. I. 332 ff., 4te I. Aufl. 323; Helmholtz, Sensations of Tone, 148 ; 
 Optik, 2te Aufl. 233, Ite Aufl. 193; Ladd, 307, 353. 
 
THE DERMAL SENSES. 9 
 
 mere contact, and give an intenser sensation when a cold 
 body is applied than do adjacent normal portions of the 
 skin) ; afterward, however, all the cutaneous nerves become 
 more or less anaesthetic. 
 
 c. Chemical stimulation of the heat nerves can be tested 
 with C0 2 . Provide two like vessels ; place them side by 
 side and fill one with C0 2 . Plunge the hand into the vessel 
 containing the gas, and for comparison into the one contain- 
 ing air. For the additional experiments necessary to prove 
 this to be a real chemical stimulation, see the literature. 
 
 Blix, Goldscheider A, J5, D, F, and Donaldson; on c, R. Du 
 Bois-Reymond. 
 
 15. The Temperature of the Skin at any moment is a 
 balance between its gain and loss of heat. Anything that 
 disturbs that balance, causing increased gain or loss, pro- 
 duces temperature sensations. It is common experience 
 that a piece of cloth, a bit of wood, a piece of metal, all of 
 the same temperature as the air that seems indifferent to 
 the hand, cause different degrees of the sensation of cold 
 when touched, because they increase the loss of heat by con- 
 duction in different degrees. If a paper bag be placed over 
 the hand held upward, a sensation of warmth is soon felt, 
 because of the decreased loss of heat. 
 
 16. The Shifting of the " Physiological Zero." a. Pro- 
 vide three vessels of water, one at 30 C., the second at 40, 
 the third at 20. Put a finger of one hand into the warmer 
 water, a finger of the other into the cooler. At first the 
 usual temperature sensations will be felt, but after a little 
 they disappear more or less completely, because of the 
 fatigue of the corresponding temperature organs. Now 
 transfer both fingers to the water of normal temperature. 
 It will seem cool to the finger from warmer* water and warm 
 to the one from cooler. This experiment has been sometimes 
 regarded as one of successive contrast. 
 
10 LABORATORY COURSE IN PSYCHOLOGY. 
 
 b. Hold the hand for one minute in water at 12 C., then 
 transfer it to water at 18. The latter will at first feel 
 warm, but after a time cold again. The water at 18 first 
 causes a decrease in the loss of heat or a slight gain, but 
 later a continued loss. 
 
 Weber; Hering; Goldscheider, B, 32 ff. 
 
 17. Effect of Extent of Surface Stimulated. The inten- 
 sity of the sensation increases as the stimulated area 
 increases. Dip the right forefinger (or hand) into hot or 
 cold water, observe the sensation, and immediately insert 
 the other forefinger to an equal depth. Vary the experiment 
 by inserting the left finger first, and by inserting both at 
 once and then withdrawing one. The original experiment 
 of Weber, who inserted first a finger, and then the whole of 
 the other hand, gives striking results, but has the fault, 
 as Goldscheider rightly observes, of adding a more sensitive 
 as well as a larger area. This experiment must not be 
 inconsiderately contrasted with Ex. 23. 
 
 Weber, 553; Goldscheider, G, 475-476. 
 
 18. Temperature Fatigue, a. Extreme temperatures fa- 
 tigue the sensory apparatus of both heat and cold. Hold a 
 finger in water at 45 C., the corresponding finger of the 
 other hand in water which feels neither cold nor hot (about 
 32). After 30 seconds dip them alternately into water at 
 10. The finger from the water at 32 will feel the cold 
 more strongly. Hold a finger in water at 10, the corre- 
 sponding finger of the other hand in water at 32. After 30 
 seconds dip them alternately in water at 45. The finger 
 from the water at 32 will feel the heat more strongly. 
 
 b. The fatigue of the temperature apparatus may produce 
 an apparent contradiction of Ex. 17. Phinge one hand 
 entirely under cold water and keep it there for a moment. 
 Then dip the finger of the other hand or the whole hand 
 
THE DERMAL SENSES. 11 
 
 several times in the same water, withdrawing it immediately 
 each time. The water seems colder to the finger or hand 
 which is only dipped. 
 
 Weber, 570; Goldscheider, B, 34 ff. 
 
 19. Temperature After-images, a. Hold a cold piece of 
 metal on the forehead or on the palm of the hand for half a 
 minute. On removing it the sensation of cold continues, 
 though the actual temperature of the skin is rising. Some- 
 times fluctuations are observed in the persisting sensation. 
 After contact with a hot body the sensation of heat con- 
 tinues in the same way, though the temperature of the skin 
 falls. Goldscheider explains this result for cold in part by 
 the persistence of the cold sensation in the manner of an 
 after-image, and in part by the lessened sensibility of the 
 nerves of heat; a similar explanation mutatis mutandis 
 holds also for heat. 
 
 b. Intermittent after-images, or those that recur after an 
 interval more or less free of sensation, have been observed 
 especially with repeated stimulation. Heat a key till it is 
 just a little short of painfully hot, touch some part of the 
 skin, e.g., the wrist, three or four times at intervals of about 
 half a second. The after-image of the heat will appear 
 several seconds later. Try the same for cold, but use a key 
 that is at the temperature of the air. 
 
 Cf. Ex. 13 c.j also the after-images of hearing and vision, 
 Chapters IV. and V., and notice that all the temperature 
 after-images are positive ; i.e., like the original sensation. 
 
 Goldscheider, B, 11, 34 ff., 38 ; on 6, Dessoir, 300. 
 
 20. Fineness of Temperature Discrimination, a. Find 
 what is the least perceptible difference in temperature 
 between two vessels of water at about 30 C., at about 0, 
 and about 55. The finest discrimination will probably be 
 found with the first mentioned, if the discrimination does 
 
12 LABORATORY COURSE IN PSYCHOLOGY. 
 
 not prove too fine at all these points to be measured with 
 the thermometers at hand. Use the same hand for these 
 tests, always dipping it to the same depth. It is better to 
 dip the hand repeatedly than to keep it in the water. 
 
 b. The different surfaces of the body vary much in their 
 sensitiveness to temperature. The mucous surfaces are 
 quite obtuse. When drinking a comfortably hot cup of 
 coffee, dip the upper lip into it so that the coffee touches 
 the skin above the red part of the lip, or dip the finger into 
 it; it will seem burning hot. Plunge the hand into water 
 at 5-10 C. The sensation of cold will be strongest at 
 first on the back of the hand where the skin is thin, but a 
 little later will come out more strongly in the palm, where 
 it will continue to be stronger and may finally approach 
 pain. 
 
 c. The middle line of the body is less sensitive to tem- 
 perature than portions at either side of it. Touch the 
 middle of the forehead, or the tip of the nose, with a piece 
 of warm or cold metal and then touch several places to the 
 right and left of that point. 
 
 Fechner; Weber, 552 ff.; Goldscheider, B, 49 ff. 
 
 SENSATIONS OF PRESSURE. 
 
 21. Pressure Points. Make an obtuse but extremely fine 
 cork point (pyramidal in shape ; for example, the pyramid a 
 quarter of an inch square on the base and of equal height), 
 set it upon the point of a pen or other convenient holder, or 
 use a match whittled down to a fine point, or even a needle. 
 Choose an area on the fore-arm and test for its pressure 
 spots somewhat as for the hot and cold spots, but this time 
 set the cork point as lightly as possible on point after point 
 of the skin instead of drawing it along. Two kinds of sen- 
 sation will be felt ; at some points a clear feeling of contact 
 with a sharp point will be felt, at others no feeling at all, or 
 
THE DERMAL SENSES. 13 
 
 a dull and vacuous one. The first are the pressure points. 
 Goldscheider describes their sensations on light contact as 
 " delicate/* " lively," " somewhat tickling ... as from mov- 
 ing a hair ; " on stronger pressure, " as if there were a resist- 
 ance at that point in the skin, which worked against the 
 pressure stimulus ; " " as if a small hard kernel lay there 
 and was pressed down into the skin." 
 
 The first are said to be more sensitive to small changes 
 of pressure, and though with sufficient increase both give 
 pain, their sensations retain their characteristics. They are 
 closer together than the temperature spots, and harder to 
 locate. The fact that our most frequent sensations of pres- 
 sure are from surfaces and not from points is perhaps the 
 reason it is difficult at first to recognize a pressure quality 
 in these sensations. 
 
 Goldscheider, B, 76 ff. 
 
 22. Minimal Pressure or Simple Contact. Mnd weights 
 that are just perceivable on the volar side of the fore-arm 
 and on the tips of the fingers. Try also, if convenient, the 
 temples, forehead, and eyelids. In applying the weights, 
 see that they are brought down slowly upon the surface of 
 the skin, that they touch equally at all points, and that 
 their presence is not betrayed by motion of the weight after 
 it touches the skin. This can be done by using a penholder 
 or small rod, with its tip put through the ring of the weight, 
 for laying it on. Compare the relative sensibility found by 
 this method with that found with Weber's compasses for 
 the same parts (Ex. 7) and note that the latter requires 
 discrimination, not mere perception. See also Exs. 29 and 
 31. 
 
 Aubert and Kammler ; Bloch. 
 
 23. Eelation of Apparent Weight to Area of Surface 
 Stimulated. Test with the equal weights of unequal size 
 
14 LABORATORY COURSE IN PSYCHOLOGY. 
 
 upon the hand, properly supported to exclude " muscle 
 sense." The smaller will seem decidedly heavier. 
 
 24. Discriminative Sensibility for Pressures. Use the 
 pressure balance if one is at hand; if not, have the subject 
 close his eyes and lay his hand, palm upward, on such a 
 support as will bring his arm into a comfortable position 
 and make his palm level ; for example, on a folded towel 
 placed on a low table or the seat of a chair. (The matter 
 of an easy position for the subject is of cardinal importance 
 in all psychological experiments.) The method of experi- 
 menting here to be used is that of the " Just Observable 
 Difference " or " Minimal Change ; " it may be applied as 
 follows : Lay in the subject's palm a piece of thick and 
 soft blotting-paper just large enough to prevent the weight 
 from touching the skin. Place the standard weight of 100 
 grams upon the paper and allow it to remain a sufficient 
 time for the subject to get a clear perception of its weight. 
 Then remove it and immediately put in its place a weight of 
 110 grams, allowing that to remain as long as the first. If 
 the subject can recognize this difference easily and surely, try 
 him with 109, 108, and so on, alternating the standard weight 
 and a weight to be compared with it till a weight is found 
 that is just recognizably different from the standard. If 
 110 grams is not recognizably different, take 111, 112 in- 
 stead of 109, 108. Occasionally follow the standard with 
 another 100 gram weight to guard against illusion on the 
 part of the subject. After having determined the just 
 observably greater weight, find the one that is just observ- 
 ably lighter in the same way. Make a good number of 
 determinations of these just observably heavier and lighter 
 weights, sometimes going toward the standard and some- 
 times away from it. Take the differences between them and 
 the standard weight and average the results. The ratio of 
 this average to the standard will be a measure of the dis- 
 
THE DERMAL SENSES. 15 
 
 criminative sensibility required. If, for example, the ratio 
 for one subject is 7 : 100 and for another 14: 100, the first 
 has a sensibility to pressure differences twice as acute as 
 the second. In half of the tests, both above and below, the 
 standard weight must be placed upon the hand first, and in 
 half the weight to be compared with it. It is well also to 
 distribute the determinations of the differences above and 
 below so that they shall be about equally affected by 
 practice and fatigue. The aim should always be to keep all 
 the conditions of the experiment as constant as possible 
 and especially to have them the same for the weights to be 
 compared. Be careful in putting on the weights that the 
 subject does not recognize a difference in the force with 
 which they strike; also that suggestions by difference of 
 temperature or by sounds made in selecting the weights 
 are avoided. 
 
 It is easy to see that this method has some disadvantages. 
 First, it leaves to the feeling of the subject what the just 
 observable difference is, and this feeling is liable to change 
 from subject to subject and in the same subject at different 
 times. In using this method the subject must know the 
 direction of the change that he is to recognize, and so is 
 somewhat exposed to the influence of expectant attention. 
 And finally, when weights are found that are just observ- 
 ably different, it is possible that they are a little larger than 
 the subject could just recognize ; that is, that he has allowed 
 himself a small margin for security. These difficulties may 
 be partially obviated by a more rigorous application of the 
 method. 
 
 Thus in making the tests for the just observable differ- 
 ences above and below, weights must first be taken that are 
 not recognizably different from the standard, and must then 
 be slowly increased or decreased till just observably different. 
 Subjective equality must be regarded rather than objective 
 
16 LABORATORY COURSE IN PSYCHOLOGY. 
 
 equality, if the two are at odds, as sometimes happens. To 
 these tests two others must be added ; namely, for the just 
 tmobservable differences above and below, the operator now 
 selecting a weight that is clearly heavier than the standard 
 and decreasing it gradually till it can just no longer be 
 recognized as different, and similarly selecting one that is at 
 first clearly lighter than the standard and increasing it till 
 it seems the same. The average of the four tests, just 
 recognizably different and just -^^recognizably different, is 
 then taken for the ratio. When great accuracy is required 
 the method must be used in this complete form. For other 
 methods and fuller literature, see the chapter on Weber's 
 Law below. 
 
 Weber, 543-549; Wundt, 3te Aufl., I., 343 ff., 350; 4te Aufl., L, 
 336 f., 341 ff. 
 
 25. Temperature and Pressure. Cold and hot bodies feel 
 heavier than bodies of equal weight at a normal temperature. 
 
 a. For cold, take two dollar pieces, warm one until it 
 ceases to seem cold ; cool the other to 10 C. Apply alter- 
 nately to the palm of the hand, letting the hand rest, mean- 
 while, on the table or some other support so as to exclude 
 "muscle sense." The cold one will seem much heavier, 
 perhaps as heavy as two at the normal temperature. The 
 same experiment may be tried on the forehead with the head 
 supported. 
 
 b. For heat take two wooden cylinders of equal weight ; 
 heat one to a high temperature by standing it on end in a 
 metal vessel floating in a water bath. Apply the cylinders 
 on end alternately to the back of the hand (supported) be- 
 tween the metacarpal bones of the thumb and first finger. 
 The hot one will seem heavier. 
 
 Weber, 512, 551; Szabadfoeldi ; Funke, 320; Dessoir, 304-306. 
 
 26. Pressure Evenly Distributed over a Considerable Area 
 is less strongly felt than pressure upon an area bordered by 
 
THE DERMAL SENSES. 17 
 
 one that is not pressed. Dip the hand up to the wrist into 
 water (or, better still, into mercury) of normal temperature, 
 and notice that the sensation of pressure is strongest in a 
 ring about the wrist at the surface of the water ; possibly 
 stronger on the volar than on the dorsal side. The ring 
 effect is unmistakable when the hand is moved up and down 
 in the water. 
 
 27. Pressures are not Equally well Perceived in all Parts 
 of the Body. This may be tested with weights applied some- 
 what as in Ex. 24, as was done by Weber, but a simpler 
 experiment may be made as follows : Find the pulse at the 
 wrist ; feel it with the finger tips, the back of the fingers, 
 the side of the hand, the other wrist, the lip, and the tip of 
 the tongue. Try the pulse in the temple with the finger 
 tips, the side of the hand, and the fore-arm. Notice that 
 when it is felt by another person the experimenter is unable 
 to feel it subjectively. 
 
 Goltz. 
 
 28. Eefinement of Active Pressure Sense. Something of 
 the refinement of the pressure sense in perceiving the un- 
 evenness of surfaces may be found by laying a hair on a 
 plate of glass or other hard, smooth surface and over it 10 
 or 15 sheets of writing-paper. The position of the hair 
 can easily be felt by passing the finger tips back and forth 
 over the paper. 
 
 29. The Hairs as Organs of Touch. The finest hairs 
 respond with a distinct sensation of anticipatory touch, 
 when they are moved, and probably this accounts for a part 
 at least of the differences between the fore-arm and finger 
 tips found in Ex. 22. Touch a few single hairs and observe 
 the sensation. 
 
 Blaschko. 
 
 30. The Feeling of Traction or Negative Pressure has 
 
18 LABORATORY COURSE IN PSYCHOLOGY. 
 
 been discriminated by some authors, but has rarely been 
 made an object of experiment. It is to be observed when 
 viscid substances are handled, when a portion of the skin is 
 brought over the mouth of a closed vessel and the air ex- 
 hausted, or when in any other way the skin is lifted from 
 the underlying portions of a member. The sensation may 
 be studied qualitatively by passing a thread through a 
 small bit of court-plaster, knotting it on. the gummed side 
 and sticking the plaster to the skin. Traction on the thread 
 now produces the sensation. 
 Hall and Motora, 93 ff. ; Bloch. 
 
 GENERAL SENSATIONS, TICKLE, AND PAIN. 
 
 These topics, though clearly of very great psychological 
 interest, have so far received comparatively little careful 
 study, and few experiments have been made upon them. 
 They are not exclusively dermal senses, but the skin offers 
 the most convenient field for the study of the two to be 
 considered here, namely, tickle and pain. In both the 
 experimenter should notice the subjective cast of the sensa- 
 tions. Our eyes and ears give us information about colored 
 and sounding things, but tickle and pain let us know that 
 we are being tickled or hurt by something. 
 
 31. Tickle. Two sorts of tickle are easily distinguish- 
 able, a deep-seated tickle located in the rib region, which 
 seems more strongly developed in children, and responds to 
 rather strong stimulation, and a superficial tickle much 
 more widely distributed, and responding to slight stimuli 
 only. The latter sort is that regarded in this group of 
 experiments. 
 
 a. Touch very lightly the different parts of the face, es- 
 pecially about the eyes, the margin of the lips and the 
 opening of the ears with the tip of a light wisp of paper 
 and notice the tickle sensations. Notice the apparent 
 
THE DERMAL SENSES. 19 
 
 disproportion between the stimulus and the resulting sen- 
 sation, the wide and indefinite irradiation, and the long 
 after-image.. 
 
 b. Touch the same parts as lightly as possible with the 
 tip of a penholder or the finger, and then with the same 
 instrument while exerting at the same time a moderate pres- 
 sure. Notice the difference in effect ; notice also that the 
 tendency to rub a tickled surface is a tendency to use a 
 greater stimulus to remove the effects of the less. Notice 
 also, when feeling a tendency to sneeze, that the sneeze 
 can be wholly prevented by firm pressure or rubbing of the 
 sides of the nose or the adjacent parts of the face. 
 
 c. Tickle is apparently a summation phenomenon. Touch 
 the tip of the tongue lightly with the prong of a tuning- 
 fork at rest and notice the after-image, which, however, has 
 no tickle in it. Then strike the fork and touch it to the tip 
 of the tongue. Compare the effects. 
 
 d. The ticklability of adjacent parts of the body is quite 
 markedly different. Test with the tuning-fork, striking it 
 and applying it gently to the tip, sides, and middle of the 
 upper surface of the tongue and to the lower surface. 
 
 32. Pain. a. Slow conduction. Eemove the shoe and 
 strike a smart blow with a light rod on the sole of the foot, 
 or on a corn ; the pain will be perceived noticeably later than 
 the first sensation of contact, separated from it perhaps by 
 an almost empty interval. This delay is probably due to 
 the same cause as the secondary after-image of touch in 
 Ex. 11. 
 
 b. Temperature pains. A given increase of heat above 
 the blood temperature is more effective in causing pain than 
 an equal decrease. Compare the effects of plunging the 
 hand into water at 10C. and at 60. Use a considerable 
 quantity of water and do not allow the hand to remain too 
 
20 LABORATORY COURSE IN PSYCHOLOGY. 
 
 long in the water, for its sensibility to pain as well as to 
 temperature is decreased by fatigue. 
 
 Experiments on pain can likewise be made with electrical 
 stimulation and pressure. These are especially suitable for 
 determining the relative sensibility of different subjects. 
 The first can easily be tried with the sliding induction coil, 
 by applying the electrodes to the surface to be tested and 
 then gradually pushing the secondary coil towards the 
 primary till the stimulation becomes painful. For appara- 
 tus, see the chapter on apparatus below. 
 
 Weber, 569 ff. ; Dessoir, Beaunis, Lombroso, Mantegazza, Preyer, 89. 
 
 BIBLIOGKAPHY. 
 
 IN the following bibliography, and those appended to later chapters, 
 the aim has not been to make an exhaustive list, but rather to give 
 a number of the more important references by which the student 
 may begin the study of original sources if he desires. For the same 
 reason the list has not been kept strictly to works where the experi- 
 ments of the chapter are discussed, but a few other important refer- 
 ences have been given. If any one wishes to increase the list, he 
 can easily do so from the reviews in the various philosophical and 
 psychological journals, and from the classified bibliography on psy- 
 chology and the physiology of the sense organs published yearly in 
 the Zeitschrift fur Psychologic, beginning with the literature of 
 1889. For the older literature the rich citations of Volkmann's 
 Lehrbuch der Psychologic may be consulted. Much psychological 
 literature appears at present in the physiological periodicals. The 
 Centralblatt fur Physiologic contains reviews and an annual bibli- 
 ography of general physiology, with sections on the physiology of the 
 senses and physiological psychology, and has done so since its begin- 
 ning, in 1887. Hermann's Handbuch der Physiologic makes many 
 references to literature, and each important section in Beaunis' s 
 Elements de Physiologic humaine is followed by a bibliography. 
 Hoffmann and Schwalbe's Jahresberichte iiber die Fortschritte der 
 
THE DERMAL SEN 
 
 Anatomie und Physiologic gives bibliographies and summaries of 
 literature since 1872. The Index Medicus, now in its fifteenth 
 volume, has listed all current medical literature since 1879 ; and in 
 connection Vith this may be mentioned the mammoth Index Cata- 
 logue of the Library of the Surgeon-General's Office, United States 
 Army, of which thirteen large volumes have so far been published 
 (September, 1893), extending from A to Sutugin. The descriptive 
 pamphlet of the Harvard Psychological Laboratory also contains, 
 in a ten-page appendix, a classified list of psychological literature. 
 
 ARISTOTLE : nepl Ei/v^W, c. 2, Bekker, 460. Also in German trans- 
 
 lation by Johannes Miiller, as an appendix to his Ueber die 
 
 phantastischen Gesichtserscheinungen. Coblenz, 1826. 
 AUBERT UND KAMMLER: MoleschotVs Untersuchungen, V., 1859 V 
 
 145. 
 
 BEAUNIS : Les Sensations internes, Paris, 1889. 
 BLASCHKO : Zur Lehre von den Druckempfindungen, Verhandl. d. 
 
 Berliner physiol. Gesell., Sitz. 27 Marz, 1885. Du Bois-Rey- 
 
 mond's Archiv, 1885, 349. 
 BLIX : Experimented Beitrage zur Losung der Frage iiber die 
 
 specifische Energie der Hautnerven, Zeitschrift fur Biologic, 
 
 XX., 1884, 141-156. 
 BLOCH: Kecherches expe"rimentales sur les sensations de traction 
 
 et de pression cutanSes, Archives de Physiologic, Ser. 5, III., 
 
 1891, 322-333. 
 BRONSON : The Sensation of Itching, Medical Record, XXXVIII., 
 
 1890, No. 1041, Oct. 18, 425-429. 
 
 DESSOIR: Ueber den Hautsinn, Du Bios-Reymond" 1 s Archiv, 1892, 
 175-339. See review of this paper by Goldscheider, Zeitschrift 
 fur Psychologic, V., 1893, 117-122. 
 
 DONALDSON: On the Temperature-sense, Mind, X., 1885, 399-416. 
 
 Du BOIS-REYMOND, EENE : Ueber chemische Reizung des Tempera- 
 tursinnes. Verhandlungen der Berliner physiol. Gesellsch., 
 Sitz. 11 Nov. 1892. Du Bois-Reymond's Archiv., 1893, 187-190. 
 
 FECHNER: Elemente der Psychophysik, L, 201-211 (temperature). 
 
 FUNKE: Der Tastsinn und die Gememgefiihle, Hermann's Hand- 
 buch der Physiol., Vol. III., pt. 2, 289-414. 
 
22 LABORATORY COURSE IN PSYCHOLOGY. 
 
 GOLDSCHEIDER: A. Ueber Warme-, Kalte- und Druckpunkte, 
 Verhandl. d. Berliner physiol. Gesell., Sitz. 13 Marz, 1885. 
 Du Bois-Reymond's Archiv, 1885, 340-345. 
 
 B. Neue Thatsachen tiber die Hautsinnesnerven. Ibid. 1885, 
 Supplement Band, 1-110, 5 plates. 
 
 C. Zur Dualitat des Tempera tursinns, Pflilger's Archiv, XXXIX., 
 
 1886, 96-120. (On Herzen's experiments.) 
 
 D. Ueber die specifische Wirkung des Menthols auf die Tempera- 
 tur-Nerven, Yerh. d. Berliner physiol. Gesell., 9 April, 1886. 
 Du Bois-Reymond's Archiv, 1886, 555. 
 
 E. Histologische Untersuchungen tiber die Endigungsweise der 
 Hautsinnesnerven beim Menschen. Ibid. 1886, Supplement- 
 Band, 191-231. 
 
 F. Die Einwirkung der Kohlensaure auf die sensiblen Nerven der 
 Haut, Verh. d. Berliner physiol. Gesell., 4 Nov. 1887. Ibid. 
 
 1887, 575-580. 
 
 G. Ueber die Topographic des Temperatursinns, Yerh. d. Ber- 
 liner physiol. Gesell., Sitz. 1 Juli, 1887. Ibid. 1887, 473-476. 
 
 H. Ueber die Summation von Hautreizen, Yerhandlungen der phy- 
 siol. Gesellsch. zu Berlin, Sitz. 31 Oct. 1890. Ibid. 1891, 164- 
 169. 
 
 L Die Lehre von den specifischen Energieen der Sinnesnerven, 
 Berlin, 1881. A forty page dissertation, containing full refer- 
 ences to literature. 
 
 GOLTZ : Ein neues Yerf ahren die Scharf e des Drucksinns der Haut 
 zuprufen, Centralblatt fur med. Wiss., 1863, No. 18, 273-276. 
 
 HALL AND DONALDSON: Motor Sensations of the Skin, Mind, X., 
 1885, 557-572. 
 
 HALL AND MOTORA: Dermal Sensitiveness to Gradual Pressure 
 Changes, American Journal of Psychology, I., 1887, 72-98. 
 
 BERING: Der Temperatursinn, Hermann's Handbuch der Physi- 
 ologic, Yol. III., pt. 2, 415-439. 
 
 HERZEN : Ueber die Spaltung der Temperatursinnes in zwei geson- 
 derte Sinne, Pflilger's Archiv, XXXYIIL, 1886, 93-103. 
 
 HOPPE : Das Aristotelische Rathsel der mit den gekreuzten Finger- 
 spitzen gefuhlten Kugel, Wiener med. Presse, 1888, Nos. 22, 23, 
 785, 827. 
 
THE DERMAL SENSES. 23 
 
 JAMES : Principles of Psychology, New York, 1890. 
 
 LADD : Elements of Physiological Psychology, New York, 1887. 
 
 LOEB: Untersuchungen tiber den Fiihlraum der Hand, Pfluger's 
 
 Archiv, XLL, 1887, 107-127. 
 LOMBBOSO : Algometria elettrica nel uomo sano e alienato, Milano, 
 
 1867. 
 (Lombroso und Ottolenghi) Die Sinne der Yerbrecher, Zeitschrift 
 
 fiir Psychologic, II., 1891, 337 ff. 
 LOTZE : A. Medicinische Psychologic, Leipzig, 1852. 
 B. Outlines of Psychology, translated by Herrick, Minneapolis; 
 
 also by Ladd, Boston, 1885. 
 
 MANTEGAZZA: La Physiologic de la Douleur, Paris, 1888. 
 PBEYER : Ueber den Farben- und Temperatur-Sinn mit besonderer 
 
 Kucksicht auf Farbenblindheit, Pfluger's Archiv, XXY., 1881, 
 
 especially pp. 75-92. 
 QUINCKE : Ueber Mitempfindungen und verwandte Yorgange, Zeitsch. 
 
 fur klin. Medicin, XYIL, 1890. 
 SCBTWANEB i Die Priifung der Hautsensibilitat vermittelst Stimm- 
 
 gabeln bei Gesunden und Kranken, Inaug. Diss., Marburg, 
 
 1890, 37. Keview with table of sensibility, Zeitschrift fiir 
 
 Psychologic, II., 1891, 398. 
 SEBGI: Su alcuni caratteri del senso tattile, Eivista di Filosojia 
 
 Scientiftca, 1891. Same paper in German, Zeitschrift fur Psy- 
 chologic, III., 1892, 175-184. 
 STUMPF: Zum Begriff der Lokalzeichen, Zeitschrift fur Psychologic, 
 
 IY., 1892, 70-73. 
 
 SZABADFOELDI: Molcschott' s Untersuchungen, IX., 1865, 631. 
 YIEBOBDT: Physiologic des Menschen, Tubingen, 1877, 340 ff. 
 
 WEBEB: Der Tastsinn und das Gemeingefiihl, Wagner's Handwor- 
 
 terbuch der Physiologic, III., 2, 481-588. 
 Yo^r WITTICH: Bemerkungen zu Preyers Abhandlung iiber die 
 
 Grenzen des Empfindungsvermogens und Willens, Pfluger's 
 
 Archiv, II., 1869, 329-350. 
 WUNDT: Grundziige der physiologischen Psychologic, 3te Aufl., 
 
 Leipzig, 1887, I., 391 ff., II., 5 ff.; 4te Aufl., Leipzig, 1893, 
 
 I., 410 ff. 
 
24 LABORATORY COURSE IN PSYCHOLOGY. 
 
 For further bibliographical references see especially the citations of 
 the following authors: On Touch, Dessoir and Goldscheider. 
 On Temperature, Dessoir, Donaldson, Goldscheider. On Pres- 
 sure, see bibliographies following the chapter on Weber's Law 
 and the account of pressure sense apparatus below. On Pain 
 in general, see Bain, Mind, Ser. 2, I., 1892, 161; Marshall, 
 Ibid. Ser. 1, XIV., 1889, 511; XYL, 1891, 327, 470; Ser. 2, L, 
 1892,358, 453; Philosophical Review, I., 1892, 625; Nichols, 
 Ibid. L, 1892, 403, 518. 
 
KIN ESTHETIC AND STATIC SENSES. 25 
 
 CHAPTER II. 
 Kinaesthetic and Static Senses. 
 
 THIS group of senses furnishes us data for the perception 
 of the positions and motions of our members and of the 
 body as a whole, and plays a leading part in the perception 
 of space. It includes some senses whose existence or 
 efficiency is disputed (Innervation Sense l and Muscle Sense), 
 and others whose independence has only of late been gener- 
 ally recognized (Joint Sense and Tendon Sense). All are 
 closely united with one another and with pressure and 
 contact, and some are hardly ever dissociated except by 
 disease. This chapter is necessarily limited to the experi- 
 mental side of the subject and to the simpler experiments 
 to be found there. Many of the most important psycho- 
 
 1 The term " innervation sense " must not be taken too strictly as meaning a 
 wholly independent sense of motor discharge, as it has often been taken. Says 
 Wundt, in his last edition (4te Aufl. I., 425): "Manifold observations make it 
 probable that the central components of the sensations accompanying active 
 movements have their origin in the memory-images of movements previously 
 executed, which partly initiate, partly accompany, each voluntary movement. 
 Since memory-images possess qualitatively the same sensory content as the 
 original perceptions, such central sensations of effort and movement (Kraft- und 
 Bewegungsempfindungeri) will under normal conditions blend completely with the 
 more intense peripheral sensations of the same kind; they will, however, produce 
 an independent effect, if from any cause the peripheral sensations fall away. It 
 would be proper, therefore, to give up the term " innervation sensations " for the 
 sensations in question ; because it is liable to convey the false impression that 
 these are sensations which in and for themselves, without any relation to the 
 peripheral components of the sensations of effort and movement, accompany 
 the motor innervation. This assumption, which as a rule has formerly been con- 
 nected with the notion of "innervation sensations," is, however, very improb- 
 able." Cf. also p. 431, and in the third edition I., p. 405 ff. 
 
26 LABORATORY COURSE IN PSYCHOLOGY. 
 
 logical problems involve the motor sensations of the eye, 
 some of which are considered in Chap. VII. 
 
 MUSCLE SENSE, Kraftsinn. 
 
 Whether there are any specific muscular sensations 
 distinct from those that come from other parts of the 
 member in motion cannot now be asserted with positive- 
 ness ; but even if there be such, the part that they play in 
 our ordinary motor perceptions is probably a minor one. 
 The term " muscle sense," however, has been used to desig- 
 nate the whole group of motor sensations, and is here re- 
 tained for that purpose. 
 
 33. Lifted Weights. a. Weights lifted slowly seem 
 heavier than the same weights lifted rapidly. Lift the 
 same weight twice, lifting it first at the most natural and 
 convenient rate, and the second time very slowly, beginning 
 with much less than the necessary effort and gradually 
 increasing it till the weight rises. 
 
 b. Lift a moderate weight with one hand and at the same 
 time clench the other sharply. The weight will seem 
 lighter than when no simultaneous effort is made. 
 
 c. Repeat Ex. 23, using active lifting instead of pressure 
 in comparing the weights. 
 
 Charpentier ; on a, Goldscheider, A, 186. 
 
 34. Discriminative Sensibility for Lifted Weights. 
 
 a. Find by the method of experiment used in Ex. 24 what 
 is the just observable difference above and below a standard 
 weight of 100 grams, when the weights are lifted instead of 
 merely being allowed to press upon the skin. In this ex- 
 periment lift the weights successively with the same hand. 
 The weights must be placed near together within convenient 
 reach, and care must be taken that both are lifted at the 
 same rate and to the same height. Let the subject lift one 
 
KIN^ESTKETIC AND STATIC SENSES. 27 
 
 weight and then the other, and render his decision after 
 once lifting each. In half of the trials let the standard 
 weight be placed at the left side of the weight to be com- 
 pared and be lifted first ; in the other half let the weight to 
 be compared stand at the left and lead in the lifting. 
 
 b. Eepeat the experiment, letting the subject lift the 
 standard with one hand, and the comparison weight with 
 the other, keeping the same hand for each during each set 
 of trials (that is, during a determination of the just ob- 
 servable difference above and below), but combining a num- 
 ber of sets with the standard in the right hand with an 
 equal number in which it is in the left. Find also from the 
 figures the ratios when the standard is in the right hand 
 and when it is in the left hand, for use in Ex. 35. Com- 
 pare the ratios found in these experiments with that found 
 in Ex. 24. 
 
 In these experiments the sense of pressure might be ex- 
 pected to co-operate ; but when it is excluded, or put at a 
 relative disadvantage, the sensibility for differences of lifted 
 weights is not diminished. Weber's method of excluding 
 the pressure sense was to wrap the weights in pieces of 
 cloth, and lift them by the four corners together. The 
 pressure on these corners can be changed at will, irrespec- 
 tive of the heaviness of the weight lifted. 
 
 For fuller literature on lifted weights, see the chapter on 
 Weber's Law below. 
 
 Weber, 546-547; Miiller und Schumann; James, II. , 189 ff., 486 ff. ; 
 Beaunis; Wundt; Fullerton and Cattell. 
 
 35. Adjustment of the Motor Discharge. After having 
 performed the second part of Ex. 34, compare the standard 
 weight with a very much heavier weight, e.g., 2 kg., with 
 all the circumstances of actual careful judgment. Practise 
 this judgment thirty times, leaving a longer time between 
 
28 LABOEATOEY COUESE IN PSYCHOLOGY. 
 
 the individual comparisons than between liftings of the 
 weights compared. Then at once return to the smaller 
 weights, giving the standard to the same hand as before, 
 and to the hand that has just been lifting the 2 kg. the 
 weight to be compared. Not only will the weight just rec- 
 ognizably heavier before seem considerably lighter than the 
 standard, but also still heavier weights will seem so. This 
 time the tests must be few, not more than three or four. If 
 more tests are desired, practise the comparison of the stand- 
 ard and 2 kg. weight again ten times before taking them. 
 By the practice the nervous centres discharging into the 
 muscles that raise the 2 kg. weight become accustomed to 
 a larger discharge than that required for the small weights 
 and do not at once re-adapt themselves, but supply too great 
 a discharge. The weight now rises with greater rapidity 
 than the standard, and is consequently pronounced lighter 
 (Miiller and Schumann), or the balance between the ex- 
 tensors and flexors that was suited to raising the heavier 
 weight is not suited to the lighter weight, and the second is 
 pronounced lighter because of the strain in the extensors 
 necessary to restore the balance (Delabarre). This experi- 
 ment seems conclusive against a well-developed and inde- 
 pendent innervation sense ; for if there were any sensation 
 of nervous discharge, we ought to know when we go from 
 a very heavy to a light weight that the discharge is dis- 
 proportionate ; but we do not. 
 
 Miiller und Schumann; but cf. also Fullerton and Cattell, 131, 
 and Delabarre. 
 
 INNERVATION SENSE, 
 
 36. Simultaneous Movements. The evidence most fre- 
 quently offered in support of a special innervation sense is 
 clinical and therefore beyond the scope of this course. Ex- 
 periments of the type of the following have been brought 
 forward, but their interpretation has been disputed. 
 
KIN^STUETIC AND STATIC SENSES. 29 
 
 a. Stand erect before the blackboard, with the eyes closed 
 and coat off, if it interferes with free motion of the arms. 
 Draw with jeach hand, using both at once, a conventional leaf 
 pattern like those in the annexed cut, drawing always from 
 a to b. In drawing, try to make the 
 
 lobes of the leaf of equal size, like 
 those in Fig. 1 ; draw each with a 
 single simultaneous " free-hand " mo- 
 tion of the arms, that is, draw each 
 with a single volitional impulse di- 
 rected equally to the two sides ; the 
 last point is important. First draw 
 a pair of leaves, beginning them with 
 the hands before the shoulders at the 
 same height; the result will be ap- \ Fig. 2. 
 
 proximately like Fig. 1. Next draw 
 a pair with one hand about a foot higher than before, the 
 other about a foot lower ; the result will be like Fig. 2. 
 
 b. Bring the hands again to the position used in drawing 
 Fig. 1, and draw a pair of leaves having their apices right 
 and left. The leaves will be symmetrical. Next begin 
 with one hand about a foot farther away from the median 
 plane than before and the other at it, but both at the same 
 level. Draw as before ; asymmetrical leaves will be the re- 
 sult. Repeat the drawing a number of times, sometimes rais- 
 ing or extending one arm, sometimes the other. In general 
 it will be found that, notwithstanding the intention to make 
 equal movements of the hands, the motions of further ex- 
 tension in the extended arm and of further flexion in the 
 flexed arm are too short, and those in the contrary direction 
 in each case too long. The argument founded on this ex- 
 periment runs as follows : We think that our hands execute 
 equal movements, when they do not, because we are con- 
 scious of willing equal movements, and unconscious, or only 
 
30 LABORATORY COURSE IN PSYCHOLOGY. 
 
 inexactly conscious, of those actually made. If we per- 
 ceived motion of our members by the skin, joint, and muscle 
 sensations that accompany their motion (as the opponents 
 of the innervation sense believe) we ought to know the ex- 
 tent to which our hands are moved each time, and not to 
 fall into the illusion that we find in these experiments. 
 Cf. Ex. 44 d. 
 Loeb, B, 15 ff. 
 
 37. Illusory Movement in an Immovable Member.. Lay 
 the hand palm downward on the edge of the table or on a 
 thick book so that the last three fingers shall be supported 
 and held extended while the thumb and first finger remain 
 free. Bend the first finger considerably at both the inner 
 joints, and hold it in position with the other hand. The 
 finger-tip is still movable, as will be found on touching it ; 
 but it is anatomically impossible to move it voluntarily. 
 When, however, the effort is made to move it (the eyes be- 
 ing closed), there is a sensation of motion, though no actual 
 motion is possible. From this, an inner sense of motion 
 (innervation sense) has been inferred. When operating 
 upon another subject, the operator may hold the finger in 
 position, and require the subject to execute with the corre- 
 sponding finger of his free hand a motion equal to that 
 which he thinks he makes with the one that is held. Ob- 
 serve, however, that the tendons in the wrist move, and 
 that there are slight movements elsewhere in the hand. 
 
 Sternberg; James, II., 105, 515, footnote; Goldscheider, A, 317. 
 
 38. Terrier's Experiment. That the feeling of effort is 
 largely, if not entirely, of peripheral rather than central ori- 
 gin, appears from such experiments as the following. Hold 
 the finger as if to pull the trigger of a pistol. Think vigor- 
 ously of bending the finger, but do not bend it j an unmis- 
 
KIN AESTHETIC AND STATIC SENSES. 31 
 
 takable feeling of effort results. Eepeat the experiment, 
 and notice that the breath is involuntarily held, and that 
 there are tensions in other muscles than those that would 
 move the finger. Eepeat the experiment again, taking care 
 to keep the breathing regular and the other muscles passive. 
 Little or no feeling of effort will now accompany the imagi- 
 nary bending of the finger. 
 
 Ferrier, 382 ff. (English Ed.). 
 
 On Innervation Sense in general, besides the authors already men- 
 tioned, see : Wundt, 3te Ann. I., 397 ff., 4te Ann. I., 423 ff.; James, 
 II., 486 ff. ; Goldscheider, A, 206 ff. 
 
 SENSATIONS OF MOTION, JOINT SENSATIONS. 
 
 39. Passive Motion at the Elbow. Let the subject rest 
 his fore-arm flat upon the arm-board of the instrument 
 (bringing his elbow 
 over the hinge), and 
 close his eyes. Let 
 the operator then raise 
 or lower the free end 
 of the arm-board 
 slowly by pressing 
 down or lifting the 
 counter weight, and 
 require the subject to 
 announce when he first 
 perceives the motion 
 of his fore-arm. Re- 
 cord the angular move- 
 ment required to produce a just observable sensation. 
 Notice that the movement seems to be located chiefly in the 
 hand. It is extremely important not to mistake the sensa- 
 tion of increased pressure or of jar for that of motion. The 
 rate of movement will be found important, and should be 
 
32 LABOEATOEY COUESE IN PSYCHOLOGY. 
 
 kept as constant as possible. The results found in this 
 way are rough ; for more exact methods see Goldscheider, A. 
 
 40. Active Movement of the Last Joint of the Finger. 
 The joint sensations of the fingers are less fine than those 
 of the elbow, but are more convenient for demonstration of 
 active flexion. Fasten a piece of straw, with court-plaster 
 or otherwise, to the finger-nail of the middle finger, and cut 
 it off at such a length that the distance from the joint of 
 the finger to the end of the straw shall be 115 mm. With 
 that radius 2 mm. corresponds to about 1 of angular meas- 
 ure. Rest the hand on a thick book, letting the last joint of 
 the finger extend beyond the edge. Set up a millimeter 
 scale at right angles with the straw. Close the eyes and 
 make the least possible flexion of the finger at the last joint, 
 having an assistant note its extent on the scale. Close at- 
 tention may perhaps be able in both the active and passive 
 movements to locate the sensation in the joint, but more 
 rigorous experiments are required to show its character 
 clearly, and to prove its location. 
 
 Goldscheider, A. 
 
 41. Location of Movements, a. Motions on the skin can 
 be interpreted either as the movement of an object over the 
 surface of the skin, or of the skin over the surface of the 
 object. This opens the way for illusions. Have an assis- 
 tant draw a pencil-point gently across the wrist or the finger- 
 tips of the observer, who sits with closed eyes. A tendency 
 to interpret the sensation as motion of the wrist or finger 
 will be observed. The hand and arm must be held free, so 
 that the illusion may not be corrected by the presence of 
 other touch sensations. 
 
 b. With the eyes closed, move the wrist or finger over 
 a stationary pencil-point. In this case the point also seems 
 to be in motion in a direction contrary to that of the hand. 
 
KIN^STHETIC AND STATIC SENSES. 33 
 
 c. When the movement may be interpreted as belonging 
 to either of two members, it may be credited to the more 
 mobile of the two, or may be shared by both. Eest the 
 finger lightly on the forehead ; then, taking pains to keep its 
 position fixed, move the head from side to side. There is a 
 strong tendency to credit the motion to the finger and arm. 
 Hold the last three fingers close together, and move the first 
 away from them and toward them again. All will seem to 
 move, but the last three in an opposite direction to the 
 first. 
 
 d. Ex. 4 above is an experiment on the location of move- 
 ments as well as of touches. If the cane is swung so as to 
 describe the surface of a cone we are conscious of the path 
 described by its point, as well as that of the hand holding it. 
 
 Cf . Ex. 39 where the motion of the whole fore-arm and 
 hand is credited chiefly to the latter. 
 Vierordt; on c, Goldsclieider, A, 181 ff. 
 
 42. Interrupted Extent may seem smaller to a moving mem- 
 ber than uninterrupted. In a piece of cardboard make three 
 pin-holes in a line separated by spaces of an inch and a half. 
 Fill one of the spaces with pin-holes a quarter of an inch apart. 
 Turn the card over, close the eyes, and move the finger-tip 
 across the little eminences made by the pin-holes. The 
 illusion seems more marked when the finger moves over the 
 interrupted half of the line first. Examine the card visu- 
 ally, and notice that the visual illusion is in the directly 
 opposite sense. As in the similar touch experiment above 
 (Ex. 8) the results are apparently not equally clear for all 
 observers. 
 
 James, II., 250. 
 
 SENSATIONS OF RESISTANCE. 
 
 43. Illusory Resistance, a. Hold a heavy weight by a 
 string so that it hangs, with the arm extended, a few inches 
 
34 LABORATORY COURSE IN PSYCHOLOGY. 
 
 above the floor, or better, have the string placed in the 
 hand by an assistant so that the length of the string may 
 not be known beforehand. Lower the weight rather rapidly 
 till it rests on the floor or other support. As it strikes, a 
 sensation of arrest will be perceived, somewhat as though 
 the hand were suddenly supported by a light rod. The illu- 
 sion is even more marked when the string, instead of being 
 held in the hand, is fastened to a small rod, and that is 
 held. The disturbing noise of the weight may be obviated 
 by having it come to rest on a cushion or in a box of sand. 
 The illusion is due to the unexpected strain put upon the 
 muscles that lower the arm by the tension of those that have 
 been holding the weight. This feeling of arrest is prob- 
 ably a joint sensation. To distinguish this sensation from 
 the motion sensations of the joints, Goldscheider has called 
 it a "joint-pressure sensation." 
 
 b. When the movement of the rod is continued downward 
 beyond the point at which the sensation of arrest is felt, a 
 certain difficulty of movement may still be observed, as 
 though the rod were moving through a resisting medium. 
 This sensation Goldscheider distinguishes from the sensa- 
 tion observed in #, believing it to be the true sensation of 
 difficult motion (of weight and heaviness also) and credit- 
 ing it to the tendons. 
 
 c. Notice something similar to b in pouring a quantity 
 of mercury rapidly from one vessel to another. 
 
 It is evident that such illusions as these speak against 
 the existence of an innervation sense in the common accep- 
 tation of the term. 
 
 Goldscheider, A, 164 ff., 172 ff., D; on &, A, 188; Mach, A, 70 ff. 
 
 BILATERAL ASYMMETRIES OF POSITION AND MOTION. 
 
 44. Apparently Symmetrical Motions of the arms. In all 
 the tests of this group, the subject should be kept in ignor- 
 
AND STATIC SENSES. 35 
 
 ance of the nature and amount of his errors till the tests 
 are finished. 
 
 a. Hold, an ordinary cork between the thumb and first 
 two fingers of each hand. Close the eyes and bring the two 
 corks together at arm's length in the median plane before 
 the face, having an assistant note the approximate amount 
 and direction of the error. The corks should be brought 
 together rather gently, so as not to betray the character of 
 the error to the operator, but the motions of the arms by 
 which they are brought up nearly to contact should be free 
 and sweeping. The error will probably be found rather con- 
 stant in direction until the operator learns to correct it. 
 Try bringing the corks together above the head, and also in 
 asymmetrical positions. 
 
 b. Let the subject seat himself at a table with the milli- 
 meter scale before him. Set a pin in the middle of the scale, 
 and bring the pin into the median plane of the subject and 
 make the scale parallel to his frontal plane. Let the sub- 
 ject place his forefingers on either side of the pin, and, with 
 closed eyes, try to measure off equal distances by moving 
 both simultaneously outward along the scale. Note the 
 result in millimeters ; for this it may be convenient to mark 
 the middle point of the finger-nails with an ink-line. A 
 constant excess in the motion of one hand or the other will 
 often be found. It is important that the subject should not 
 open his eyes till his fingers are removed from the scale ; 
 for he will find it difficult not to correct his error if he 
 knows its nature. The finger-tips should rest lightly on 
 the scale, and the motions should be made from the shoulder 
 by a single impulse ; if they are too slow, and the subject 
 attends to his sensations of position, the errors will be small 
 and uncertain. The left hand, it is said, generally makes the 
 greater excursion in right-handed persons not mechanics. 
 
 c. Eepeat the tests, having the motions of the hands made 
 
36 LABORATORY COURSE IN PSYCHOLOGY. 
 
 successively instead of simultaneously. The constant differ- 
 ence between the hands will probably not appear. 
 
 d. Let the subject start with his right and left hand each 
 20 cm. toward its own side of the median plane, and try 
 to measure off equal distances on either side of those 
 points, moving both hands at once in the same direction. 
 Distances inward will be made too large, distances outward 
 too small. In all these experiments with closed eyes we 
 seem inclined to judge distance rather from the intention 
 of equal motion and the continuance of motor sensations 
 for equal times, than from the actual peripheral sensations. 
 
 The judgments of symmetry of position and motion rest 
 upon very complex combinations of the dermal and kin- 
 aesthetic sensations, already made the subject of experiment 
 above. As a result of this complexity the experiments of 
 this group will be found to give rather variable results, 
 from one subject to another, and in the same subject at 
 different times. 
 
 Hall and Hartwell; Loeb; Delabarre; Block. 
 
 RECOGNITION OF THE POSITION OF THE BODY AS A WHOLE. 
 45. Recognition of Direction. In this experiment- it is 
 especially desirable that the subject should know as little as 
 possible of the purpose of the experiment. Cause him to 
 stand erect with his back against a wall. Choose a point 
 on the opposite wall about the height of his shoulders. Let 
 him look at it, and then require him, having closed his eyes, 
 to point to it as exactly as possible with a light rod held 
 symmetrically in both hands. Cause him also to hold the 
 rod vertically and horizontally in the median plane ; also 
 horizontally parallel to the frontal plane. All these he will 
 probably be able to do with much accuracy ; or if, as some- 
 times happens, he shows a "personal equation," his error 
 will be constant. 
 
KIN^STHETIC AND STATIC SENSES. 37 
 
 a. Cause the subject to repeat the experiment, this time 
 turning his head as far as possible to the left after closing 
 his eyes, taking pains to keep his shoulders square. Eepeat, 
 causing the subject to turn to the right. In both cases an 
 error will be observed, the subject pointing too far in a 
 direction opposite to that of the turning of the head. The 
 subjeckwill be able to hold the rod vertically, or horizontally, 
 without error. Cause the subject to hold the rod in what 
 he thinks is a horizontal position, in the median plane when 
 his head is thrown well back; when bowed well forward. 
 Illusions like those observed above, affecting directions in 
 the plane of movement of the head, will result. Cause the 
 subject to hold the rod in what he thinks is a horizontal 
 position, parallel to the frontal plane, when his head is 
 bowed to the right ; when bowed to the left. Illusions sim- 
 ilar to those in the previous experiments will appear. In all 
 these cases judgment of one cardinal direction in space alone 
 is affected ; the other two show little or no errors. 
 
 b. Eepeat the first part of experiment a; but instead of 
 having the subject point to the designated object, have him 
 walk toward it, keeping his shoulders square, his eyes shut, 
 and his head turned to one side. He will walk more and 
 more too far toward the side away from which his head is 
 turned. 
 
 c. The illusion is due, at least in the case of turning the 
 head about a vertical axis, to the position of the eyes ; the 
 eyes turn farther than the head in the direction in which 
 it is turned, as may easily be observed upon any other per- 
 son. From the eyes we judge the position of the head, and 
 thus over judging it, point too far in a contrary direction in 
 trying to point to the required object (Delage). The illu- 
 sions can be produced by motion of the eyes alone. Holding 
 the head erect, and taking pains not to move it when moving 
 the eyes, turn the closed eyes as far as possible_fcQjhe right 
 
38 LABORATORY COURSE IN PSYCHOLOGY. 
 
 or left, and then try to point to some determined object. An 
 error like that in a will be observed. Turning of the eyes 
 upward or downward has a doubtful result. Instead of 
 closing the eyes, they may be kept open if an opaque 
 screen is held close before the face. Eepeat a, voluntarily 
 turning the eyes as far as possible in the direction opposite 
 to that of the turning of the head. The original error will 
 probably disappear, or be found to have changed its sign. 
 
 For this illusion another eye explanation is suggested by 
 Breuer, namely, that in such extreme turnings of the eyes, 
 their actual position does not correspond with the intended 
 position, but conies short of it. We infer the direction, 
 however, from the intended position, and thus fall into the 
 error in pointing. For the illusion in other positions of the 
 head and even for this, his own preferred explanation is 
 again different, and is partly based on the following experi- 
 ment. 
 
 d. Close the eyes, and touch the tip of the nose or the 
 forehead with a pin or a pencil while the head is in the 
 usual position, and after a little try to touch the same spot 
 again. The error, if any, will be very small. Eepeat the 
 touch in the normal position, and then turn the head to the 
 right or left or incline it toward the shoulder or forward or 
 backward. After holding it in the chosen position for half 
 a minute, attempt to touch the spot again. Gross errors 
 will result till corrected by practice. The error is one of 
 underestimation, and should by itself alone produce a result 
 directly the reverse of that found by Delage. Breuer, how- 
 ever, introduces another factor. His explanation for the 
 inclined positions of the head is somewhat as follows : by 
 means of the otolith-apparatus of the ear, we get a true 
 perception of the amount of inclination of the head, at the 
 same time that we get the erroneous perception just men- 
 tioned. The only way in which we can harmonize the 
 
KIN^STHETIC AND STATIC SENSES. 89 
 
 Conflicting perceptions is by altering our judgment of the 
 vertical, and with that, of course, of the horizontal. For the 
 movements of rotation about a vertical axis the semi-circular 
 canals (See Exs. 47-49) would furnish the knowledge of 
 the true amount of turning, and from a similar combination 
 of the true and false the illusions in that case would result. 
 
 This group of experiments, except perhaps the last, when 
 tried under the ordinary conditions of the practice labora- 
 tory, seems liable to considerable individual variation ; but 
 sufficient care, especially as to the position of the eyes in 
 turning to the right and left, should lead to a tolerable 
 degree of success. 
 
 Aubert (Delage), 17 ff.; Loeb, B, 20 f., 31 f.; Breuer, 270 ff. 
 
 46. Vertical and Horizontal Positions of the Body. Secure 
 the subject properly upon the tilt-board, and have him close 
 his eyes. Start with 
 the board vertical (head 
 up). Require the sub- 
 ject to describe his po- 
 sition. He will prob- 
 ably announce that he 
 is then leaning forward 
 slightly. As a matter 
 of fact he is, if his heels 
 are against the board. 
 Turn him slowly back- 
 ward, and require him 
 to say when he seems to 
 himself vertical (head 
 up), when he seems 
 tilted backward at an angle of 45 from the vertical, when at 
 an angle of 60, when at 90, when at 180. Two classes of 
 illusions will be found : angles of less than 40 will prob- 
 ably seem too small ; those from 40 to 60 will be rightly 
 
40 LABORATORY COURSE IN PSYCHOLOGY. 
 
 i 
 
 judged ; those beyond 60 will seem too large. The subject 
 will say that he is vertical, head downward, when he is yet 
 30-60 from it. The subject may be allowed a pillow if he 
 desires it. 
 
 The illusions depend in large measure on the distribution 
 of pressure on the soles and other surfaces of the body and 
 the direction of pressure of the movable viscera and the 
 blood. 
 
 Aubert (Delage), 40 ff. ; Breuer, 270 f. 
 
 SENSATIONS OF ROTATION. 
 
 47. Perception of Uniform Eotations. Let the subject 
 be seated upon the rotation table with closed eyes, blind- 
 folded if necessary. Turn the table slowly and evenly in 
 one direction or the other. The subject will immediately 
 recognize the direction and approximately the amount of 
 rotation when the rate is as m slow as 2 per second, or even 
 slower. After continued rotation at a regular rate the 
 sensation becomes much less exact or entirely fails. This 
 fact has been generalized by Mach in the law that only 
 change of rate, not continuous rotation, is perceived. After 
 some pauses and short movements in one direction and the 
 other, the subject may become quite lost, and give a totally 
 wrong judgment of the direction of motion, if it is slow. 
 
 48. Illusion of Backward Eotation. Let the subject be 
 seated as before. Rotate him a little more rapidly for half 
 a turn, and then stop him suddenly. A distinct sensation 
 of rotation in the opposite direction will result. Repeat, 
 and when the illusory rotation begins, open the eyes. It 
 immediately ceases. Close the eyes again, and, if strong, it 
 again returns. 
 
 49. Location of the Organs for the Perception of Rotation. 
 a. Repeat the first part of Ex. 48, letting the subject 
 
KIN ESTHETIC AND STATIC SENSES. 41 
 
 give the word for stopping. At the same instant let him 
 incline his head suddenly backward or forward, or lay it 
 upon one shoulder or the other. The axis of rotation of 
 the body will appear to change in a direction opposite to 
 that of the inclination of the head ; i.e., if the head is in- 
 clined to the right, the axis seems to incline to the left. 
 The feeling is as if the body were rotating in the surface 
 of a cone in a direction contrary to that of the first rotation. 
 The head dictates the apparent axis of rotation. The same 
 illusion occurs if the head is inclined during the actual 
 rotation and straightened at the word for stopping. Turning 
 the head to the right or left introduces no such illusion, 
 because it does not change the axis of rotation of the head. 
 The illusion comes out with very disagreeable strength 
 when the rotation is rapid, and the subject changes the 
 position of his head during the rotation. 
 
 b. Let the subject lie upon his side, and rotate him rather 
 rapidly till the sensation of rotation becomes faint or disap- 
 pears. Then let him turn suddenly upon his back or upon 
 his other side. Turning upon his back starts rotation about 
 a new axis, and it is felt in its true sense, while the rotation 
 about the previous axis is felt as an illusion in its reverse 
 sense. The resulting perception combines both. Turning 
 completely over reverses the direction of motion completely, 
 and the combined sensation and illusion produce a corre- 
 spondingly powerful effect. 
 
 The change of the apparent axis of rotation with the 
 change of position of the head points to the location in the 
 head of the organ for such sensations. For the experiments 
 by which the semicircular canals are indicated as this organ, 
 and the arguments pro and con, see the literature cited by 
 Aubertj Ayres, and others. 
 
 On the last three experiments, see: Aubert (Delage), 49 ff. ; 
 Brown; Mach; Wundt, 3te Aufl., L, 211 f.;.IL, 24,139. 
 
42 LABORATORY COURSE IN PSYCHOLOGY. 
 
 .. 50. Another Illusion of Eotation (Purkinje's dizziness) 
 is due to involuntary motions of the eyes. Let the subject 
 whirl rapidly on his heels with his eyes open till he begins 
 to be dizzy. At first objects about him seem at rest, then 
 to be turning in the opposite direction. Let him now stop 
 and look at an even surfaced wall while the experimenter 
 carefully observes his eyes, picking out some clearly marked 
 fleck or spot as a point of observation. To the subject the 
 surrounding objects will seem to continue to move in the 
 same direction as before ; i.e., in a direction contrary to his 
 previous rotation; the experimenter will see the subject's 
 eyes executing slow motions in one direction (in the direc- 
 tion of the original motion of the subject) alternating with 
 rapid motions in the other. The subject himself may be 
 able to perceive a corresponding irregularity of motion in 
 the spots upon the wall at which he looks. He can easily 
 observe the motions of his own eyes if he looks fixedly for 
 twenty or thirty seconds at a flame or a strip of white paper 
 in a bright light before beginning his rotation ; the after- 
 image (see Chapter Y.) thus produced remains fixed on the 
 retina, and its apparent movements betray the motions of 
 the eye. If the eyes are closed after the rotation, the image 
 will seem to move in one direction, and rather slowly. The 
 illusion rests upon the subject's unconsciousness of the 
 slow motions of his eyes. It is probable that these eye 
 motions and the sensations of attempted restoration of equi- 
 librium in other parts of the body are reflexly caused by the 
 disturbance in the semicircular canals. 
 
 It should be noticed that this illusion is the exact 
 reverse of that found with closed eyes in Ex. 48. There the 
 subject feels a rotation of his own body contrary to that it 
 previously received. If he was turned at first in the direction 
 of the hands of a watch, on being stopped he would seem to 
 be turning in a direction contrary to the hands. If these 
 
KIN AESTHETIC AND STATIC SENSES. 43 
 
 motions were transferred to objects about him, they would, 
 during the rotation, seem to move contrary to the hands, 
 and after stopping, in the direction of the hands. In the 
 Purkiiije experiment the motion of objects is not thus re- 
 versed. 
 
 Those who try these rotational experiments should do so 
 with caution, for the unpleasant effects of them sometimes 
 last several hours. 
 
 Aubert (Delage), 52, 100 ff. ; Mach. Aubert reprints Purkinje's 
 paper on dizziness as an appendix to the translation of Delage. 
 
 SENSATIONS OF PROGRESSIVE MOTION. 
 
 51. Progressive motions, so far as they do not involve 
 rotation, probably give us combinations of sensations from 
 several different sources. The principle holds for progres- 
 sive motions as for rotations, that we perceive changes of rate 
 of motion, and not uniform motion ; as long as the motion 
 remains uniform we can by an effort of imagination conceive 
 ourselves to be moving in either direction or to be standing 
 still, except for what jarring there may be. The apparatus 
 for the study of these phenomena will be found in railroad 
 trains and elevators. See also Mach for special laboratory 
 apparatus. 
 
 Aubert (Delage), 75 ff. ; Mach; Brown; Breuer, 283. 
 
 BIBLIOGKAPHY. 
 
 AUBERT : Physiologische Studien iiber die Orientierung, Tubingen, 
 1888, 122. This is a translation of the paper of Delage 
 below, with full notes and appendix containing Purkinje's Bul- 
 letin von 1825, Ueber den Schwindel, See criticism by Breuer, 
 270 ff. 
 
44 LABORATORY COURSE IN PSYCHOLOGY. 
 
 AYRES : A. A Contribution to the Morphology of the Vertebrate 
 Ear, with a Keconsideration of its Functions, Journal of 
 Morphology, VI., Nos. 1 and 2, May, 1892. Ayres gives a 
 bibliography of nearly three hundred titles, many upon the 
 psycho-physiology of the semicircular canals, but not a complete 
 list. 
 B. The Ear of Man : Its Past, Present, and Future. Wood's 
 
 Holl Biological Lectures, 1890. An Abstract of A. 
 BASTIAN : "The Muscular Sense; " Its Nature and Cortical Locali- 
 sation, .Brain, X., 1887-88, 1-89. Discussion on the paper by 
 Ferrier, Sully, and others, 89-137. 
 BEAUNIS: Les Sensations internes, Paris, 1889. 
 BLOCK : Experiences sur les sensations musculaires, Revue Scien* 
 tifique, XLV., No. 10, 1890, 294-301. 
 
 BBEUEB: Ueber die Function der Otolithenapparate, Pfluger's 
 Archiv, XL VIII., 1890-91, 195-304. 
 
 BROWN: A. On the Sense of Kotation and the Anatomy and Physi- 
 ology of the Semicircular Canals of the Internal Ear, Journal 
 of Anatomy and Physiology, VIII., 1874, 327. Reprinted by 
 Mach, A, 100. 
 B. On Sensations of Motion, Nature, XL., 1889, 449. 
 
 CHARPENTIER : Analyse experimental de quelques elements de la 
 sensation de poids, Archives de Physiologic, Ser. 5, III., 1891, 
 122-135. 
 
 DELABARRE: Ueber Bewegungsempfindungen, Inaug. Diss., Frei- 
 burg, 1891, 111. The author has also published a portion of 
 the same matter in Mind, Ser. 2, 1892, 379-396. 
 
 DELAGE: fitudes experimentales sur les illusions statiques et dy- 
 namiques de direction pour servir a determiner les fonctions 
 des canaux demicirculaires de 1'oreille interne, Archives de 
 Zool. Exper., No. 4, 1886, 535-624 (with index). Translated 
 by Aubert above. See also abstract of this paper in Comptes 
 rendus, CIIL, 1886, 749. 
 
 EWALD: Physiologische Untersuchungen iiber das Endorgan des 
 Nervus octavus, Wiesbaden, 1892. Gives a very extended 
 bibliography. 
 
 FERBIER : Functions of the Brain, London, 1886. 
 
KIN^ESTHETIC AND STATIC SENSES. 45 
 
 FULLERTON AND CATTELLi On the Perception of Small Differ- 
 ences, Publications of the University of Pennsylvania, Philo- 
 sophical Series, No. 2, Philadelphia, 1892. 
 
 FUNKE: Der Tastsinn und die Gemeingefuhle, Hermann's Handbuch 
 der Physiologic, III., pt. 2, 289-414. 
 
 GOLDSCHEIDER i A. Untersuchungen iiber den Muskelsinn, Du Bois- 
 Reymond's Archiv, 1889, 369 ff. and 540, also Supplement- 
 Band, 1889, 141 ff. 
 B. Ueber den Muskelsinn und die Theorie der Ataxie, Zeitschrift 
 
 fur klin. Med., XV., 1888-89. 
 (7. Ueber die Grenzen der Wahrnehmung passiver Bewegungen, 
 
 Centralblattfur Physiologic, I., 1887, 223-225. 
 D. Ueber paradoxe Widerstandsempfindung, Ibid, III., 1889, 90-91. 
 
 HALL AND HARTWELL: Bilateral Asymmetry of Function, Mind, 
 IX., 1884, 93-109. 
 
 JAMES : Principles of Psychology, New York, 1890. 
 
 KREIDL : Beitrage zur Physiologic des Ohrlabyrinthes auf Grand von 
 Versuchen an Taubstuminen, Pfluger's Archiv, LI., 1891-92, 
 119-150. 
 
 LOEB: A. Untersuchungen iiber den Fiihlraum der Hand; Erste 
 Mittheilung, Gleiche Fiihlstrecken, Pfluger's Archiv, XLL, 
 1887, 107-127. 
 
 B. Untersuchungen iiber die Orientirung im Fiihlraum der Hand 
 und im Blickraum, Ibid. XL VI., 1890, 1-46. 
 
 MACH: A. Grundlinien der Lehre von den Bewegungsempfindungen, 
 
 Leipzig, 1875, 128. Gives bibliography of thirty-one titles. 
 B. Analyse der Empfindungen, Jena, 1886, 69 ff. 
 
 MULLER UND SCHUMANN: Ueber die psychologischen Grundlagen 
 der Vergleichung gehobener Gewichte, Pfliiger's Archiv, XLY., 
 1889, 37-112. 
 
 MUNSTERBERG: Die Willenshandlung, Freiburg, 1888. 
 
 SCHAEFER: Die Erklarung der Bewegungsempfindungen durch den 
 Muskelsinn, Inaug. Diss., Jena, 1889; also an article of similar 
 title, Pfluger's Archiv, LXL, 1887, 566-640. 
 
 STERNBERG : Zur Lehre von den Vorstellungen iiber die Lage un- 
 serer Glieder, Pfluger's Archiv, XXXYII., 1885, 1. Gives 
 bibliography of fifty-two titles. 
 
46 LABOEATOEY COURSE IN PSYCHOLOGY. 
 
 VIERORDT: Die Bewegungsempfindung, Zeitschrift filr Biologie, 
 XII. , 1876, 226-240. See also Vierordt's Physiologic, 5te Aufl., 
 329 ff. 
 
 WALLER: The Sense of Effort, Brain, XIY., 1891, 179-249, 
 433-436, especially 229 ff. This study is accompanied by a 
 bibliography of fifty titles. Nearly the same portion of the 
 paper indicated here as of special importance will be found 
 under the following title : Experiments on Weight-discrimina- 
 tion, Proceedings of the Physiological Society, Session of Jan. 
 30, 1892, Journal of Physiology, XIII. , May, 1892. 
 
 WEBER: Work cited in bibliography of Chap. I. 
 
 WLASSAK: Die statischen Functionen des Ohrlabyrinthes und ihre 
 Beziehungen zu den Eaumempfindungen, Vierteljahr. fur iviss. 
 Philosophic, XVL, 1892, 385-403, XVII., 1893, 15-29. 
 
 WUNDT: Work cited in bibliography of Chap. I., 3te Aufl., I., 397 ff., 
 II., 21 ff.; 4te Aufl., I., 419 ff. 
 
SENSATIONS OF TASTE AND SMELL. 47 
 
 CHAPTER III. 
 Sensations of Taste and Smell. 
 
 THESE sensations are of secondary importance in psy- 
 chology, and have received a correspondingly small share 
 of investigation. In subjective quality they seem to stand 
 midway between the general senses mentioned at the end 
 of Chapter I. and the higher senses of Hearing and Vision. 
 
 SENSATIONS OF TASTE. 
 
 52. Tastes and Smells. Much of what is commonly 
 called taste is really a combination of taste with smell and 
 with touch in its various forms. With the nostrils held, try 
 to distinguish by taste alone between small quantities of 
 water and a weak solution of essence of clove in water. 
 A discrimination that is easily possible with the nostrils 
 open is difficult or impossible with the nostrils closed. 
 The solution should not be swallowed, for then the olfac- 
 tory region may be reached from the back of the nose. 
 
 53. Distribution of the Organs of Taste, a. Using the 
 weaker taste solutions, and operating upon yourself with a 
 mirror or on another person, find out as nearly as you can 
 in what part of the tongue the strongest sensations are pro- 
 duced by each. Test the tip, the sides, the back, and the 
 middle, putting the solutions on with a cameFs-hair brush, 
 and rinsing the mouth as often as necessary. Try also the 
 hard and soft palates. 
 
 b. Dry the tongue with a handkerchief, and test the in- 
 dividual fungiform papillae with the stronger solutions, 
 
48 LABORATORY COURSE IN PSYCHOLOGY. 
 
 applying them with fine camel's-hair pencils. It will be 
 found possible to get taste sensations from the single 
 papillae, though perhaps not all four from each. Einse the 
 mouth as needed. Test the surface of the tongue between 
 the papillae and observe that no taste sensations follow. 
 
 a. Rittmeyer; b. Oehrwall. 
 
 54. Minimal Tastes, a. Find what is the greatest dilution 
 of the weaker solutions in which the characteristic tastes can 
 still be recognized. The same quantity, e.g., half a teaspoon- 
 ful, should be taken into the mouth at each trial, and may 
 be swallowed with advantage. Einse the mouth thoroughly 
 as required. The following are the average proportions 
 found by Bailey and Nichols for male observers : Quinine, 
 1 : 390 000 ; Sugar, 1 : 199 ; Salt, 1 : 2240 ; for Sulphuric Acid, 
 which they used instead of Tartaric, the proportion was 
 1 : 2080. 
 
 b. The intensity of the sensation and the greatest dilution 
 still tastable depend on the number of taste organs stimu- 
 lated. Take a portion of one of the solutions of just tast- 
 able strength, found in a, add an equal quantity of water, 
 and take a large mouthful of the mixture. The character- 
 istic taste will still be perceived, perhaps more strongly 
 than before. 
 
 a. Bailey and Nichols, A\ Lombroso und Ottolenghi; Camerer, A. 
 b. Camerer, B. 
 
 55. Discriminative Sensibility for Taste. For a rough 
 determination, test with solutions of sugar, taking first a 
 small quantity of the standard 20% solution, then an equal 
 quantity (the equality is important) of one of the weaker 
 solutions, or first one of the weaker and then the standard, 
 until a solution is found that is just recognizably different 
 from the standard. Make this determination several times. 
 The excess of sugar in the standard solution over the 
 
SENSATIONS OF TASTE AND SMELL. 49 
 
 amount in the solution just observably weaker, set in a ratio 
 to the total percentage of sugar in the standard, measures 
 the sensibility. Some experimenters may be able to dis- 
 tinguish the 18% from the 20% solution; their sensibility 
 would then be expressed by the ratio 2 : 20. 
 Keppler. 
 
 56. Electrical Stimulation, a. Using a constant current, 
 from a single Grenet cell, for example, and two small zinc 
 electrodes, one applied to the inner surface of the under lip 
 and the other to the tongue, notice the sour taste at the 
 positive pole and the alkaline at the negative. 
 
 Von Yintschgau, 181 ff. ; Oehrwall ; Hermann. 
 
 SENSATIONS OF SMELL. 
 
 57. Minimal Odors. The keenness of smell may be 
 tested with dilute solutions of odorous substances or with 
 the olfactometer. 
 
 a. Test with solutions. Pour small quantities of the 
 solutions of oil of cloves into little wide-mouthed bottles, 
 filling each to about the same height. Mark all in an in- 
 conspicuous manner. Set the bottles a foot apart on a 
 table in a place where there is moderate circulation of 
 air, in the order of the strength of their solutions, be- 
 ginning with the water and following with the weakest so- 
 lution and so on. Require the subject to smell of the 
 bottles in succession without lifting them from the table, 
 beginning with the water, and to indicate that in which he 
 first recognizes a characteristic odor. If the solutions stand 
 for any length of time where they are subject to evaporation, 
 it will be safer to prepare fresh ones before undertaking a 
 new test. Other precautions will suggest themselves, such 
 as the use of similar bottles, and care in filling them that 
 none of the solution is left clinging near the mouth. 
 
50 LABORATORY COURSE IN PSYCHOLOGY. 
 
 b. Test with the olfactometer. Test the sides of the nose 
 separately. Push the odor-tube on till its end is flush with 
 that of the glass tube, insert the bent end of the latter into 
 the nostril, and gradually lengthen the exposed surface 
 of the odor-tube till its odor is just discernible. Note in 
 millimeters the length exposed. 
 
 a. Bailey and Nichols, B\ Lombroso und Ottolenghi; Savelieff; 
 b. Zwaardemaker, A and C. 
 
 58. Discriminative Sensibility for Odors. Using the 
 double olfactometer with both odor-tubes drawn out far 
 enough to give an unmistakable odor, but not too strong a 
 one, say both drawn out 5 cm., find how far one or the other 
 must be drawn out (or pushed back) to make the odor which 
 it gives just observably stronger (or weaker) than that of 
 the other. The test should be made with the sides of the 
 nose separately (there is frequently a difference in sensi- 
 tiveness between the two sides, due to mechanical obstruc- 
 tion or other cause), unless for some reason a bilateral form 
 of experiment is desirable. Try a number of times, in half 
 the tests smelling the weaker before the stronger, and in 
 half the stronger before the weaker, but be careful to avoid 
 fatigue. 
 
 59. Fatigue of Smell, a. Hold a piece of camphor gum 
 to the nose, and smell of it continuously, breathing in 
 through the nose and out through the mouth, for five or ten 
 minutes. A very marked decrease in the intensity of the 
 sensation will be observed, reaching perhaps even to com- 
 plete loss of the odor. 
 
 b. It is important, however, to observe that fatigue for one 
 substance does not cause obtuseness for all other substances, 
 though it does for some. Smell of some essence of cloves 
 and of some yellow wax, then fatigue for camphor as in a, 
 and smell of the essence of cloves and of the wax again. 
 
SENSATIONS OF TASTE AND SMELL. 51 
 
 The odor of the wax will probably be fainter, that of the 
 essence of cloves unaffected. 
 Aronsohn. 
 
 60. Combination of Odors. Experiment with the olfac- 
 tometer on one side of the nose as follows. Hold against 
 the end of the rubber odor-tube another odor-tube of wax 
 (partly covered on the inside by a glass tube of the same 
 size as that used in the olfactometer), in such a way that 
 the air must pass through both to reach the nose. Then 
 gradually increase the length of the rubber tube exposed 
 till the odor of the wax is no longer perceived. If the 
 experiment is carefully performed, a point may be found 
 where the odors nearly balance. If the rubber is length- 
 ened beyond this point, its odor overpowers that of the 
 wax ; if it is shortened, it is overpowered by that of the 
 wax. A mixture of the odors in which both can be detected 
 is difficult to find. Care should of course be taken to avoid 
 fatigue. 
 
 A similar balance of odors was found by Zwaardemaker 
 when the double olfactometer was used and the two sides of 
 the nose received separate stimuli. 
 
 Zwaardemaker, B. 
 
 BIBLIOGRAPHY. 
 
 ARONSOHN: Experimentelle Untersuchungen zur Physiologic des 
 
 Geruchs, Du Bois-Reymond' s Archiv, 1886, 321-357. 
 BAILEY AND NICHOLS: A. The Delicacy of the Sense of Taste, 
 
 Nature, XXXVII., 1887-88, 557; also Science, 1888, 145. 
 B. The Sense of Smell, Nature, XXX V., 1886-87, 74. 
 CAMERER: A. Die Grenzen der Schmeckharkeit von Chlornatrium 
 
 in wasseriger Losung, P finger's Archiv, II., 1869, 322. 
 B. Die Methode der richtigen und falschen Fiille angewendet auf 
 den Geschmackssinn, Zeitschrift fur Biologie, XXL, 570. 
 
52 LABORATORY COURSE IN PSYCHOLOGY. 
 
 COBIN: Action des acides sur le gout, Archives de Biologie, VIII. , 
 1888, fasc. 1, 121-138. 
 
 GOLDSCHEIDER UNO SCHMIDT : Bemerkungen iiber den Geschmack- 
 sinn, Centralblattfur Physiologic, IV., 1890, 10-12. 
 
 HAYCRAFT: The Nature of the Objective Cause of Sensation; Taste, 
 Brain, X., 1887, 145-163; Smell, Ibid., XL, 1888-89, 166-178. 
 
 HERMANN: Beitrage zur Kenntniss des elektrischen Geschmacks, 
 nach Versuchen von Laserstein, Pfliiger's Archiv, XLIX., 1891, 
 519-538. 
 
 HOWELL AND KASTLE i Note on the Specific Energy of the Nerves 
 of Taste, Studies from the Biological Laboratory, Johns Hopkins 
 University, IV., No. 1. 
 
 KEPPLER: Das Unatrscheidungsvermogen des Geschmacksinnes fiir 
 Concentrationsdifferenzen der schmeckbaren Korper, Pfluger's 
 Archiv, II., 1869, 449. 
 
 LOMBROSO UND OxTOLENGHi : Die Sinne der Verbrecher, Zeitschrift 
 fiir Psychologic, II., 1891, 342-348. 
 
 OEHRWALL: Untersuchungen liber den Geschmackssinn, Scandinav. 
 Archiv f. PhysioL, II. , 1890, 1-69; see also abstract by the 
 author in the Zeitschrift f. Psych., I., 1890, 141. 
 
 PASSY: Sur les minimums perceptibles de quelques odeurs, Comptes 
 rendus, CXIV., 1892, 306. 
 
 KAMSEY: On Smell, Nature, XXVI., 1882, 187. 
 
 KITTMEYER: Geschmackspriifungen, Inaug. Diss., Gottingen, 1885. 
 
 SAVELIEFF : Untersuchung des Geruchsinnes zu klinischen Zwecken, 
 Neurologisches Centralblatt, XII., 1893, 340-345. 
 
 SHORE: A Contribution to our Knowledge of Taste Sensations, 
 Journal of Physiology, 1892, 191-217. 
 
 VON VINTSCHGAU: Physiologic des Geschmackssinns und des Ge- 
 ruchssinns, Hermann's Handbuch der Physiologic, III., pt. 2, 
 143-286. For a general account of the physiology and psychol- 
 ogy of taste and smell to 1880, and references to the earlier 
 literature, see this work. 
 
 WUNDT: Work cited in bibliography of Chap. I., 3te Aufl., I., 411 ff.; 
 4te Aufl., 438 ff. 
 
SENSATIONS OF TASTE AND SMELL. 53 
 
 Z WAARDEMAKER : A. Die Bestimmung der Geruchscharfe, Berliner 
 kiln. Wochenschrift, XXV., 1888, No. 47, 950 (abstract of the 
 same, British Med. Journal, 1888, ii. 1295); also Lancet, 
 London, 1889, i. 1300. 
 
 B. Compensation von Geriichen mittelst des Doppelreichmessers, 
 Fortschritte der Medicin, VII., 1889, 721 if. 
 
 C. Sur la norme de 1'acuite olf active (olfactie), Archives neer- 
 landaises, XXV,, 131-148. 
 
 D. La niesure des sensations olf actives et Tolfactometre, Revue 
 scientifique, 1889, ii. 810-812. Extract from the Archives 
 neerlandaises. 
 
54 LABORATORY COURSE IN PSYCHOLOGY. 
 
 CHAPTER IV. 
 Sensations of Hearing. 
 
 - JN these experiments a little knowledge of the physics of 
 sound is presupposed as much as would be given in an 
 elementary course in physics. A very little knowledge of 
 musical notation is also required, but hardly more than 
 everybody has. No special musical skill is needed except 
 in Exs. 70 and 93 b. It is also the author's belief that most 
 persons calling themselves " unmusical," however truly 
 they may rate themselves as performers, are very much 
 in error as to their ability to discriminate musical sounds. 
 The greatest difficulty in some of these experiments will be 
 found in the continuous intrusion of outside sounds, and 
 some even may have to be tried at night. 
 
 SOUNDS IN GENERAL. 
 
 61. Minimal Sounds, a. Experiment in a large room as 
 free as possible from noise. Let the subject be seated with 
 his side toward the experimenter, his eyes closed, and his 
 ear upon the other side plugged with cotton. Let the 
 experimenter then find what is the greatest distance at 
 which the subject can still hear the tick of a watch held 
 at the level of his ear and on the prolongation of the line 
 joining the two ears. This is easily done with sufficient 
 accuracy by drawing a chalk line on the floor, marking off 
 feet or meters and fractions upon it, and estimating by eye 
 the point of the line directly under the watch. Try several 
 times for each ear, both when the watch is being brought 
 
SENSATIONS OF HEARING. 55 
 
 toward the ear and when it is being carried away. The 
 experimenter should from time to time cover the watch with 
 his hand to discover whether or not the subject really hears, 
 or is under illusion. For normal ears the distance found 
 may vary from 2.5 m. to 4.5 m., and may even rise to as 
 much as 9 m. 
 
 b. The subject should notice in this experiment the very 
 marked intermittences of the sound when just upon the 
 limit of audibility. It will for a few seconds be heard d^. 
 tinctly, and a few seconds later will as distinctly not be 
 heard. 
 
 c. Faint sounds are apt to be underestimated. Place a 
 sounding tuning-fork on the head and let the sound die 
 away to almost complete extinction ; then remove it. The 
 drop to complete silence will often seem larger than the 
 apparent intensity of the tone would justify. 
 
 On a, von Bezold, -4; on 6, Urbantschitsch, A* Lange; Miinster- 
 berg, A\ on c, Stumpf, I., 388, who quotes from Fechner. 
 
 62. Discriminative Sensibility for Intensity of Sounds. 
 Exact experiments on this topic are difficult to make, be- 
 cause of the very great difficulty of determining objectively 
 the intensity of the sounds used. A rough determination 
 can easily be made, however, with the sound pendulum (see 
 chapter on apparatus). Choose a medium sound as a 
 standard, and by the Method of Just Observable Difference 
 explained under Ex. 24, find a sound that is just recogni- 
 zably different from it. The discriminative sensibility is 
 very much finer, apparently, when the question is not one 
 of recognizing a difference, but of locating a sound as right 
 or left of the median plane. Cf. Ex. 101 and Eayleigh. 
 
 Wundt, 3te Aufl., L, 364 ff. ; 4te Aufl., I., 360'ff. ; Stumpf, I., 345 ff. 
 
 63. Auditory Fatigue, a. Cause an assistant to strike 
 once with a hammer on the floor, or to clap his hands. 
 
56 LABORATORY COURSE IN PSYCHOLOGY. 
 
 With the ears open a single sound, or at most a single sound 
 and transient echoes are heard. If, however, the ears are 
 kept closed with the fingers till half a second or more after 
 the stroke (the time may easily be fixed by rapid counting), 
 the fainter echoes will be heard on the opening of the ears, 
 like a new stroke. In the first case, fatigue from the origi- 
 nal sound deadens the ears to the fainter echoes, though 
 they may still be heard by attentive listening; in the 
 second case they are more strongly heard because the closed 
 ears are unfatigued. The sound produced by the simple 
 opening of the ears without any objective stroke will be less 
 if the finger is not put into the ears, but presses the tragus 
 back upon the opening. 
 
 b. Strike a tuning-fork, press the stem firmly upon the 
 mastoid process, or the crown of the head, and hold it 
 there till the tone is no longer heard. Then instantly 
 remove it, and after a second or two replace it upon the 
 same spot, taking pains to press no harder than before. 
 The fork will be heard again sounding faintly. The experi- 
 ment may not succeed at first, but a few trials should not 
 fail to show the effect. 
 
 c. Insert in the openings of the ears the ends of a rubber 
 tube. Strike a tuning-fork and set it upon the tube at such 
 a point that it sounds equally intense to the two ears. 
 The sound will then probably appear to be located in the 
 head midway between the ears at least not nearer one 
 than the other. After a few seconds strike the tuning-fork 
 again, pinch the tube on one side, say the left, so as to shut 
 off the sound from the ear on that side, set the tuning-fork 
 at the proper place on the tube and keep it there till the 
 sound has become rather faint. Then allow the pinched 
 tube to open, and notice that the sound is now stronger on 
 the left than the right and apparently located on the left. 
 Try the experiment in reverse form, pinching the tube on 
 the right. 
 
SENSATIONS OF HEARING. 57 
 
 Cf. later experiments on the analysis of compound tones 
 by the fatigue method, Ex. 89 e. 
 
 Stumpf,..!., 360-363. On a, Mach; on 6, Corradi; on c, Urbant- 
 schitsch, B. 
 
 64. Inertia of the Auditory Apparatus, a. Inertia tend- 
 ing to keep the auditory apparatus out of function can be 
 demonstrated as follows. Place the ends of a rubber tube 
 in the ears, and set upon the middle of it a low tuning-fork 
 sounding as faintly as possible. Notice that the sound does 
 not reach its maximum intensity for an appreciable length 
 of time ; if the fork is barely audible, this may be as much 
 as a second or two. Be careful not to increase the pressure 
 of the fork upon the tube after first setting it on, for that 
 will produce an objective strengthening of the tone ; and 
 allow an interval of several seconds between the tests so 
 that the auditory apparatus may again come completely to 
 rest. A tuning-fork that will preserve these minimal vibra- 
 tions for some seconds, and complete freedom from distract- 
 ing noises, will be found necessary for success. 
 
 b. Inertia tending to keep the auditory apparatus in func- 
 tion (positive auditory after-images) can be demonstrated as 
 follows. Fasten upon the front of a rather solid pendulum 
 a small tuning-fork, so that it shall project forward at right 
 angles to the pendulum bar and the tines of the fork shall 
 be vertically one above the other. On the three arms of a 
 Y-tube attach three pieces of small rubber tubing, say quar- 
 ter inch outside measurement. Those fitting on the upper 
 arms of the Y should be of the same length, that fitting upon 
 the stem may be of any convenient length. Insert the free 
 end of the last mentioned tube in the outer passage of the 
 ear, and hold the tips of the other tubes about half an inch 
 apart, open end upward, in such a way that the tip of the 
 tuning-fork, as the pendulum swings, will pass close over 
 them. Strike the fork with a small rubber hammer as the 
 
58 LABORATORY COURSE IN PSYCHOLOGY. 
 
 pendulum swings and notice the sound produced by the 
 fork as it passes the ends of the tube. If a single continu- 
 ous sound is heard, separate the tubes a little ; if a double 
 sound is heard, bring them together ; and thus by shifting 
 them back and forth find the place where the sounds just 
 fuse into one. The auditory disturbance occasioned by the 
 first pulse of sound outlasts the interval between the two, 
 and blends with the second. Move the pendulum slowly 
 over the end of one tube and then of the other, meantime 
 pinching the tube over which the fork is sounding, to con- 
 vince yourself that the tone is not heard at substantially 
 the same instant in both tubes. It is possible from the 
 rate of the pendulum and the separation of the tubes to find 
 approximately the length of time through which the sensa- 
 tion persists. 
 
 c. Sometimes it is possible to get more lasting after-im- 
 ages and even those that are recurrent. Try with a tuning- 
 fork struck and held a few seconds before one ear. Stop 
 the fork by touching it, without removing it from the ear. 
 The after-image is not very easy to observe; the lowest 
 degree of it seems to be the transforming of faint outer 
 noises into something qualitatively like the tone heard, or 
 perhaps a selection of certain of those noises. The usual 
 interval between the stimulus and the after-image is under 
 fifteen seconds. The number of recurrences of the after- 
 image differs in different subjects ; for Stumpf, they seem 
 to come by preference in the unstimulated ear. 
 
 Stumpf, L, 211 ff, 278; Urbantschitsch, C. For methods of dem- 
 onstration permitting more accurate measurement of the persistence 
 of tone, see Urbantschitsch, C, and Mayer, A. 
 
 65. Noise. Whether or not there is a distinctive sensa- 
 tion of noise different from that of a mass of short, disso- 
 nant, and irregularly changing tones, is yet under debate. 
 A little attention to the noises constantly occurring, espe- 
 
SENSATIONS OF HEARING. 59 
 
 eially to their pitch, will easily convince the observer that a 
 tonal element is present. This is striking when resonators 
 (cf. notes on apparatus for simultaneous tones) are used, 
 for they pick out and prolong somewhat the tones to which 
 they correspond, but they are not indispensable. On the 
 other hand, attention to musical tones will often discover 
 the presence of accompanying noises. 
 
 Wundt, 3te Aufl., L, 420; 4te Aufl., I., 447 f ; Stumpf, II., 497- 
 515; Brucke; Exner; Mach, B, 117. 
 
 66. Silence. When circumstances promise absence of 
 external sounds, notice that many are still present and 
 distinct, though faintly heard. Notice also the pitch and 
 changing character of the subjective sounds to be heard. 
 Our nearest approach to the experience of absolute stillness 
 is this mass of faint inner and outer sensations. 
 
 Preyer, A, 67-72; Stumpf, L, 380 ff. 
 
 SINGLE AND SUCCESSIVE TONES. 
 
 67. Highest Tones. With the apparatus at hand for the 
 purpose, find what is the highest audible tone ; i. e., if the 
 cylinders are used, the shortest cylinder which still gives a 
 ringing sound when struck with the hammer, or if the whis- 
 tle is used, the closest position of the plunger at which a 
 tone can still be heard beside the rush of air. If a number 
 of persons are tested, it is not improbable that some will yet 
 hear the tone after it has become inaudible for the rest. 
 
 Same references as Ex. 68. 
 
 68. Lowest Tones. If low-pitched tuning-forks or other 
 vibrators are at hand, find what is the slowest rate of vibra- 
 tion that can yet be perceived as a tone. In some physio- 
 logical laboratories electric tuning-forks or interrupters may 
 be found that have vibration rates of twenty-five per second. 
 Low tones can be heard from these, though they have many 
 
60 LABORATORY COURSE IN PSYCHOLOGY. 
 
 overtones. The latter can be partly damped by touching the 
 tines midway of their length with the finger, and partly 
 avoided by bringing the ear not to the free end, but to a 
 point somewhat nearer the handle. The determination of 
 the lower limit of audible pitch is difficult and uncertain 
 because of the great difficulty which observers, even those 
 of trained ear, find in distinguishing these lowest tones from 
 the next higher octaves. The general character of these 
 deep tones can be demonstrated with sufficient clearness 
 upon the contra octave (Ci-C) of a church organ, if one is 
 accessible and tuning-forks are lacking. 
 
 Von Bezold, B; Wundt, 3te Aufl., I., 423; 4te Aufl., I. , 450; 
 Preyer, A and D; Stumpf, L, 263, II., 551. 
 
 69. Some Characteristics of High and Low Tones. 
 
 a. High tones are smoother than low tones. This is clear 
 with almost all tones used in music, and particularly so with 
 those of reed instruments. The roughness of low tones is 
 largely due to the beating of their partials among them- 
 selves (see Exs. 86 ff. and 79 ff.) and even with the funda- 
 mental tones ; the high tones having fewer audible partials 
 are freer from it. Play the scale of any instrument from its 
 lowest to its highest tone, or sing the ascending scale. The 
 difference of roughness is observable also with simple tones, 
 but only at lower pitches, and is even there less marked. 
 
 b. In spite of the generally accepted fact that high tones 
 produce a more intense sensation than low tones of equal 
 physical energy, high tones are more readily suppressed by 
 stronger lower tones than vice versa. Place an ordinary 
 clock at a distance of a few feet and hold close before the 
 ear a watch. When the watch is near the ear all the ticks 
 will be heard. As it is gradually removed, a position can 
 be found where the watch-tick that coincides with the clock- 
 tick will be suppressed. When both make an equal number 
 
SENSATIONS OF HEARING. 61 
 
 of ticks to a second, and one gains a little on the other, 
 there will occur periods in which no watch-ticks are heard, 
 and, alternating with them, periods in which all are heard. 
 If the watch beats oftener than the clock and both run at 
 the same rate, a single watch-tick will be lost at regular in- 
 tervals. When the clock is removed, all the ticks of the 
 watch can easily be heard at the distance used. The phe- 
 nomenon can be observed when the watch is on the opposite 
 side of the head from the clock. To demonstrate weakness 
 of high tones in suppressing lower tones, sound together a 
 large and a small tuning-fork on their resonance cases, e. g., 
 c and c", a' 1 ', or #", sounding the first very faintly and the 
 second as loudly as possible. The first will still be heard 
 even when the second is brought close to the ear. In this 
 connection compare the difficulty of analyzing the compound 
 tones in Exs. 86 if., also Exs. 83 b and 84. 
 
 c. Some high tones are particularly strengthened by the 
 resonance of the outer passage of the ear. These generally 
 lie between c 4 and c 5 , and give to the tones of this octave a 
 superior strength and ear-piercing quality. They may be 
 demonstrated easily with a small piston whistle. Find by 
 adjustment of the piston the point at which the tone is most 
 piercing. Insert in the outer ends of the ear-passages bits 
 of rubber tubing half an inch long (which will change the 
 resonance of the passages, making them responsive to a 
 lower tone) and sound the whistle again. The piercing 
 quality will be gone and the tone appear decidedly weaker. 
 Remove the bits of tubing and sound the whistle as before ; 
 the original quality and intensity reappear. 
 
 d. Very closely associated with the pure tonal sensations 
 are certain of a spatial quality. Compare in this respect 
 the sensations of the tones observed in c above ; or, better 
 still, those of Ex. 67 with those of Ex. 68, or any other 
 deep tones. Play the scale through the complete compass of 
 any instrument, keeping this quality in mind. 
 
62 LABORATORY COURSE IN PSYCHOLOGY 
 
 e. Under certain conditions, low tones seem to be located 
 in the head, high tones outside of it. Close the ears with 
 the fingers and have an assistant strike a low tuning-fork 
 (e.g., 50 vibrations per sec.), and set the stem of it upon the 
 crown of the head ; notice the location. Try the same 
 with a high fork. 
 
 /. The emotional shading of tones changes with their 
 pitch. Recall the descriptive terms used : Deep, low, 
 bright, sharp, acute. Play the scale, and judge of the ap- 
 propriateness of these terms to match the shades of feeling 
 that mark the tones of low, middle, and high pitch, distin- 
 guishing those that refer to pitch from those enumerated 
 in Ex. 90, which refer to timbre. 
 
 Stumpf, L, 202-220, II., 56-59, 227; also Mach, B, 120 ff. On 6, 
 Mayer, 7*; on c and /, Helmholtz, 116, 179, and 69 ff.; on d, James, 
 II. , 134 ff.; on e, Kessel. 
 
 70. Eecognition of Absolute Pitch, a. This experiment 
 gives accurate results only with those of very decided 
 musical skill, but it may be tried with any subject that 
 knows the names of the notes. Strike various notes in dif- 
 ferent parts of the scale of the instrument and require the 
 subject to name the note given. Record the note struck 
 and the subject's answer. He should be seated with his 
 back toward the experimenter, or should keep his eyes 
 closed. 
 
 b. Pitch differences in the perceptions of the two ears. 
 The same tone, heard first with one ear and then with the 
 other, seems to many observers, even professional musicians, 
 somewhat different in pitch. Take two small rubber tubes 
 of equal size and length (e. g., quarter inch tubes, two feet 
 long), place an end of one in the right ear, an end of the 
 other in the left, and bring the free ends near together on 
 the table. Then have an assistant strike a tuning-fork and 
 
SENSATIONS OF HEARING. 63 
 
 present it alternately to the ends of the tubes. The 
 difference between the two ears is said to vary more or less 
 from day to day and to be different in amount for tones of 
 different^ pitch. Such differences may be observed by the 
 unmusical. 
 
 Stumpf, I., 305-313, also II., index, Hohenurteile, for experiments 
 on trained musicians; von Kries, J5; on 6, Stumpf, II., 319 f. 
 
 71. Just Observable Difference in Pitch. Test as follows 
 with the set of mistuned forks. Let the subject pick out 
 from the mistuned forks that which sounds to him just 
 noticeably different from the normal fork, striking and hold- 
 ing them successively (never simultaneously) over a reson- 
 ance bottle. If all of them seem more than just observably 
 different, let him put the riders on the one that is next 
 higher, and gradually lower the pitch by sliding them 
 toward the ends of the fork till the two forks, heard suc- 
 cessively, are just different and no more. The experimenter 
 <may then determine the error of the subject in vibrations 
 per second approximately by counting the number of beats 
 produced by the forks when sounded together. If the 
 number of beats per second is less than 2 or more than 6, 
 it will be best to get the difference in pitch with some other 
 of the forks first, so as to avoid too slow or too rapid count- 
 ing, and from that to arrive at the difference from the 
 standard fork. Repeat the test several times, sometimes 
 sounding the standard fork first, and sometimes that to be 
 compared with it, and average the result. Take care to 
 avoid fatigue. This experiment will not be refined enough 
 for testing those of keen musical ear. 
 
 Preyer, A, 26 ff., D, 64; Stumpf, I., 296-305; Luft. 
 
 72. Differences in Pitch that are Just Eecognizable as 
 Higher or Lower. It is easier to recognize a difference 
 than to tell its direction. Experiment as in Ex. 71, but 
 
64 LABORATORY COURSE IN PSYCHOLOGY. 
 
 require the subject this time to pick out and adjust a fork 
 that is just observably sharper or natter than the standard. 
 
 Preyer, A, 28, 36. For experiments on extremely unmusical sub- 
 jects, see Stumpf, L, 313-335. 
 
 73. Number of Vibrations Necessary to Produce a Sensa- 
 tion of Pitch. Arrange an apparatus for blowing soap- 
 bubbles with a mixture of hydrogen and air. Blow bubbles 
 of different sizes and touch them off with a match, either in 
 the air, or (if proper precaution is taken to prevent the igni- 
 tion of the mixed gases in the vessel and any resonance in 
 the pipe), while still hanging. The explosion of these 
 bubbles is supposed to produce a single sound wave. The 
 pitch of the sounds produced cannot be accurately given, 
 but the report of the large bubbles is distinctly deeper than 
 that of the small ones. 
 
 Briicke; Cross and Maltby; Herroun and Yeo. 
 
 74. The Apparent Pitch of Tones is Affected by their^ 
 Quality. Tones of dull and soft character seem lower in 
 pitch than those that are brighter and more incisive. Re- 
 quire the subject to pick out on some stringed or reed instru- 
 ment the tone corresponding to that produced by blowing 
 across the mouth of a medium-sized bottle. Too low a note 
 at first will generally be chosen, at least by those without 
 special musical training. The tones should be sounded suc- 
 cessively, not at the same time, during the test. Afterward 
 they may be sounded together, and the pitch of the bottle 
 determined approximately by finding with which tone of 
 the instrument its tone makes the slowest beats (cf. Ex. 
 79). It should be remembered, however, that it will be 
 possible to get beats also with tones an octave lower and an 
 octave higher than that corresponding most nearly with the 
 true pitch of the bottle tone. 
 
 Stumpf, I., 227-247, especially, 235-245. 
 
SENSATIONS OF 11 EARING, f 65 
 
 75. Recognition of Musical IntervalsT'^^rffse a familiar 
 air to be played, first in the octave of c and then in that of 
 c" in the same or another key. Even those of no musical 
 training .will easily recognize that the air (i. e., the succes- 
 sion of musical intervals in fixed rhythmical relations), is 
 the same in both cases ; and any mistake or variation will 
 be noticed as easily as if the air had been repeated at the 
 first pitch. With the unmusical, however, the recognition 
 is often rather of the rhythm than the intervals ; try there- 
 fore a repetition of the air changing some of the intervals 
 but preserving the original rhythm. The power of recog- 
 nizing intervals is very much more highly developed in 
 persons of musical training, but any one that can whistle 
 a tune at one pitch and repeat it recognizably at another 
 undoubtedly has the rudiments of interval recognition. 
 
 For exact methods of testing the accuracy of the power of recog- 
 nizing intervals, see Preyer, A, 38-64; and Schischmanow, and the 
 references given by them. 
 
 76. Pitch Distances. Beside the interval relations of 
 tones, and overshadowed by them in musicians, are certain 
 relations of separateness or distinctness or distance in pitch, 
 which do not depend on the ratios of vibration rates. Equa- 
 musical intervals (i.e., intervals between tones that have 
 vibration rates in a fixed ratio to each other, e.g., CD and 
 c" d") do not correspond to equal pitch distances. Sound 
 the half-tone interval c c-sharp through the range of the in- 
 strument, beginning in the bass and ascending. Notice the 
 increasing distinctness and separation of the tones as the 
 interval is taken higher and higher. For the very highest 
 tones there is probably a decrease of separateness agaiD. 
 The difference is most striking, hovfever, with intervals 
 smaller than those in common use, e.g., with quarter or 
 eighth tones. On the harmonical (cf. notes on apparatus) 
 strike in succession the c-sharp and d keys in the four lower 
 
66 LABORATORY COURSE IN PSYCHOLOGY. 
 
 octaves, beginning with the lowest. In this instrument the 
 osharp key is given to another d, a comma, or about one- 
 ninth of a tone, natter than the regular d of the scale. 
 
 Stumpf, I., 247-253; Lorenz, and the discussion between Wundt, 
 Stumpf, and Engel; Helmholtz, 264-265; Miinsterberg, C. 
 
 77. The Effect of a Given Tone in a Melody depends in 
 part on the succession of tones in which it stands. Cause a 
 simple air, in which the same tone recurs in different suc- 
 cessions of tones, to be played, and notice the difference 
 in effect in the different circumstances, or simply play the 
 ascending and descending scales. 
 
 Mach, B, 130-131. 
 
 78. Tones that Vary Irregularly in time and in pitch are 
 unpleasant. Test with a piston whistle. 
 
 SIMULTANEOUS TONES. 
 
 79. Beats. When tones that are different in pitch are 
 sounded at the same time, they mutually interfere, and 
 make the total sensation at one instant more intense and 
 the next instant less intense. This regular variation in 
 intensity is called " beating." Exs. 71 and 74, where beats 
 have been used incidentally, are a sufficient introduction to 
 them. 
 
 a. The rapidity of beats depends on the difference in the 
 vibration rates of the beating tones. Prepare two bottle 
 whistles of the same size, and blow both at the same time. 
 Slow beats will probably be heard. If not, pour a little 
 water into one bottle (thus raising the pitch of its tone), and 
 blow as before. Continue adding water, a little at a time, 
 till the beats lose themselves in the general roughness of 
 the tone. Blow the bottles separately now and then to 
 observe the increasing difference in pitch. The same may 
 be shown with a couple of piston whistles, if they are first 
 
SENSATIONS OF REARING. 67 
 
 adjusted to unison, and then the piston of one or the other 
 is slowly pushed in or pulled out. 
 
 b. Tones that are a little more or a little less than an 
 octave apart may give beats. Try with a pair of octave- 
 forks on resonance boxes or held over resonance bottles, one 
 of which has been slightly lowered in pitch by weighting 
 the prongs with wax or a bit of rubber tubing. In this 
 case the beating-tones are the tone of the lower fork and 
 the difference tone (see Ex. 82). Eepeat the experiment on 
 a reed instrument. In this case beats may be heard be- 
 tween the higher tone and the first over-tone of the lower 
 (see Ex. 86). 
 
 c. The rate at which the roughness of rapid beats dis- 
 appears, as also the rate which produces the greatest rough- 
 ness, differs with the pitch of the beating-tones. Sound the 
 following pairs of tones which have somewhat near the 
 same difference in vibration rates per sec., namely, 33 ; and 
 observe that the roughness from the beats decreases and 
 finally disappears entirely at about the fourth pair ; V c", 
 c f d', e g, c e, Go, C G. The a! and c" tuning-forks give a 
 vanish of roughness, representing a rate of 80-88 per sec. 
 
 Helmholtz, 159-173; Stumpf., II., 449-497, especially 461-465 ; 
 Mayer, A ; Cross and Goodwin. 
 
 80. Beats Betray the Presence of very Faint Tones, both 
 because the total stimulus is actually stronger in the phase 
 of increased intensity, and because intermittent stimuli are 
 themselves more effective than continuous ones. 
 
 a. Strike a pair of beating tuning-forks, and hold one at 
 such a distance from the ear that it is very faint or quite 
 inaudible. Then bring the other fork gradually toward the 
 ear, and notice the unmistakable beats. 
 
 b. Strike a tuning-fork and hold it at a distance, being 
 careful to have the fork sidewise or edgewise, not corner- 
 
68 LABORATORY COURSE IN PSYCHOLOGY. 
 
 ing, toward the ear. Rotate the fork one way and the 
 other about its long axis, and observe the greater distinct- 
 ness of the tone, due in this case simply to its intermit- 
 tence. 
 
 81. Beats are in general Attributed to the Tone that 
 Receives Attention ; in the absence of other determining 
 causes, to the louder tone, to the lower tone, or to the 
 whole mass of an unanalyzed compound tone (see intro- 
 duction to Ex. 86). 
 
 a. Set two properly tuned resonance bottles about a foot 
 apart on the table. Strike two forks that beat, and hold 
 them over the bottles. While both are about equally in- 
 tense, it is easy, by mere direction of the attention, to make 
 the beats shift from one to the other. 
 
 b. Turn one of the forks an eighth of a turn about its 
 long axis, which will weaken its tone, and observe that 
 the beats seem to come from the other fork. By turning 
 first one fork and then the other, the location of the beats 
 may again be made to shift at pleasure. If tuning-forks on 
 resonance boxes are at hand they may be used, and the tone 
 of one weakened by covering the opening of the box with a 
 bit of cardboard. 
 
 c. Warm the c r fork in any convenient way (holding it 
 clasped in the hand will do). This will flatten it some- 
 what. Strike it and the c" fork, and press the stems of both 
 on the table at the same time ; or, better, on the sounding- 
 board of the sonometer. Observe that the beats seem to 
 come from the c r fork unless it is very faint. 
 
 d. Tune a string of the sonometer so that its third partial 
 (or corresponding harmonic) beats slowly with the c" fork. 
 (On partials and harmonics cf. Exs. 86-89.) Strike the 
 tuning-fork, and hold it over a resonance bottle, or press its 
 stem against the table at arm's length from the string. 
 Then pluck the string and attend to its tone ; the beats may 
 
SENSATIONS OF HEARING. . 69 
 
 seem to affect the whole compound tone of the string. 
 But this will not happen if the tone of the string is an- 
 alyzed, or if the attention is directed to the fork. The 
 same may be tried on the piano by picking out from the 
 mistimed c" forks one that beats slowly with c" on the 
 piano. Strike the / key and hold it down ; strike the fork, 
 and observe the beats as before. Cf. Ex. 69 a. 
 Stumpf, II., 489-497. 
 
 82. Combination Tones: Difference Tones. 1 When two 
 tones are loudly sounded at the same time they produce by 
 their combination other tones, one of a pitch represented by 
 the difference of the vibration rates of the two original or 
 generating tones, and one of a pitch corresponding to their 
 sum. The existence of the summation tones has been dis- 
 puted, and they are hard to hear. The difference tones, 
 however, are easy to hear, at least when they are consider- 
 ably lower in pitch than the generators, when the latter are 
 loud and sustained, and when they make a consonant in- 
 terval though the last is not essential. A loud difference 
 tone may itself take the part of a generator and produce yet 
 another difference tone a difference tone of the second 
 order and so on, though difference tones of higher orders 
 are heard with difficulty even by skilled observers. Differ- 
 ence tones are hard to hear on the piano and similar stringed 
 instruments because of the rapid decline in the strength of 
 the generators. The difference tones are sometimes called 
 Tartini's tones, after an early observer of them. 
 
 a. Repeat Ex. 79 &, continuing to pour water into one of 
 the bottles till the difference tone appears. At first the 
 roughness of the beats and the difference tone may both be 
 
 1 Konig distinguishes between " difference tones " and "beat tones." Both 
 tones, however, generally have the same pitch, and the older term for them has 
 here been retained ; strictly speaking, however, the "difference tones " heard in 
 these experiments are " beat tones." 
 
70 LABORATORY COURSE IN PSYCHOLOGY. 
 
 heard at once. Try the same with the piston whistles, first 
 setting them at unison, and then slowly pushing the piston 
 of one in or out while blowing rather hard. The beats will 
 almost immediately give place to a low difference tone 
 which may be heard ascending through several octaves 
 before becoming indistinguishable from the generators. 
 The double warning whistles used by bicyclists give a fine 
 difference tone, to which indeed they owe their deep and 
 locomotive-like quality. 
 
 b. Difference tones are strong on reed instruments. Press 
 the adjacent white keys of a parlor organ, or the harmonical, 
 by twos, beginning at c and going up a couple of octaves. 
 If there is difficulty in hearing the difference tone, sound 
 the upper tone intermittently and listen for the difference 
 tone at the instant of pressing the key. 
 
 c. Sound c" and d" which should give C as a difference 
 tone (594528=66). Sound also d" and e" which should 
 give the same (660594=66). If, however, the tuning is 
 inexact, as it is intentionally in the tempered tuning of 
 keyed instruments, these difference tones will be somewhat 
 different and may be heard to beat with each other when c", 
 d" and e" are sounded at once. Notice that these beats are 
 not heard when the tones are sounded in pairs. On the 
 harmonical this difference may be brought about by sound- 
 ing one of the tones flat by pressing its key only a little 
 way down. The same thing may be shown with three 
 piston whistles blown at once, by a little careful adjustment 
 of the pistons. 
 
 d. In the case of reed instruments the difference tones 
 probably owe part of their intensity to the vibrations of the 
 air in the wind chest. When two whistles are blown by one 
 person something of the same kind may happen. In order 
 to make a clean experiment, have the whistles blown by 
 two assistants, or observe the difference tones from tuning- 
 forks. 
 
SENSATIONS OF HEARING. 71 
 
 e. The location of difference tones. The location of these 
 tones is sometimes influenced by the location of their 
 generators, but under favorable circumstances they seem to 
 arise in the ears or even in the head. This is strikingly the 
 case, both for the blower and the listeners, with the differ- 
 ence tones produced with the piston whistles. Cf. Ex. 69 e. 
 
 Helmholtz, 152-159; Stumpf, II., 243-257; Konig; Preyer, C and 
 D; Hermann. 
 
 83. Blending of Tones. The degree to which tones blend 
 with one another differs with the interval relation of the 
 tones taken. It is, according to Stumpf, greatest with the 
 octave, less with the fifth, less again with the fourth, slight 
 with the thirds and sixths, and least of all with the remain- 
 ing intervals. 
 
 a. Try on the instrument the extent to which the tones 
 forming these intervals blend, also those forming intervals 
 greater than the octave : double octave, twelfth, etc. 
 
 b. The blending in case of the octave is so complete under 
 favorable circumstances as to escape the analysis of trained 
 ears. Use two tuning-forks, one an octave higher than the 
 other, on resonance cases or held over resonance bottles. 
 Sound the forks, first the higher, then the lower. For a 
 while the higher fork will be heard sounding in its proper 
 tone, but by degrees it will become completely lost in the 
 lower, and a subject with closed eyes will be unable to say 
 whether or not it yet sounds. Cf. Ex. 69 b. Stop the lower 
 fork, or remove it from its resonance bottle, and notice that 
 the higher is still sounding. Notice the change in timbre 
 (cf . Ex. 90) produced by the stopping of the higher fork 
 something like the change from the vowel to the vowel 
 U (oo). 
 
 On a, Stumpf, II., 127-218, especially 135-142; for his experi- 
 ments on the unmusical confirming his grades of blending, 142-173. 
 On 6, Stumpf, II., 352-358, and Helmholtz, 60-61. 
 
72 LABORATORY COURSE IN PSYCHOLOGY. 
 
 84. Analysis of Groups of Simultaneous Tones. Ease of 
 analysis depends on a number of conditions, among others 
 on the following. 
 
 a. Analysis is easier for tones far distant in the scale. 
 Compare the ease of recognizing the sound of the c" fork 
 when c' and c" are sounded together, with that of recogniz- 
 ing c m when sounded with c'. Compare also the ease of 
 distinguishing c' and a! with that of distinguishing c' and 
 a". 
 
 b. Analysis is made easier by loudness in the tone to be 
 separated. Eepeat Ex. 83 b, sounding the c f faintly, the c" 
 strongly. Little difficulty will be found in keeping the lat- 
 ter distinct. 
 
 c. Analysis is easier when the tones make intervals with 
 little tendency to blend. Compare the ease of analysis of 
 c f c" and c' a' or a' c" . Also notice that the addition of d" 
 (octave of df, fifth of g 1 r , fourth below g"} to the chord g d f 
 g r g" produces a less striking change than the addition of b' 
 (major third of gf, minor sixth below /') to the same chord. 
 
 d. Analysis is easier with sustained than with short 
 chords. Eepeat the last experiment, making the chords very 
 short, and notice that the difference made by inserting either 
 d" or V is less marked. Cf. also Ex. 100. 
 
 Stumpf, II., 318-361; also his experiments, 362-382. 
 
 85. The Lower Tone of a Chord Fixes the Apparent 
 Pitch of the Whole, a. Repeat Ex. 83 #, and notice that 
 when the c' fork is stopped, the tone appears to jump up- 
 ward an octave in pitch (i.e., it takes the pitch of the c" still 
 sounding) ; but when the c" fork is removed, the quality of 
 the tone is changed, but not its pitch. 
 
 b. Strike the chord C c" e" g" or G e' g r c" ', and compare 
 the effect upon the pitch of the whole mass of tone pro- 
 duced by omitting C or G alone with that of omitting any 
 one or all three of the higher tones. See also the function 
 
SENSATIONS OF HEARING. 73 
 
 of the lowest partial of a compound tone in fixing the 
 pitch, noticed below. 
 Stumpf, II., 383-392. 
 
 86. Compound Tones. Almost all tones heard, and in- 
 deed all those used in music, are not simple tones, but com- 
 pound. The tone given by the C string of a piano is made 
 up of at least C, c, g, c', e' and g', and generally other tones. 
 The lowest tone of the group gives the pitch attributed to 
 the whole, and is known as the fundamental, the other tones 
 as over-tones. In another way of naming them, the com- 
 ponent tones are all partial tones or partials, the fundamen- 
 tal being called the first or prime partial, the next higher 
 the second partial and so on. The first over-tone is thus 
 the second partial tone, the second over-tone the third partial, 
 and in general the same tone receives as a partial tone a 
 number one higher than as an over-tone. The vibration 
 rates of the partial tones of a compound are generally once, 
 twice, three times, four times, the rate of the fundamental, 
 and so on. In some cases, however, e.g., in bells and tun- 
 ing-forks, one or more of the partial tones may have a vibra- 
 tion rate not represented in this series, and discordant 
 with the fundamental tone. In what follows, the regular 
 series of partial tones is meant except where the contrary 
 is specified. 
 
 Partial Tones. If resonators are at hand, the demon- 
 stration of the partial tones will be easy. Sound on a 
 stringed or reed instrument the tones to which the resona- 
 tors are tuned, and notice that they resound strongly to 
 these tones and less strongly or not at all to other tones 
 adjacent in pitch. Then sound the tone to which the lar- 
 gest of the resonators is tuned (or a tone an octave lower), 
 and try the resonators in succession. Notice that others 
 also resound (at their own proper pitch), thus betraying 
 the presence of the tones to which they are tuned, and 
 
74 LABORATORY COURSE IN PSYCHOLOGY. 
 
 thus the composite character of the tone under examination. 
 Which resonators will " speak " will depend on the instru- 
 ment used ; reed instruments give a long and perfect series, 
 piano and stretched wires a perfect series generally as far 
 as the ninth or tenth partial, and stopped organ-pipes a 
 short series. If difficulty is found in knowing when the 
 resonator is resounding, it will be found useful to apply it 
 to the ear intermittently, alternating, for example, two sec- 
 onds of application with two seconds of withdrawal. 
 
 87. Partial Tones : Analysis by indirect means, a. By 
 sympathetic vibration. This succeeds especially well with 
 the piano. Press the c key and hold it down so as to leave 
 its strings free to vibrate ; then strike the C key forcibly, 
 and after one or two seconds release it. The c strings will be 
 found to be sounding. Eepeat, trying c-sharp or b instead 
 of c; they will be found not to respond. Eepeat the experi- 
 ment, substituting </, c', e', g', or c" ; all will be found to re- 
 spond but in lessening degrees. Other keys between C and 
 c" may be tried but will be found in very faint vibration, if 
 at all. 
 
 b. By beats. This will succeed best with a reed instru- 
 ment, e. g., a parlor organ or the harmonica!. By pressing 
 the keys of the instrument only a little way down, any of its 
 tones may be sounded a little flatter than its true pitch and 
 so in condition to beat with any other tone having that true 
 pitch. Sound at this flattened pitch the over-tones of C in 
 succession while C is sounding, and notice the slow beats 
 that result. For verification sound other tones not over-tones 
 of (7, and notice that the beats when present are much more 
 rapid. 
 
 88. Partial Tones : Direct analysis without special appara- 
 tus. The directions given here apply to the sonometer, but 
 will be readily adaptable to any stringed instrument in which 
 
SENSATIONS OF HEARING. 75 
 
 the strings can be exposed. It is easier to hear any partial 
 tone in the compound, if the partial is first heard by itself, 
 and then immediately in combination with the rest. On 
 strings this is easily done by sounding the partials as 
 "harmonics." Pluck the string near one end (say about 
 one-seventh of the length of the string from the end), and 
 immediately touch it in the middle with the finger or a 
 camel' s-hair brush. The fundamental will cease to sound 
 and its octave (the second partial) will be left sounding, as 
 a " harmonic." With it sound also other even-numbered 
 partials, but less strongly. Pluck as before, and touch the 
 string at one-third its length ; the third partial will now 
 sound out strongest, with the sixth, ninth, etc., more faintly. 
 Thus by plucking the string and touching it respectively 
 at one half, one third, one fourth, one fifth, one sixth, one 
 seventh, one eighth, one ninth, and one tenth its length from 
 the end, the series of tones corresponding to the 2d, 3d, 4th, 
 5th, 6th, 7th, 8th, 9th, and 10th partials can be heard, each 
 in large measure by itself. In getting the higher "harmon- 
 ics " it will be found better to pluck nearer the end than 
 one seventh, and in no case should the string be plucked at 
 the point at which it is presently to be touched. (Cf. Ex. 
 90S.) 
 
 To hear the partial tones when sounding in the compound, 
 proceed as follows. Sound the required tone as a " har- 
 monic," and then keeping the attention fixed on that tone, 
 stop the string and pluck it again, this time letting it vibrate 
 freely. The tone just heard as a " harmonic " will now be 
 heard sounding with the rest as a partial. When the partial 
 is thus made out, verify the analysis by touching the string 
 again and letting the tone sound once more as a "harmonic." 
 Try in this way for the partials up to the tenth ; first for 
 the 3d, 5th, and 7th, afterward for the 6th, 4th, and the 2d, 
 which is the most difficult of all. It is said that analysis 
 
76 LABORATORY COURSE IN PSYCHOLOGY. 
 
 is easier at night (not alone on account of the greater still- 
 ness) and when one ear is used, and that certain positions 
 of the head favor certain partials. 
 
 89. Partial Tones : Direct analysis without apparatus. 
 Certain parts of a compound tone are sometimes so sepa- 
 rated by their dissonance, intensity, or pitch that they stand 
 out with striking clearness. 
 
 a. Strike a tuning-fork on a hard surface, and observe the 
 high, ringing, dissonant partials. They fade out before the 
 proper tone of the fork, and are heard best when the fork 
 is not held near the ear. 
 
 b. As the tone of a string is allowed to die away of itself, 
 different partial tones come successively into prominence. 
 Try with a low piano string, keeping the key pressed down 
 while the sound fades, or with the sonometer. Something 
 of the same kind, but less marked, happens in the dying 
 away of a low tone on a reed instrument when the air is 
 allowed to run low in the bellows. 
 
 c. When a tone is sounded continuously for some time on 
 a reed instrument with one of the keys clamped down, dif- 
 ferent partials come successively into prominence, either 
 through varying fatigue or the wandering of attention. 
 
 Helmholtz, 36-65; Stumpf, II., 231-243; see also the index under 
 Obertone ; Mach, A, 58, B, 127. 
 
 90. Timbre. The peculiar differences in quality of tones 
 (distinct from pitch and intensity) which are known as 
 differences in timbre (tone-color, clang-tint, Klangfarbe) , are 
 due largely to differences in the number, pitch, and intensity 
 of the partial tones present. Compare in this respect the 
 dull-sounding bottle-tones or the tones of tuning-forks held 
 over resonance bottles, and the more brilliant tones of a reed 
 or stringed instrument ; the first are nearly simple tones, 
 while the second have strong and numerous over-tones. 
 
SENSATIONS OF HEARING. 77 
 
 a. Notice the difference in quality between the tone given 
 by a tuning-fork held before the ear and that given by the 
 same fork when its stem is pressed upon the table. In the 
 second position the over-tones are relatively stronger. 
 
 b. Notice the differences in quality in the tone of a 
 string when it is plucked in the middle, at one third its 
 length and at about one seventh. When plucked in the 
 middle, many odd-numbered partials are present, and the 
 even-numbered partials are either absent or extremely faint, 
 and the tone is hollow and nasal ; when plucked at one third, 
 the third, sixth, and ninth partials are wanting, and the tone 
 is hollow, but not so much so as before ; when plucked at 
 one seventh all the partials up to the seventh are present. 
 For their theoretical intensities, cf. Helmholtz, 79. 
 
 c. Try also plucking very near one end, plucking with 
 the finger-nail and striking the string with a hard body, e. g., 
 the back of a knife-blade ; all these bring out the higher and 
 mutually discordant partials strongly, and produce a .brassy 
 timbre. 
 
 Helmholtz, 65-119 ; Stumpf, II., 514-549. 
 
 91. In Successive Chords the Whole Mass of Tone seems 
 to move in the same direction as the part that changes most. 
 Strike in succession the chords e' g r -sharp l> e'' ', a a! c" -sharp 
 e" , or a c r e' c" , a c r f c" . If the attention is directed to the 
 bass in the first example and to the alto in the second the 
 whole mass of tone will appear to descend in the first case 
 and to ascend in the second. If the attention is kept on the 
 soprano part the illusion will not appear, as also when the 
 observer examines his sensations critically. Cf. also Ex. 81 
 d, where beats of a partial tone are attributed to the whole 
 compound tone. 
 
 Mach, B, 126-127; Stuinpf, II., 393-395, 
 
78 
 
 LABORATORY COURSE IN PSYCHOLOGY. 
 
 92. Simultaneous Tones interfere somewhat with one 
 another in Intensity. 
 
 la. 
 
 a. Play the groups of notes numbered 1, 2, and 3 and ob- 
 serve the slight increase in the apparent intensity of the 
 remaining tones as one after another drops out, making 1 
 sound like la, 2 like 2a, and so on. On the piano it will be 
 well to play the notes an octave or two lower than they 
 are written. 
 
 b. Play the notes marked 4, and notice that the increase 
 of loudness seems to affect the note (highest or lowest) that 
 receives particular attention, making the effect in one case 
 like 4a, in the other like 4&. 
 
 Mach, J3, 126; Stumpf, II., 418-423. 
 
 93. Consonant and Dissonant Intervals, a. The conso- 
 nant intervals within the octave are the unison, octave, fifth, 
 fourth, major sixth, major third, minor third, and minor 
 sixth. They will be found to decrease in smoothness about 
 in the order given. Try them beginning with the octave 
 and at c, as follows : c c', eg, cf,ca, c e, c e-flat, c a-flat. 
 Try the last four intervals also in the octave of c" or c"' 
 and notice that they are less rough than when taken in the 
 
SENSATIONS OF HEADING. 79 
 
 octave of c. Any other intervals within the octave are dis- 
 sonant. Try c c-sharp, c d, c b, c b-flat, c f-sharp. The 
 roughness is due to beating partial tones and in general 
 is greater when these stand low in the partial tone series 
 and are loud, and when they lie within a half-tone of each 
 other. Work out for the tones of several of the intervals 
 the series of partial tones up to the eighth. In general the 
 extension of intervals into the second octave (taking the 
 higher tone an octave higher or the lower tone an octave 
 lower) does not change the fact of consonance or dissonance, 
 though it may change the relative roughness. 
 
 b. Those fitted by musical training to pronounce upon 
 questions of consonance and dissonance hold that dissonance 
 can be perceived between simple tones under conditions that 
 exclude beats, and that consonance is something more than 
 the smooth flowing of tones undisturbed by beats. The 
 test is easy to make. Hold tuning-forks making the inter- 
 val to be tested one before each ear, and if there are 
 beats, carry the forks far enough away in each direction 
 to make the beats inaudible. Only those of musical ear, 
 however, can pronounce upon the result. 
 
 Helmholtz, 179-197; Stumpf, II., 470, 460; Wundt, 3te Aufl., I., 
 439, II., 47 ff ; Mach, B, 129-130; Preyer, D, 44 ff. 
 
 94. Consonant and Dissonant Chords. In order to form 
 a consonant chord, all the intervals among the tones must 
 also be consonant. The only chords of three tones which 
 fulfil this condition within the octave are represented by 
 the following : Major c e g, cf a, c e-flat a-flat, minor c e- 
 flat g, c f a-flat, c e a. Try these and for comparison any 
 other chord of three tones having c for its lowest tone. 
 
 Helmholtz, 211 ff.; Wundt, 3te Aufl., II., 61, 63 ff. 
 
 95. Major and Minor Chords. Compare the chords c" e" 
 g" and c" e"-flat g". This unmistakable difference in effect 
 
80 LABORATORY COURSE IN PSYCHOLOGY. 
 
 depends in part at least on the fact that in the major chord 
 the difference tones of the first order are lower octaves of c" 
 itself , while in the minor chord one difference tone is not 
 such at all, and if taken in the same octave with the chord 
 would be highly dissonant. For the major chord, when 
 taken in the octave of c" ', the difference tones are c and c", 
 for the minor chord c e-flat, A-flat. Try on a reed instru- 
 ment the difference tones generated by c" e", e" g", c" e"-flat, 
 e"-flat g", first separately ; and then, while c" and g" are 
 kept sounding strike e" and e"-flat alternately. 
 
 Helmholtz, 215-217; Stumpf, II., 335, 376 ff.; Wundt, 3te. Aufl., 
 II., 61 ff., 67 ff. 
 
 96. Cadences. Modern music requires the prominence of 
 the key note or tonic and of the chord in which it holds the 
 chief place at the beginning of a piece of music and at the 
 end. The feeling of the appropriateness of this close, and 
 especially of the succession of chords in the cadences above, 
 can hardly fail to appeal even to the unmusical. 
 
 Helmholtz, 293. 
 
 97. The Absolute Time Eelations of music have much to 
 do with its emotional effect. Have a familiar piece of music 
 played in its proper time, then very slowly and very rapidly. 
 
SENSATIONS OF BEARING. 8l 
 
 BINAURAL AUDITION AND THE LOCATION OF SOUNDS. 
 
 98. Unison Tones Heard with the Two Ears. a. Strike 
 a pair of unison forks that will sound equally loud and 
 vibrate an equal length of time, and hold one before each 
 ear, three or four inches away ; a single tone of rather in- 
 definite location will be heard. As the forks are brought 
 nearer, their tone seems to draw by degrees toward the 
 median plane ; and when they are very loud and near, the 
 tone may seem to be in the head. Return the forks to 
 their first position and then move one a little nearer or a 
 little farther away, and notice that the sound moves to the 
 side of the nearer fork. When the difference in distance 
 has become considerable that fork alone will be heard. 
 
 b. Bring the forks again into the positions last mentioned 
 one near and one far, (or better, place one fork on a rub- 
 ber tube one end of which has been inserted in the opening 
 of the ear and hold the other fork before the other ear), 
 and then with the free or more distant fork make slow 
 rhythmical motions toward and away from the ear, or rotate 
 the fork slowly about its long axis, attending meantime to 
 the fork on the other side. Alternate variations in the 
 intensity of the tone of this fork corresponding to the ap- 
 proach and recession of the other and apparently unheard 
 fork can be observed. 
 
 c. Repeat b and notice that when the changes in intensity 
 are considerable there is a simultaneous shifting of the place 
 of the tone, towards the median plane when the tone grows 
 stronger, and away when it grows fainter. These changes of 
 place are, however, less marked than the changes in intensity 
 and those accompanying slight changes in intensity gener- 
 ally escape observation. 
 
 Schaefer, J5; Thompson; Urbantschitsch, B. 
 
82 LABORATORY COURSE IN PSYCHOLOGY. 
 
 99. Beats Heard with Two Ears. a. Operate as in Ex. 
 98 a, with forks beating three or four times a second. 
 
 b. Try with a pair of very slow beating forks (once in 
 two or three seconds). Notice a shifting of the sound from 
 ear to ear corresponding to the rate of beating. 
 
 c. Try again with a pair of rapid beating forks (twenty or 
 thirty a second), and notice that the beats are heard in both 
 ears. 
 
 Schaefer, A, B, and O; Thompson; Cross and Goodwin. 
 
 100. Difference of Location Helps in the Analysis of 
 Simultaneous Tones. Compare the ease with which the 
 tones of a pair of octave forks are distinguished when the 
 forks are held on opposite sides of the head with the diffi- 
 culty of analysis in Ex. 83 b. 
 
 Stumpf, II. , 336, 363. 
 
 101. Judgments of the Direction of Sounds. These 
 depend in general on the relative intensity of the sounds 
 reaching the two ears, but there is pretty good reason to 
 believe that other factors co-operate and that tolerably cor- 
 rect judgments, both as to distance and direction, can some- 
 times be made from the sensations of one ear. 
 
 a. Let the subject be seated with closed eyes. Snap the 
 telegraph snapper at different points in space a foot or two 
 distant from his head, being very careful not to betray the 
 place in any way, and require him to indicate the direc- 
 tion of the sound. Try points both in and out of the median 
 plane. Observe that the subject seldom or never confuses 
 right and left but often makes gross errors in other direc- 
 tions. Constant tendencies to certain locations are by no 
 means uncommon. 
 
 b. Have the subject hold his hands against the sides of 
 his head like another pair of ears, hollow backward, and 
 try the effect upon his judgment of the direction of the 
 snapper. 
 
SENSATIONS OF HEARING. 83 
 
 c. Find approximately how far the snapper must be 
 moved vertically from the following points in order to make 
 a just observable change in location : on a level with the ears 
 in the median plane two feet in front ; opposite one ear, 
 same distance ; in the median plane behind the head, same 
 distance. Find the just observable horizontal displace- 
 ments at the same points. A convenient way of measuring 
 these distances is to clamp a yard-stick to a retort-stand, 
 bring it into the line along which measurements are to be 
 made and hold the snapper over the divisions of the stick. 
 Snap once at the point of departure, then at a point a little 
 way distant in the direction to be studied ; again at the 
 first point, so that the subject may keep it in mind, and 
 then at a point a little more distant, and so on till a point 
 is finally found which the subject recognizes as just obser- 
 vably different. Eepeat, alternating snaps at the point of 
 departure with those at a greater distance than that just 
 found, decreasing the latter till a point is found where the 
 directions can be no longer distinguished. Make a number 
 of tests each way and take their average. 
 
 d. Continuous simple tones are very difficult to locate. 
 Place a tuning-fork on its resonance case at some distance 
 in front of the subject (seated with closed eyes), another at 
 an equal distance behind him. With the help of an assis- 
 tant strike both forks, and after a little have one of them 
 stopped and the mouth of its resonance box covered. Ee- 
 quire the subject to say which has been stopped. His 
 errors will be very frequent. Compare with this his ability 
 to distinguish whether a speaker is before or behind him. 
 
 On a, Preyer, B ; von Kries, A ; on c, Miinsterberg, B ; on d, Kay- 
 leigh. 
 
 102. Intercranial Location of Sounds, a. Sounds origi- 
 nating outside the head are not located in the head when 
 heard with one ear. Hold a loud-sounding tuning-fork 
 
84 LABORATORY COURSE IN PSYCHOLOGY. 
 
 near the ear, or place it on a rubber tube, one end of which 
 is inserted in the opening of the ear, and notice that the 
 sound when strong may be located in the ear, but does not 
 penetrate farther. Insert the other end of the tube in the 
 opening of the other ear and repeat. The tone, if loud, will 
 appear to come from the inside of the head. Removing and 
 replacing the fork several times will help to give definite- 
 ness to the location. 
 
 b. Repeat the experiment, but use a fork sounding as 
 faintly as possible (e.g., set in vibration by blowing smartly 
 against it), and notice that the location, when a single ear 
 receives the sound, is not so clearly in the ear, and, when 
 both receive it, not so clearly in the head, perhaps even 
 outside of it. Cf. also Ex. 103 b. Both a and b may also 
 be made with beating tones instead of a single one. See 
 also Ex. 69 e. 
 
 Schaefer, B. 
 
 103. Location of the Tones of Tuning-forks Pressed 
 against the Head. a. Strike a large and loud-sounding 
 tuning-fork, and press its stem against the vertex. The 
 tone will seem to come from the interior of the head, chiefly 
 from the back. While the fork is in the same position, 
 close one of the ears with the finger, not pressing it too 
 tight; the sound will immediately seem to concentrate in 
 the closed ear. Have an assistant manage the fork, and 
 close the ears alternately. Something of the same kind 
 happens when a deep note is sung ; close first one ear and 
 then both, and notice the passage of the tone from the 
 throat to the ear and finally to the middle of the head. 
 
 b. Have an assistant manage the fork, and close both ears. 
 Notice that when the fork is pressed on so as to make the 
 tone loud the intercranial location is exact, but when the 
 pressure is relaxed and the tone is faint the location tends 
 to be extracranial. 
 
SENSATIONS OF HEARING. 85 
 
 c. Try setting the fork on other places than the vertex. 
 Notice that in the occipital and parietal regions the sound 
 appears in the opposite ear, though closing the ear as in 
 a may bring it back to the same side as the fork. 
 
 d. Take a long pencil in the teeth like a bit and rest the 
 stem of a vibrating tuning-fork vertically on it near one 
 end and close the ear on the other side ; the sound will 
 seem to be located in the closed ear. Then gradually tilt 
 the fork backward toward a horizontal position, keeping it 
 in contact with the pencil, till its tip is opposite the open 
 ear. The tone will change its place from the closed to the 
 open ear. 
 
 On a and 6, Schaefer, B and C ; on c, Thompson. 
 
 BIBLIOGKAPHY. 
 
 BRUCKE: Ueber die Wahrnehmung der Gerausche, Wien., Sitzb. 3te 
 
 Abth., XC., 1884, 199-230. 
 VON BEZOLD: A. Schuluntersuchungen iiber das kindliche Gehoror- 
 
 gan, Zcitsch.f. Ohrenheilkynde, XIV., 1884-85, and XV., 1885- 
 
 86; also in English translation in the Archives of Otology, XIV. 
 
 This paper gives the results of numerous tests on Munich 
 
 school-children, not only with the watch but also with the acou- 
 
 meter of Politzer and with whispered speech. 
 J5. Einige weitere Mitteilungen iiber die kontinuierliche Tonreihe, 
 
 insbesondere iiber die physiologische obere und untere Ton- 
 
 grenze, ibid., XXIII., 1892, 254-267; also in English translation, 
 
 Archives of Otology, 1893, 216-225. 
 CORRADI: Zur Priifung der Schallperception durch die Knochen, 
 
 Archiv fur Ohrenheilkunde, XXX., 1890, 175-182. Review 
 
 with extract in the Zeitschrift fiir Psychologic, II., 1891, 124. 
 CHARPENTIER: Recherches sur 1'intensite comparative des sons 
 
 d'apres leur tonalite, Archives de physiologic, normale etpatho- 
 
 logique, 1890, No. 3, 496-507. 
 
86 LABORATORY COURSE IN PSYCHOLOGY. 
 
 CROSS AND GOODWIN: Some Considerations regarding Helniholtz's 
 Theory of Consonance, Proceedings of the American Academy 
 of Arts and Sciences, 1891-92, 1-12. 
 
 CROSS AND MALTBY: On the Least Number of Vibrations Neces- 
 sary to Determine Pitch, ibid., 222-235. 
 
 DOCQ: Kecherches physico-physiologique sur la fonction collective 
 des deux organs de 1'appareil auditif. Memoir es couronnes de 
 I' Academic royale de Belgique, XX5QV., 1870. 
 
 EXNER: Zur Lehre von den Gehorsempfindungen, Pfluger's Archiv, 
 XIII. , 1876, 228-253. 
 
 HELMHOLTZ : Sensations of Tone, English translation by Ellis, 2d 
 Ed., London, 1885. This is the great classic of the subject. 
 
 HENSEN: Physiologic des Gehors, Hermann's Handbuch der Physio- 
 logic, III., pt. 2, 1-137. 
 
 HERMANN: Zur Theorie der Combinationstone, Pjluger's Archiv, 
 XLIX., 1891, 499-518. 
 
 HERROUN AND YEO: Note on the Audibility of single Sound 
 Waves and the Number of Vibrations necessary to produce a 
 Tone, Proc. Royal Soc., L., No. 305, 1892, 318-323. 
 
 JAMES : Principles of Psychology, New York, 1890. 
 
 KESSEL : Ueber die vordere Tenotomie, Archiv fur Ohrenheilkunde, 
 XXXI., 1891, 131-143, Keviewed, Zeitschrift fur Psychologic, 
 II., 1891, 398. 
 
 KONIG: Quelques experiences d'acoustique, Paris, 1882. 
 
 VON KRIES: A. Ueber das Erkennen der Schallrichtung, Zeitschrift 
 
 fur Psychologie, I., 1890, 235-251, 488. 
 B. Ueber das absolute Gehor, Ibid., III., 1892, 257-279. 
 
 LANGE : Beitrage zur Theorie der sinnlichen Aufmerksamkeit und 
 der activen Apperception, Wundfs Philosophiscfie Studien, 
 IV., 1888, 390-422. 
 
 LORENZ : Untersuchungen iiber die Auffassung von Tondistanzen, 
 WundV s Philos. Studien, VI., 1890, 26-103. 
 
 LUFT : Ueber die Unterschiedsempfindlichkeit fur Tonhohen. 
 Wundfs Philos. Studien, IV., 1888, 511-540. 
 
 MACH: Works cited with same letters in bibliography of Chap. II. 
 
SENSATIONS OF HEARING. 87 
 
 MAYEB: A. Kesearches in Acoustics, Amer. Jour. Science, 3d Ser. 
 
 VIII., 1874, 241-255, IX., 1875, 267-269, also Phil. Mag., 4th 
 
 Ser. XLIX., Jan.-June, 1875, 352. 
 B. Kesearches in Acoustics, No. VIII., Amer. Jour. Sc., 3d Ser. 
 
 XII!, 1876, 329-336, also Phil. Mag., Ser. 5, II. , July-Dec., 
 
 1876, 500-507. 
 MUNSTERBERG: A. Schwanktmgen der Aufmerksamkeit, Beitrdge 
 
 zur experimentellen Psychologic, Heft 2, 1889, 69-124. 
 
 B. Raumsinn des Ohres, Ibid., 182-234. 
 
 C. Vergleichung von Tondistanzen, Ibid., Heft 4, 1892, 147-177. 
 PREYER: A. Ueber die Grenzen der Tonwahrnehmung, Sammlung 
 
 physiologischer Abhandlungen, I., Jena, 1877, 1-72. 
 
 B. Die Wahrnehmung der Schallrichtung mittelst der Bogen- 
 gange, Pfluger's Archiv, XL., 1887, 586-622. 
 
 C. Ueber Combinationstone, Wiedemann's Annalen, XXXVIII., 
 1889, 131-136. 
 
 D. Akustische Untersuchungen, Sammlung physiologischer Ab- 
 handlungen, II., Jena, 1882, 175-244. 
 
 RAYLEIGH: Our Perception of the Direction of a Source of Sound, 
 Nature, XIV., 1876, 32. See also Acoustical Observations, 
 Phil. Mag., Ser. 5, III., Jan.-June, 1877, 456-458. 
 
 RUTHERFORD : A Lecture on the Sense of Hearing, delivered before 
 the British Association at Birmingham on Sept. 6, 1886, 
 Lancet, 1887, i. 2-6. 
 
 SCHAEFER: A. Ueber die Wahrnehmung und Lokalisation von 
 Schwebungen und Differenztonen, Zeitschrift fur Psychologie, 
 I., 1890, 81-98. 
 
 B. Zur interaurealen Lokalisation diotischer Wahrnehmungen, 
 Ibid., I., 1890, 300-309. 
 
 C. Ein Versuch iiber die intrakranielle Leitung leisester Tone von 
 Ohr zu Ohr, Ibid., II., 1891, 111-114. See also discussion of 
 Schaefer, Scripture and Wundt, Ibid., IV., 348; V., 397; and 
 WundV s Philos. Studien, VII., 630; VIII., 638, 641. 
 
 SCHISCHMANOW: Untersuchungen iiber die Empfindlichkeit des In- 
 tervallsinnes, Wundt 1 s Philos. Studien, V., 1889, 558-600. 
 
 STUMPF: Tonpsychologie, Leipzig, 1883 and 1890. This work of 
 Stumpf s is by far the most complete upon the Psychology of 
 Tone. The two volumes so far published (the work is to be 
 complete in four) cover the psychology of successive and of 
 simultaneous tones. 
 
88 LABORATORY COURSE IN PSYCHOLOGY. 
 
 THOMPSON, SYLVANUS P.: A. On Binaural Audition, Phil. Mag., 
 
 Ser. 5, IV., July-Dec., 1877, 274-276; VI., July-Dec., 1878, 
 
 383-391; XII. , July-Dec., 1881, 351-355. 
 B. On the Function of the Two Ears in the Perception of Space, 
 
 Ibid., XIII., Jan.- June, 1882, 406-416. 
 UKBANTSCHITSCH: A. Ueber eine Eigentiimlichkeit der Schallem- 
 
 pfindungen geringster Intensitat, Centralblatt f. d. med. Wis- 
 
 sens., 1875, 625-628. 
 
 B. Zur Lehre von der Schallempfindung, Pflilger's Archiv, XXIV., 
 1881, 574-595. 
 
 C. Ueber das An- und Abklingen acustischer Empfindungen, 
 Ibid., XXV., 1881, 323-342. 
 
 WUNDT: Work cited in bibliography of Chap. I., 3teAufl., I., 415 
 ff., II., 42 ff.; 4te Aufl., I., 443 ff. 
 
 On the physics and physiology of sound, reference may be made, in 
 addition to the works already mentioned, to Tyndall, On 
 Sound; Blaserna, Theory of Sound in its Relations to Music; 
 Zahn, Sound and Music; and Taylor, Sound and Music. The 
 last is very simple and untechnical, and is perhaps the best for 
 those approaching the subject for the first time. 
 
 For the Stumpf-Wundt discussion on pitch distances consult the 
 following: Stumpf, Zeitschrift fur Psychologic, I., 1890, 419; 
 II., 1891, 266,426, 438; Engel, Ibid, II., 1891, 361; Wundt, 
 Philos. Studien, VI., 1890-91, 605; VII., 1891, 298, 633; also 
 Miinsterberg, C, above. 
 
THE MECHANISM OF THE EYE. 89 
 
 CHAPTER V. 
 The Mechanism of the Eye and Vision in General. 
 
 THE mechanism of the eye accomplishes two things : the 
 projection of a sharp image on the retina, and the ready 
 shifting of the eye so as to bring successive portions of the 
 image into the best position for seeing. To the study of 
 these mechanisms and other physiological phenomena of 
 importance for the psychology of vision, this chapter is 
 devoted. 
 
 THE KETINAL IMAGE AND ACCOMMODATION. 
 
 104. The Eetinal Image. This is easily seen in the 
 unpigmented eye of a pink-eyed rabbit. 
 
 a. Chloroform the rabbit, remove the eyes, and mount 
 them in clay for readier handling. The mounting is done 
 as follows : Make a thick ring of clay with an internal 
 diameter a little greater than that of the cornea of the 
 rabbit's eye ; place the eye, cornea downward, in the ring ; 
 lay a similar ring upon it to keep it in place, and press the 
 edges of the rings together. The eye can now be handled 
 easily and turned in any direction. Turn the cornea 
 toward the window, and observe, from behind, the inverted 
 image on the retina. Bring the hand into range and move 
 it to and fro ; observe that the image of distant objects is 
 more distinct than that of the hand. The dead eye is 
 adjusted for distant vision. If convex and concave lenses 
 are at hand (spectacle lenses will answer), bring them 
 before the eye, and observe that the effect upon the 
 
90 LABORATORY COURSE IN PSYCHOLOGY. 
 
 retinal image is similar to that seen subjectively when 
 they are held before the observer's own eye, provided 
 that that is normal. 
 
 Reverse the eye, holding it retina side toward the win- 
 dow, and observe the radiating and circular fibres of the 
 iris. The eye must be fresh, for if long removed it loses 
 its transparency. 
 
 105. Accommodation. The sharpness of the retinal 
 image depends on the adjustment of the crystalline lens, 
 which must be such as to focus upon the retina the light 
 from the object under regard. The lens must be thicker 
 and rounder for near objects, thinner and flatter for more 
 distant ones. These adaptations of the eye are known as 
 Accommodation. The changes in the clearness of the retinal 
 image are easy to observe subjectively. Hold up a pin or 
 other small object six or eight inches away from the eyes. 
 Close one eye and look at the pin with the other. The out- 
 line of the pin is sharp, but the outlines of things on the 
 other side of the room behind it are blurred. Look at these, 
 and the outline of the pin becomes blurred. Notice the 
 feeling of greater strain when looking at the nearer object. 
 The experiment is somewhat more striking when the nearer 
 object is a piece of veiling or wire gauze, and the farther, a 
 printed page held at such a distance that it can just be 
 read. 
 
 On this and the next two experiments, see Helmholtz, A, 112-118, 
 Fr. 119-126 (90-96). 
 
 106. Schemer's Experiment, a. Pierce a card with two 
 fine holes separated by a less distance than the diameter of 
 the pupil, say, a sixteenth of an inch. Set up two pins in 
 corks, distant respectively eight and twenty inches from 
 the eye in the line of sight ; close one eye, and holding the 
 card close before the other with the holes in the same hori- 
 
THE MECHANISM OF THE EYE. 
 
 91 
 
 zontal line, look at the nearer pin; the farther pin will 
 appear double. Look again at the nearer pin, and while 
 looking, cover one of the holes with another card ; one of 
 the images of the farther pin will disappear the left when 
 the left hole is covered, and the right when the right is 
 covered. Look at the farther pin or beyond it ; the nearer 
 pin appears double. Repeat the covering ; closing the left 
 hole now destroys the right image, and covering the right 
 destroys the left. 
 
 Why this should be so will be clear from the diagrams 
 above. The upper diagram illustrates the course of the rays 
 of light when the eye is accommodated for the nearer pin ; 
 the lower diagram when it is accommodated for the farther 
 pin. A and B represent the pins; S and S the pierced 
 screen ; d and df the holes in the screen ; c and c the lens ; 
 a' b a" and V a V the retinae ; A, A', B r and B" ', the positions 
 of the double images. The solid lines represent the course of 
 the rays from the pin that is accommodated for ; the lines 
 of short dashes, the course of the rays from the other pin ; 
 
92 LABORATORY COURSE IN PSYCHOLOGY. 
 
 the lines of long dashes, the lines of direction ; i.e., approxi- 
 mately those giving the direction in which the images 
 appear to the observer. In the upper diagram the rays 
 from B are focused to a single retinal image at b, while 
 those from A, being less divergent at first, are brought to 
 a focus nearer the lens, cross over and meet the retina at 
 a! and a", and, since each hole in the screen suffices to 
 produce an image, cause the pin to appear double. Its two 
 images are referred outward as all retinal images are, along 
 the lines of direction (which cross a little forward of the 
 back surface of the lens, in the crossing point of the lines of 
 direction), the right retinal image corresponding with the 
 left of the double images and vice versa. If now the right 
 hole (d) in the screen be closed, the left retinal image and 
 the right double image disappear. The case of accommo- 
 dation for the farther pin will be clear from the lower 
 diagram, if attention is given to the dotted and dashed 
 lines. It will also be easy to explain why moving the card 
 when looking through a single pin-hole causes apparent 
 movements of the pin not accommodated for, and why in 
 one case the movement seems to be with the card, and in the 
 other case against it. 
 
 b. Stick the pins into the corks so that they shall extend 
 horizontally, and examine them with the card held so as to 
 bring the holes one above the other. 
 
 c. Arrange the holes thus : . . and observe that the 
 triple image of the nearer pin (when the farther is fixated) 
 has the reverse figure 
 
 Schemer's experiment can easily be illustrated with any 
 convex lens and a pierced screen of suitable size. 
 
 107. Range of Accommodation, a. Find by trial the 
 nearest point at which a pin seen as in Schemer's experi- 
 ment can be seen single. This is the near point of accom- 
 
THE MECHANISM OF THE EYE. 93 
 
 modation. For the short-sighted a far point may also be 
 found, beyond which double images reappear. 
 
 b. Find how far apart in the line of sight two pins may 
 be, and yet both be seen single at one and the same time. 
 Try with the nearer at 20 cm., at 50 cm., at 2 m. That 
 portion of the line of sight, for points in which the same 
 degree of accommodation is sufficient, is called the Line of 
 Accommodation. The length of the line increases rapidly as 
 the distance of the object from the eye increases. 
 
 Helmholtz, A, 114, 119, Fr. 122 (93), 128 (97). 
 
 108. Mechanism of Accommodation. The change in the 
 lens in accommodation is chiefly a bulging forward of its 
 anterior surface. This may be observed as follows : 
 
 a. Let the subject choose a far and a near point of fixation 
 in exactly the same line of vision ; close one eye and fix the 
 other upon the far point. Let the observer place himself 
 so that he sees the eye of the subject in profile with about 
 half the pupil showing. Let the subject change his fixation 
 at request, from the far to the near point, and vice versa, 
 being careful to avoid any sidewise motion of the eye. 
 The observer will notice, when the eye is accommodated for 
 the near point, that more of the pupil shows and that the 
 farther side of the iris seems narrower. This change is 
 due to the bulging forward of the front of the lens. If 
 the change were due to accidental turning of the eye 
 toward the observer, the farther edge of the iris should 
 appear wider instead of narrower. Notice also that the 
 diameter of the pupil changes with the accommodation. 
 
 b. Purkinje's Images. The changes in the curvature of 
 the lens may also be observed by means of the images 
 reflected from its front and back surfaces and from the front 
 of the cornea. Operate in a darkened room. Let the sub- 
 ject choose far and near fixation points as^beJor^Let the 
 
94 LABORATORY COURSE IN PSYCHOLOGY. 
 
 observer bring a candle near the eye of the subject at a 
 level with it and a little to one sideband place his own eye 
 in a position symmetrical to the candle on the other side of 
 the subject's line of sight. Careful examination and some 
 shifting about of the place of the candle and of the observer 
 will show three reflected images of the flame : one on the 
 side of the pupil next the light, easily recognizable, bright 
 and erect, reflected from the surface of the cornea ; a second, 
 nearer the centre of the pupil and apparently the farthest 
 back of the three, erect like the first, but very indistinct 
 (more like a light cloud than an image), reflected from the 
 anterior surface of the lens ; and a third, a mere point of 
 light, near the side of the pupil farthest from the flame, 
 inverted and reflected from the posterior surface of the 
 lens. When the observer has found these three images, the 
 subject should fixate alternately the near and far points 
 chosen. As he fixates the near point, the middle image 
 will grow smaller, advance, and draw toward the corneal 
 image ; when he fixates the far point, the image will enlarge, 
 recede, and move away from the corneal image. The follow- 
 ing diagram, after Aubert, illustrates the movement of the 
 
 middle image ; the full 
 lines indicate the posi- 
 tions of the cornea and 
 lens and the course of 
 the rays of light when 
 the eye is accommodated 
 for the far point ; the 
 dotted lines indicate 
 the anterior surface of 
 the lens and the direc- 
 tion of the ray reflected 
 from its surface when the eye is accommodated for the near 
 point. Three images similar to those in question can be 
 
THE MECHANISM OF THE EYE. 95 
 
 observed on a watch glass and a double convex lens held in 
 the relation of the cornea and crystalline. 1 
 
 Helmholtz, A, 131-141, especially 131-134, Fr. 142-154 (104-112), 
 especially ,142-146 (104-107); Aubert, A, 444; Tscherning. 
 
 109. Dioptrical Defects of the Eye. Of these defects 
 only two will be considered here : Astigmatism and Chro- 
 matic Aberration. The first is an error in the form or set- 
 ting of the refracting surfaces, which prevents their bringing 
 parallel light to a focus in a single point. If the curvature 
 of the lens, for example, (or of the cornea), is greater on the 
 vertical meridian than on the horizontal, parallel light fall- 
 ing upon the first will be brought to a focus nearer the lens 
 than that falling upon the second. This makes it impossible 
 for the astigmatic eye to see all parts of a plane figure with 
 equal distinctness at the same time. Chromatic Aberration 
 depends upon the different degrees of refraction which dif- 
 ferent colored lights experience in traversing the lens ; 
 those of short wave-length (violet and blue) are most re- 
 fracted, those of long wave-length (red and orange) least, 
 and the others in order between. The point at which paral- 
 lel violet rays are brought to a focus is therefore nearer the 
 lens than the point for red. In order, therefore, that the 
 same degree of accommodation may serve to show a red 
 lighted object and a violet lighted object at the same time 
 and both with full distinctness, the red light must be less 
 divergent than the violet ; in other words, the red lighted 
 object must be somewhat farther away. 
 
 a. Astigmatism. Make a fine pin-hole in a card ; hold it 
 at arm's length against a bright background and accommo- 
 
 1 By using a magnifying-glass a second faint corneal image very close to the 
 first can be seen, when the light strikes the cornea well toward one side. When 
 this is counted, as it is by Tscherning, there are four Purkinje images, those from 
 the front and back of the lens becoming the third and fourth in the enumeration, 
 instead of the second and third. 
 
96 
 
 LABORATORY COURSE IN PSYCHOLOGY. 
 
 date the eye for a nearer point, or put on convex glasses. 
 The spot will not appear as a little circle of light, as it 
 would if the lens and cornea were perfect in form, but as a 
 more or less irregular star or flower-shaped figure in which 
 portions of several images of the hole may be made out. 
 Accommodate for a point considerably beyond the card 
 and notice the change in the figure. 
 
 These irregularities (phenomena of Irregular Astigma- 
 tism) disappear, however, with exact accommodation, but 
 another kind (Regular Astigmatism) is then to be observed. 
 Close one eye and look with the other at the centre of the 
 radiating figure below. Notice which lines appear with 
 greatest blackness and distinctness. Try the effect of 
 increasing and decreasing the distance. Try also the other 
 eye. 
 
 Something of the same kind is to be seen in the set of 
 concentric circles ; also evidences of irregular astigmatism 
 when accommodation is changed or when the distance of 
 the diagram is increased or decreased. Notice especially 
 the rayed appearance and the distortion of the inner circles 
 when the eye is accommodated for a greater distance than 
 
THE MECHANISM OF THE EYE. 97 
 
 that of the diagram. On the latter peculiarity, see von 
 Bezold. 
 
 b. Chromatic Aberration. Bend a fine platinum wire 
 into a ring half an inch in diameter, and heat it white hot 
 in the flame of a Bunsen burner. Look at the ring through 
 a pin-hole in a black card held at such a distance that the 
 ring lies close to the edge of the field of the pin-hole all 
 around. Accommodate the eye for the centre of the ring, 
 and observe that the outer edge of the ring appears bright 
 red, the inner edge blue or violet. Substitute for the card 
 .a bit of blue glass, and accommodate first for the glass, then 
 for a point some distance beyond the ring. In the first 
 case the outer and inner edges of the ring (except as astig- 
 matism interferes) will both be blue; in the second case 
 they will be red. The ordinary blue glass allows both red 
 and blue light to pass through it. 
 
 Look at the edge of the window frame next the pane, 
 and bring a card before the eye so that about half the 
 pupil is covered ; if the card has been brought up from the 
 frame side, the frame will be bordered with yellow ; if 
 from the pane side, with blue. In ordinary vision these 
 fringes do not appear, because the colors partially overlap 
 and produce a practically colorless mixture. 
 
 Yon Bezold's Experiment. Look at the parallel lines of 
 the left figure in Ex. 118 with imperfect accommodation, 
 e.g., through convex spectacles, and observe the aberra- 
 tion colors. If a set of heavy concentric circles (separated 
 by equal spaces, and beginning with a central black dot 
 of a diameter equal to the width of the lines) is used 
 instead of the straight line figure, it will be possible by 
 changing its distance from the eye to find a position in 
 which the aberration colors so overlap that dark and light 
 seem to have changed places, and the central spot is light 
 instead of dark. The spiral figure with Ex. 128 will show 
 
98 LABORATORY COURSE IN PSYCHOLOGY. 
 
 something of the effect, but the central black spot is too 
 large to show it completely. 
 
 Both astigmatic differences and the aberration colors may 
 at times influence judgments of distance. 
 
 On a, Helmholtz, A, 169 ff., Fr. 187 (138) ft. On 6, Helmholtz 
 A, 156-164, Fr. 172-179 (125-131); von Bezold; Tumlirz. 
 
 ENTOPTIC APPEARANCES. 
 
 110. Floating Particles in the Media of the Eye and on 
 its Surface; Muscce Volitantes. Fix a lens of short focus 
 at some distance from a bright gas or candle flame. Set up- 
 in the focus of the lens a card pierced with a very fine hole ; 
 bring the eye close to the hole and look toward the light. 
 The eye should be far enough from the hole to prevent the 
 edge of the lens from being seen. The rays of light that 
 now reach the eye are strongly divergent, and the crystalline 
 lens does not bring them to a focus on the retina, but only 
 refracts them to such a degree that they traverse the eye 
 nearly parallel, and thus in suitable condition for casting 
 sharp shadows upon the retina of objects on or in the eye. 
 
 a. The lens will appear full of light, and in it will be 
 seen a variety of shadings, blotches, and specks, single or in 
 strings, the outward projection of the shadows just men- 
 tioned. The figures in this luminous field will vary from 
 person to person, even from eye to eye, but in almost every 
 eye some will be found that move and some that remain 
 fixed or only move with the eye. Of the moving figures 
 some are due to particles and viscous fluids on the surface 
 of the eye ; they seem to move downward, and &re changed 
 by winking. Notice, for example, the horizontal bands that 
 follow a slow dropping and raising of the upper lid. Such 
 appearances as these, since their cause is not really in the 
 eye but outside of it, have been called pseudentoptic by 
 Laqueur. Others, the muscce volitantes, are frequently 
 
THE MECHANISM OF THE EYE. 99 
 
 noticed without any apparatus ; they appear as bright 
 irregular threads, strings^ of beads, groups of points, or 
 single minute circles with light centres. They seem to 
 move downward in the field, but actually move upward in 
 the vitreous humor where they are found. Of the per- 
 manent figures, some are due to irregularities of structure 
 or small bodies in the crystalline and its capsule (spots 
 with dark or bright centres, bright irregular lines, or 
 dark radiating lines corresponding probably to the radial 
 structure of the lens) ; others of a relatively permanent 
 character, it is said, can be produced on the cornea by 
 continued rubbing or pressure on the eyeball. 
 
 b. The rouncj spot of light in which these things are seen 
 represents the pupil, and the dark ground around it is the 
 shadow of the iris. Notice the change in the size of the 
 spot of light, as the eye is accommodated for different dis- 
 tances (cf. Ex. 108), or as the other eye is exposed to, or 
 covered from, the light. The change begins in about half 
 a second. It shows the close connection of the iris 
 mechanisms of the two eyes, and is typical of the way in 
 which the two eyes co-operate as parts of a single visual 
 organ. 
 
 Some of these entoptic observations may be made with a 
 pierced card alone, or simply by looking directly at a broad 
 expanse of clear sky without any apparatus at all. 
 
 Helmholtz, A, 184-192, and Tafel I., which shows the appearance 
 of several of these entoptic objects, Fr. 204-214 (149-156) and PL V., 
 also 548-558 (419-427); Laqueur. 
 
 111. Eetinal Blood-vessels, Purkinje's Vessel Figures. 
 a. Concentrate a strong light (preferably in a dark room), 
 or even direct sunlight, with a double convex lens of short 
 focus on the sclerotic in the outer corner of the eye of the 
 subject, requesting him to turn the eye toward the nose 
 
100 LABORATORY COURSE IN PSYCHOLOGY. 
 
 / 
 
 and giving him a dark background to look toward. Mate 
 the spot of light on the sclerotic as small and sharp as 
 possible, and give to the lens a gentle to and fro or circular 
 motion. After a little the subject will see upon the field, 
 which the light makes reddish-yellow, the dark branching 
 figure of the shadows of the retinal vessels. Notice that 
 the spot directly looked at is partially surrounded, but not 
 crossed, by the vessels. In this lies the yellow spot (macula 
 luted) , the retinal area of clearest vision. The centre- from 
 which the vessels radiate lies in the point of entrance of 
 the optic nerve. In this form of the experiment the light 
 radiates in all directions within the eye from the illumi- 
 nated point of the sclerotic. 
 
 b. Somewhat the same sort of image is to be secured by 
 moving a candle about near the eye, below it and a little to 
 one side. In this experiment some indication of the region 
 of the yellow spot is to be seen. This time the light enters 
 by the pupil, forms an image on a part of the retina some- 
 what remote from the centre, and this retinal image is 
 itself the source of the light by which the vessel shadows 
 are cast. 
 
 c. Look through a pin-hole in a card, held close before 
 the eye, at the sky or some other illuminated surface, or at 
 a broad gas-flame. Give the card a rather rapid circular 
 motion, and the finer retinal vessels in the region of the 
 yellow spot will readily be seen, among them also a small 
 colored or slightly tinted spot (best seen, perhaps, by gas- 
 light) representing the macula, and in its centre a shadowy 
 dot (representing the fovea, the point of clearest vision), 
 which appears to rotate when the motion of the card is 
 circular. If the card is moved horizontally, the vertical 
 vessels alone appear; if vertically, the horizontal vessels. 
 Notice also the granular appearance of the macula; the 
 granulations have been supposed to represent the visual 
 
THE MECHANISM OF THE EYE. 101 
 
 cones of that region. The finer retinal vessels can also be 
 seen when looking at the vacant field of a compound micro- 
 scope, if the eye is moved about rapidly. 
 
 In ail cases it is important that the shadows be kept 
 moving ; if they stand still, they are lost. The explanation 
 is partly physiological (the portions of the retina on which 
 the shadows rest soon gain in sensitiveness enough to com- 
 pensate for the less light received) and partly psychological 
 (moving objects in general arouse spontaneous attention, and 
 those whose images rest continuously on the retina without 
 motion are particularly subject to neglect). 
 
 Once having become familiar with these vessel figures, it 
 is often possible for the observer to see traces of them 
 without any apparatus. Parts of them, with something of 
 the yellow spot, may sometimes be seen for an instant as 
 dark figures on the diffusely lighted walls and ceiling, or as 
 light figures on the dark field of the closed eyes, when the 
 eyes are opened and closed after a glance at the window on 
 first waking in the morning, or as blue figures when looking 
 at the snow and winking on a bright winter morning. 
 
 Helmholtz, A, 192-198, 555,Fr. 214-221 (156-161), 528 (402). 
 
 112. Eetinal Circulation. Look steadily through two or 
 three thicknesses of blue glass at the clear sky or a bright 
 cloud, and observe the bright points darting hither and 
 thither like bees in a swarm or snowflakes on a windy day. 
 Careful observation will also establish that the bright points 
 are followed by shadowy darker ones. Pick out a speck on 
 the window to steady the eyes, and observe that while the 
 movements of the points seem irregular the same lines are 
 retraced by them from time to time. When several of 
 their courses have been accurately determined for one of 
 the eyes, repeat the experiment for demonstrating the finer 
 retinal vessels (Ex. Ill, c), and notice that fine vessels 
 
102 LABORATORY COURSE IN PSYCHOLOGY. 
 
 are found which correspond to the courses that the points 
 seem to follow. These flying points can be seen without 
 the glass by a steady gaze at an evenly lighted bright sur- 
 face, and sometimes a rhythmical acceleration of their 
 movements will be found, corresponding to the pulse. 
 Helmholtz explains the phenomenon by a temporary clog- 
 ging of fine capillary vessels by large blood corpuscles. 
 The bright lines (the apparent tracks of bright points) are 
 really the relatively empty capillary tubes ahead of the cor- 
 puscles, which, after an instant, are driven onward by others 
 crowding behind, which in turn give the shadow that ap- 
 parently follows the bright points. 
 
 Helmholtz, A, 198 f., Fr. 221 (837), 555 (425); Kood. 
 
 113. The Blind Spot. Mariotte's Experiment. The point 
 of entrance of the optic nerve is unprovided with visual 
 end-organs and is irresponsive to light. This insensitive- 
 ness is easily demonstrated with the diagrams below. 
 
 a. Close the left eye, and keeping the right fixed on the 
 upper asterisk in the diagram move the latter toward the 
 eye and away from it till a point is found where the black 
 oval disappears. For the blind spot of the left eye, turn 
 the diagram upside down and close the right eye. 
 
 The blind spot may be demonstrated simultaneously in 
 both eyes with the figure on the next page. The experi- 
 menter should look at the asterisk while he holds a card 
 
THE MECHANISM OF THE EYE. 103 
 
 in the median plane of his head, to prevent each eye 
 from seeing the other's part of the diagram. 
 
 O O 
 
 b. To draw the projection of the blind spot, arrange a 
 head-rest opposite a vertical sheet of white paper, and 15 or 
 18 inches distant from it. Put a dot on the paper for a 
 fixation point. Fasten upon the end of a light rod a bit of 
 black paper about 2 mm. square or blacken the end of the 
 rod with ink. Bring the face into position, close one eye, 
 and fix the other upon the dot. Move the rod slowly so as 
 to bring the little square over the part of the paper corre- 
 sponding to the blind spot, dotting on the paper the points 
 where the square disappears or reappears. Repeat at vari- 
 ous points till the outline of the projection of the blind spot 
 is complete. If the mapping is carefully carried out, the 
 map will probably also show the points of departure of the 
 large blood-vessels that enter with the nerve. 
 
 Helmholtz, A, 250-254, Fr. 284-289 (210-214). 
 
 114. The Filling-out of the Blind Spot is of considerable 
 psychological interest. The mind supplies what is lacking 
 in the sense, and in doing so is influenced both by the sensa- 
 tions of the parts of the retina surrounding the spot and 
 by previous experience. In ordinary two-eyed vision the 
 blind spot of one eye corresponds to a seeing spot in the 
 other, and this with the movements of the eyes amply sup- 
 plies the defect. The spot, furthermore, lies so far out 
 of the range of clear vision that its existence is habitually 
 overlooked, even in monocular vision. 
 
 a. When the image of the oval in a of the last experi- 
 
104 LABORATORY COURSE IN PSYCHOLOGY. 
 
 ment is brought wholly upon the spot, the paper seems an 
 unbroken white, because the adjacent parts of the retina 
 are stimulated with white. When, however, the diagram 
 is held a little nearer so that the edge of the black oval can 
 be seen, the filling is part black and part white. 
 
 b. The effect of experience appears when the oval is 
 replaced by such a figure as that below, or any other in 
 which the bars stand out well from one another and the 
 background. 
 
 When the image of the middle of this diagram falls upon 
 the blind spot, one bar will seem to cross completely over 
 the other. Bars that cross are so much more frequent in 
 experience than those that are mitered together that the 
 sensations of the adjacent parts are thus interpreted. 
 Skill in observation in indirect vision seems to hinder this 
 filling-out process somewhat, probably by aiding in more 
 exact distinguishing of the character of the sensations 
 received. Both Helmholtz and Aubert find themselves 
 unable to determine how the parts of the figure resting on 
 the blind spot are related. 
 
 Helmholtz, A, Fr. 734-745 (574-583); Aubert, A, 595. 
 
THE MECHANISM OF THE EYE. 105 
 
 115. The Yellow Spot, the Macula Lutea. The projection 
 of the yellow spot in the visual field can be made visible in 
 several ways. Two have already been mentioned in Ex. 
 Ill ; Others are as Jj^ws : Close the eyes for a few 
 seconds and then loolMHough a flat-sided bottle of chrome 
 alum solution at a brightly lighted surface or at the clear 
 sky. In the blue-green solution a rose-colored spot will be 
 seen which corresponds to the yellow spot. The light that 
 comes through the chrome alum solution is chiefly a mixture 
 of red and green and blue. The pigment of the yellow spot 
 absorbs a portion of the blue and green and transmits the 
 rest, which makes a rose-colored mixture, to the visual 
 organs behind it. The same can be very beautifully de- 
 monstrated with violet or purple gelatine sheets. 
 
 Helmholtz, A, Fr. 548-551 (419-421); Maxwell; Sachs; Hering, C. 
 
 116. Intermittent Illumination. Tl^ region of the yel- 
 low spot can be seen, together with^many other curious 
 figures and patterns, when the illumination of a single eye 
 is made intermittent by moving the spread fingers rapidly 
 to and fro before it. Something may be seen when the 
 open eyes are fixed on a uniformly lighted surface, but more 
 when they are turned with closed lids toward a bright sky 
 or the sun itself. The figures probably differ in different 
 eyes and some are beautiful and elaborate. Sometimes with 
 steady fixation the figures give place more or less completely 
 to a general streaming of fine particles, suggesting the flying 
 specks of Ex. 112, but finer and of less regular course. 
 Vierordt credited the appearance to the circulation of the 
 blood in the retinal vessels ; Helmholtz is inclined to think 
 the fine particles lymph corpuscles rather than blood corpus- 
 cles. Similar phenomena are to be observed with black and 
 white disks when rotated at less speed than that required 
 for uniform mixing of the black and white. 
 
 Helmholtz, A, 532 f., Fr. 502 (381) f.; Exner, F. 
 
106 LABORATORY COURSE IN PSYCHOLOGY. 
 
 117. Acuteness of Vision, Minimum Visibile. a. Place 
 the parallel line diagram used in Ex. 118 in a good light 
 and walk backward from it till the lines can just no longer 
 be distinguished as separate. If ti^^perimeiiter's eyes are 
 not normal, he should use glasse^^Blt fit his eyes for dis- 
 tinct vision at the distance requireor Measure the distance 
 between the eye and the diagram, and calculate the angle 
 whose apex lies in the crossing point of the lines of direc- 
 tion (about 7.2 mm. back of the cornea and 15.6 mm. in 
 front of the retina) and whose base is the distance from 
 the middle of one line of the diagram to the middle of the 
 next; in this diagram 1.6 mm. This angle measures the 
 least visible extent when discrimination is involved ; the 
 least luminous extent that can still impress the retina is far 
 smaller, as witness the visibility of the stars. On the sup- 
 position that if the sensations of two cones are to be separ- 
 able they must be s^arated by an unstimulated cone, or at- 
 least by a less stimulated one, it has generally been consid- 
 ered that the cones could not subtend a greater angle 
 than that found in this experiment, 60" 90", represent- 
 ing 0.004 0.006 mm. on the retina, and this agrees 
 well with microscopical measurements. But as Helm- 
 holtz notices (Phys. Opt., 2d ed., p. 260), this experiment 
 does no more than prove that there are on the retina rows 
 of sensitive elements, the middle lines of which are sepa- 
 rated by the angular distance found in the experiment. 
 The elements themselves, if properly arranged, may be 
 somewhat larger. Calculation of the number of such ele- 
 ments in a sq. mm. of the retina, based on this view of the 
 experiment, agrees well in the case of Helmholtz's own 
 determination with the result of microscopical counting. 
 
 b. The discriminative power of the retina falls off rapidly 
 in all directions from the fovea more rapidly above and 
 below than in a horizontal direction. Arrange a head-rest 
 
THE MECHANISM OF THE EYE. 
 
 107 
 
 and perpendicular plane as in Ex. 113, b (or if a perimeter 
 is at hand use that). Place upon the end of the rod used in 
 that experiment a card on which have been made two black 
 dots 2 mm. in diameter and 4 mm. from centre to centre. 
 Move the card horizontally toward the fixation point, begin- 
 ning beyond the point at which the two dots can be dis- 
 tinguished and moving inward till they can just be 
 distinguished. Measure the distance from the fixation 
 point, and repeat several times both to the right and left 
 of the fixation point, holding the card so that both dots are 
 in each case equally distant from that point. Try the same 
 for the vertical meridian. 
 
 Helmholtz, A, 255-264, Fr. 291-301 (215-223); Uhthoff. On a, 
 Aubert, A, 579-585; on 6, 585-591. On 6, see also Exner, D, 242 ff. 
 
 118. Bergmann's Experiment. Place the left hand dia- 
 gram in a good light, and look at it from a distance of a yard 
 and a half or two yards. Observe the apparent bending and 
 beading of the lines. This is believed by Helmholtz to be due 
 to the mosaic arrangement of the visual cones. The cones 
 that are touched by the image of one of the white lines are 
 stimulated in proportion as they are more or less touched. 
 Those that are much stimulated furnish the sensation of 
 the white line and its irregularities; those that are little 
 
108 LABOEATOEY COUESE IN PSYCHOLOGY. 
 
 stimulated join with those that are not touched at all to 
 give the image of the black line and its irregularities. 
 This is schematically represented in the right hand cut. 
 Von Fleischl, on the other hand, has made experiments to 
 show that the bending and beading of the lines is not con- 
 nected with the retinal mosaic, but rather with movements 
 of the eyes that sweep the point of fixation backward and 
 forward across the lines. Further than this his explanation 
 does not go. 
 
 Helmholtz, A, 257-258, Fr. 293 294 (217-218); von Fleischl. 
 
 119. Mechanical Stimulation of the Retina, a. Phos- 
 phenes. Turn the open or closed eye as far as possible 
 toward the nose and press on the eyelid at the outer corner 
 with the finger or the tip of a penholder. On the opposite 
 side of the visual field will be seen a more or less complete 
 circle of light surrounded by a narrow dark band, outside of 
 which again is a narrow band of light. Notice the color of 
 the light seen. Get phosphenes by pressure at other points 
 of the eyeball. 
 
 b. Press the eye moderately with some large object, say, 
 the angle of the wrist when the hand is bent backward, and 
 continue the pressure for a minute or two. Peculiar palpi- 
 tating figures will be observed and strange color effects. 
 The former Helmholtz compares to the tingling of a mem- 
 ber that is " asleep." 
 
 c. Standing before a window, close the eyes and turn 
 them sharply from side to side. As they reach the extreme 
 position in either direction, observe immediately in front of 
 the face a sudden blue spot surrounded by a yellow band. 
 A second fainter spot farther from the centre in the direc- 
 tion of motion may also be seen. The appearance of the 
 first spot is due to a mechanical stimulation of a portion of 
 the retina at the edge of the blind spot in the eye that turns 
 
THE MECHANISM OF THE EYE. 109 
 
 inward. The second spot belongs to the corresponding area 
 in the other eye. 
 
 Helmholtz, A, 235-239, Fr. 266-270 (196-200), 744 (583) f. 
 
 120. Idio-retinal Light, Light Chaos, Light Dust. a. Close 
 and cover the eyes so as to exclude all light, taking care 
 not to press them, or experiment in a perfectly dark room. 
 Let the after-effects of objective light fade away, and then 
 watch the shifting clouds of retinal light. The cause of 
 the retinal light is not altogether clear, but it is supposed 
 to be a chemical action of the blood on the nervous portion 
 of the visual apparatus. Aubert estimates its brightness 
 at about half the brightness of a sheet of paper illuminated 
 by the planet Venus when at its brightest. 
 
 b. When awake in the night time in a room that is almost 
 perfectly dark (e.g., in which the form of the window and 
 the large pieces of furniture cannot be made out), notice 
 that the white clothing of the arms can be seen faintly 
 when they are moved about, but not when they are still. In 
 the last case the very faint light they reflect is not sufficient 
 to make them distinguishable from clouds of idio-retinal 
 light. 
 
 Helmholtz, A, 242-243, Fr. 274-275 (202-203). On 6, Helm- 
 holtz, jB. 
 
 121. Electrical Stimulation of the Visual Apparatus. 
 Moisten thoroughly with salt water both the electrodes and 
 the portions of the skin to which they are to be applied. 
 Place one of the electrodes on the forehead (or on the edge 
 of the table and lay the forehead upon it), the other on the 
 back of the neck ; or, if the current is strong enough, hold 
 it in the hand or lay it on the table with the hand upon it. 
 At each opening or closing of the circuit, a bright flash will 
 be seen, whether the eyes are closed or open. With the eyes 
 closed and covered, the effects of the continuous current 
 
110 LABORATORY COURSE IN PSYCHOLOGY. 
 
 may be observed. In this case it is well to apply the elec- 
 trode slowly and carefully so as to avoid as much as possi- 
 ble the flash caused by the sudden closing of the circuit. 
 When the positive electrode is on the forehead, the nega- 
 tive on the back of the neck, a transient pale violet light 
 will be seen distributed generally over the field and forming 
 a small bright spot at its centre. Sometimes traces of the 
 blind spot also appear. The violet light soon fades, and on 
 opening the circuit there is a notable darkening of the 
 field, with a momentary view of the blind spots as bright 
 disks. When the negative electrode is on the forehead, the 
 positive on the back of the neck, the phenomena are in 
 general reversed, the darkening occurring on closing the cir- 
 cuit, the violet light on opening it. Helmholtz sums up 
 these and other experiments in the following law : " Con- 
 stant electrical circulation through the retina from the 
 cones toward the ganglion cells gives the sensation of 
 darkness ; circulation in the contrary direction gives the 
 sensation of brightness." (Phys. Opt., 2d ed., p. 247.) That 
 the blind spot should appear as a disk of different color 
 from the rest of the field seems to be due to the fact that 
 the sensitive parts of the retina immediately surrounding 
 it are somewhat shielded from the electric current, and as 
 usual their condition is attributed to the blind spot also. 
 The experiment is not altogether a pleasant one, on account 
 of the feeling which the current produces in the head, the 
 " electrical taste " in the mouth, and. the reddening of the 
 skin under the electrodes. 
 
 Helmholtz, A, 243-248, Fr. 275-281 (203-207), 744 (583). 
 
 RETINAL FATIGUE AND ADAPTATION. 
 
 122. Eetinal Fatigue. Stare with perfectly fixed and 
 motionless eyes at a selected spot on a variegated carpet or 
 wall paper, and notice the levelling effect of fatigue. The 
 
THE MECHANISM OF THE EYE. Ill 
 
 differences in color and pattern gradually disappear, and 
 the whole field becomes a nearly uniform cloud. The parts 
 of the recina that are strongly stimulated are brought 
 down to the general level ; those that are little stimulated 
 are built up to it. Every wink or slight movement of the 
 eyes causes a general brightening up of the field and 
 restoration of vision. The experiment is particularly easy 
 to make when looking at a uniform surface with faint 
 shadows lying on it. 
 
 Helmholtz, A, 508, 555 ff., Fr. 478 (362), 527 (402) ff.; Fick, B, 
 222; Treitel; Hering, C. See also the discussion on this topic by 
 A. E. Fick and Hering. 
 
 123. Adaptation of the Eye. a. The adjustment of the 
 eye to the intensity of its illumination is effected partly by 
 change in the size of the pupil, and partly by changes in 
 the retina itself. The first is of common observation, and 
 the connection of the two eyes in this respect has been 
 noticed in Ex. 110, b. The effects of going from a dark 
 room into a light room and vice versa, and the gradual im- 
 provement of vision on remaining in one or the other, are 
 also familiar. 
 
 b. It has not, however, been so generally observed that 
 adaptation to very weak lights is much more favorable to 
 the perception of colorless light than to colored. This may 
 easily be observed in a dark room with single flashes of a 
 rather faint Geissler tube. Before the room is darkened, 
 and for a short time after, the colors of the light are readily 
 perceived. After some time, however, they nearly or quite 
 fail, seeming to be lost in the increased brilliancy of the 
 white light. It is important that there should be an inter- 
 val between the flashes sufficient to allow all the effects of 
 one to disappear before another is given. If the room is 
 not completely dark, the head of the observer and the tube 
 
112 LABORATORY COURSE IN PSYCHOLOGY. 
 
 must be covered closely with an opaque cloth to allow full 
 adaptation. 
 
 Aubert, A, 483 f., B, 25 ff. ; Charpentier, A, 154 ff. ; Treitel; Bering, 
 C. On 6, Hillebrand. 
 
 AFTER-IMAGES. 
 
 After-images, Accidental or Consecutive Images. After- 
 images in which the relations of light and shade of the 
 original object are preserved are called Positive After-images. 
 Those in which these relations are reversed (as in a photo- 
 graphic negative) are called Negative After-images. Posi- 
 tive after-images are of various colors, but most important 
 to notice here are those of the color of the object (like- 
 colored), and of the complementary color (opposite-colored). 
 Negative after-images, so far as observed, are always oppo- 
 site-colored. All after-images, especially the positive, can 
 best be observed in the morning when the eyes are well 
 rested. 
 
 124. Negative After-images, a. Look steadily for a minute 
 at a fixed point of the window, then at a white screen or an 
 evenly lighted, unfigured wall ; the dark parts of the win- 
 dow will now appear light and the light dark. 
 
 b. Get a lasting after-image and look at a corner of the 
 room, or at a chair or other object of uneven surface ; 
 notice how the image seems to fit itself to the surface upon 
 which it rests. After a little practice it is also possible at 
 will to see the image floating in the air instead of lying on 
 the background. 
 
 c. Look steadily at a bright-colored object or some bits 
 of colored paper, then at the screen ; observe that the colors 
 of the after-images are approximately complementary to the 
 colors of the objects producing them. 
 
 d. Negative After-images upon a Background faintly Tinged 
 with the Stimulating Color. Fasten upon the color-mixer a 
 
THE MECHANISM OF THE EYE. 118 
 
 white disk upon which has been painted a six rayed star of 
 red. Set the disk in rapid rotation, bring the eyes within 
 eight or ten inches of the disk, and after half a minute sud- 
 denly withdraw them to thirty or forty inches. As the 
 head is drawn back the complementary color will be seen 
 to press in upon the disk from all sides while the red con- 
 tracts. When the head is again approached to the disk the 
 red will enlarge and the blue-green disappear. The cause 
 of the rushing in of the blue in the first case is the contrac- 
 tion of the retinal image, which of course decreases in size 
 as the head is drawn back, and is thus brought upon parts 
 of the retina that have been more strongly stimulated. 
 When the head approaches the disk the retinal image 
 enlarges and its outer portion lies on a fresh area. 1 
 
 Negative after-images are sometimes very lasting, and for 
 that reason are those most frequently noticed in ordinary 
 experience. They are phenomena of retinal fatigue (Helm- 
 holtz), or of retinal restitution (Hering). 
 
 125. Positive After-images. These images are not diffi- 
 cult to see, if after a brief stimulation the eye is shielded 
 from further action of light. Thus, when the gas is sud- 
 denly turned off in a dark room, the positive image of the 
 flame and the burner is very easily seen. 
 
 a. Look for an instant (one-third of a second) at the win- 
 dow, then close and cover the eyes. Notice that the after- 
 image is like the window in distribution of light and shade, 
 bright panes and dark bars, and at first like it also in color. 
 After some practice it is also possible to see, for a small 
 fraction of a second, the positive after-image of almost any 
 bright object on suddenly turning the eyes from the object 
 to some other part of the field, especially if the latter is 
 dark. The positive after-image is of short duration and 
 less readily observed than the negative. It has generally 
 
 1 For a still simpler experiment, see Mind, Ser. 2, IT., 1893, 485, note. 
 
114 LABORATORY COURSE IN PSYCHOLOGY. 
 
 been considered a phenomenon of retinal inertia, a prolon- 
 gation of the original retinal excitation, and such a prolonga- 
 tion does undoubtedly exist. Charpentier and Hess, however, 
 in experiments with very brief stimulation, have found 
 a transient negative image coming between the original 
 impression and the ordinary positive after-image observed 
 with longer stimulation. The full series would then be : 
 1. Prolongation of the original stimulus ; 2. First Negative 
 Image ; 3. Ordinary Positive After-image ; 4 Ordinary 
 Negative After-image. 
 
 b. Colored Positive After-images. Look for an instant at 
 a gas flame through a piece of red glass, then close and 
 cover the eyes and observe the red image ; repeat the exper- 
 iment, continuing the fixation of the " flame for half a min- 
 ute ; the resulting after-image will be bright as before but 
 of the opposite color. 
 
 c. After-images on Dark and Light Backgrounds. Get 
 an after-image of the window of not too great intensity, and 
 project it alternately on a sheet of white paper and the dark 
 field of the closed and covered eyes ; it will be found nega- 
 tive on the white background and positive on the dark. 
 Some observers find a periodic reappearance of positive 
 after-images, or an alternation of positive and negative 
 images, without a change of background. 
 
 d. Sequence of Colors. Get a good after-image of the 
 window, and observe with closed and covered eyes the play 
 of colors as the image fades. Try several times and observe 
 that the order of succession is the same. According to 
 Hering, this play of colors would not take place if the origi- 
 nal stimulus were absolutely colorless. 
 
 On Exs. 124 and 125, consult the following: Helmholtz, A, 480 ff., 
 501 ff., Fr. 446 (338), 471-500 (357-380); Wundt, A, 3te Aufl., I., 
 472-476, 4te Aufl., I., 512 ff.; Hess; Charpentier, B. See also ref- 
 erences given in Chap. VI. for Successive Contrast. 
 
THE MECHANISM OF THE EYE. 115 
 
 126. Effect of Eye-motions on After-images. Get a mod- 
 erately strong after-image of the window ; look at the wall 
 and keep the eyes actively in motion. The image will be 
 seen with difficulty while the eye is in motion ; when, how- 
 ever, the eye is brought to rest, it will soon appear. In 
 general, any visual stimulus that moves with the eye is less 
 effective than one that does not. 
 
 Exner, A. 
 
 127. The Seat of the After-image. An after-image due to 
 stimulation of one eye may, under proper conditions, some- 
 times seem to be seen with the other. From this it has 
 been inferred that the seat of after-images is central, not 
 peripheral ; that is, in the visual centres of the brain, 
 higher or lower, not in the retina. The following experi- 
 ments show, however, that the after-image is really seen 
 with the eye first stimulated, and so render the hypothesis 
 of a central location unnecessary. 
 
 a. Look steadily for a considerable time at a bit of red 
 paper on a white ground, using only one eye, say the right, 
 and keeping the other closed; when a strong after-image 
 has been secured, remove the paper, close the right eye, 
 open the left, and again look steadily at a fixed point on the 
 white ground ; after a little the field will darken and the 
 after-image will reappear. If the red does not produce a 
 sufficiently lasting image, substitute for it a gas flame or 
 some other bright object. 
 
 b. That we have really to do with the eye originally 
 stimulated (its present dark field suppressing the light one of 
 the other eye), appears from such experiments as the fol- 
 lowing : Get the after-image as before ; then open both 
 eyes and bring a bit of cardboard before the eyes alter- 
 nately. Bringing it before the left eye rather brightens the 
 image ; bringing it before the right dims or abolishes it. 
 
116 LABORATORY COURSE IN PSYCHOLOGY. 
 
 The image is thus chiefly affected by what affects the right 
 eye. 
 
 c. Get the after-image again, and close and cover both 
 eyes ; observe the color of the after-image, as projected on 
 the dark field ; then open the left eye, letting the right eye 
 remain closed and covered. The after-image will be seen, 
 not in the color it has when the right eye is open and the 
 image is projected in the light field, but in that which it 
 has in the dark field of the closed eye. 
 
 These experiments prove that after-images belong to the 
 stimulated half of the visual apparatus, but they do not 
 show whether the images belong to the retina of that half 
 or to the nervous centres connected with it. Other consid- 
 erations, such, for example, as the fact that the image fol- 
 lows every motion of the eye, even those that are usually 
 unconscious, is affected by pressures exerted on the eyeball 
 and by electric currents sent through it, together with Ex- 
 ner's direct experiments on retinal and optic nerve stim- 
 ulation, support the retinal location, in favor of which 
 current opinion is practically unanimous. Some observers, 
 however, have been able to get a binocular after-image of 
 a somewhat different character ; see binocular section of 
 Chap. VI. 
 
 Delabarre ; Exner, D, 246 ff. and E ; Fick and Giirber, 296 ff. 
 
 128. After-images of Motion. These after-images can be 
 secured from almost any continuously moving object. They 
 are often unpleasantly striking after looking at the water 
 from the deck of a vessel or at the landscape from a car 
 window. In the experiments below, variations of one of 
 the laboratory methods of producing them are given. 
 
 a. Fasten upon the rotation apparatus a disk bearing a 
 large number of equal black and white sectors ; set it in 
 slow rotation and gaze fixedly at it. The rate must not 
 
TlfE MECHANISM OF THE EYE. 117 
 
 be fast enough to blur the outlines of the sectors very 
 much. After a moment or two of steady fixation, bring it 
 suddenly to rest and observe its slow illusory backward 
 movement. 
 
 b. Fasten on the apparatus a disk like that in the accom- 
 panying cut, and get an after-image as before, fixating the 
 centre. Bring the disk 
 
 suddenly to rest, or 
 
 look away from it to a 
 
 page of print or into 
 
 the f a,ce of a bystander 
 
 and notice the apparent 
 
 shrinking or swelling, 
 
 reversing the previous 
 
 motion of the spiral. 
 
 Illusions of increase 
 
 or decrease of distance 
 
 sometimes accompany 
 
 those of motion with 
 
 this disk. Eepeat the experiment, but this time instead of 
 
 looking at some object, close the eyes and turn them toward 
 
 the sky or other source of bright light. The apparent motion 
 
 will be observed again in the red-yellow field. 
 
 c. Hold over half of the disk while in rotation a piece of 
 cardboard, fixate the centre of the disk, and get the after- 
 image. Observe that the after-image is limited to the 
 portion of the retina stimulated. 
 
 d. Get a monocular after-image of the spiral, with the 
 right eye, for example. Then close the right eye and open 
 the left ; the after-image of motion will be projected like 
 that of color in Ex. 127. 
 
 e. Hold just above the spiral disk a larger disk of paste- 
 board, cut with a radial slot an inch or two wide. When 
 the spiral is now revolved a narrow strip will be seen in 
 
118 LABORATORY COURSE IN PSYCHOLOGY. 
 
 which the motion is in one direction only. Get a strong 
 after-image and observe it with closed eyes as in b above. 
 It will sometimes be possible, at least for a short time, to 
 get a reversal of the previous illusion ; the part of the image 
 corresponding to the slot will appear to stand still while the 
 adjacent parts move, or both will appear in motion in op- 
 posite directions. This experiment is apparently easier 
 to get with the antirrheoscope, where the moving field is 
 larger. With that instrument the effect mentioned can be 
 seen in the ordinary projected after-image. 
 
 When a strong after-image is projected upon a set of 
 straight lines at right angles to the direction of movement, 
 some observers have seen the lines more or less distorted by 
 it (Budde saw them thus affected when the lines did not 
 cross, but only entered the moving part of the field) ; others 
 have found the lines entirely unaffected. It seems prob- 
 able that the breadth and distinctness of the lines have 
 something to do with this difference of results. 
 
 Exner, who believes in the retinal seat of color after- 
 images, is inclined to give a more central location to these 
 of motion. In his opinion such experiments as those above 
 indicate also that our knowledge of such motions is a sensa- 
 tion, not a perception. 
 
 After-images of motion have been explained by actual, 
 though unconscious, movements of the eyes, like the ap- 
 parent movements of objects in dizziness. This is certainly 
 incorrect ; for in b it would seem necessary that the eyes 
 should move in all directions at once, and c shows that the 
 effect is limited to a portion of the field, which would be 
 impossible if it were due to actual eye motions. The same 
 was demonstrated by Dvorak by means of a disk with three 
 concentric spirals, the inner and outer ones being drawn 
 in the same way, (right-handed spirals, for example), while 
 that between was drawn in the reverse direction. How far 
 
THE MECHANISM OF THE EYE. 119 
 
 some psychical representation of ocular motions co-operates 
 in the illusion would be hard to say. 
 
 Helmhojtz, A, Fr. 766-769 (603-605); Bowditch and Hall; Mach, 
 A, 59-61 (see also 61-65 for yet another kind of after-image), and I?, 
 65-67; Exner, B and O, 440 ff.; Dvorak; Budde; von Fleischl; 
 Heuse; Zehfuss. 
 
 MOVEMENTS OF THE EYES. 
 
 The eye is a moving as well as a seeing member ; and its 
 motor functions are of great importance for psychology, es- 
 pecially for the theory of the visual perception of space. 
 The experiences of the eye in motion have a controlling 
 influence upon its perceptions even when at rest, as will 
 appear in some of the experiments of Chap. VII. 
 
 All motions of the eye may be conceived as rotations of 
 greater or less extent about one or more of three axes : a 
 sagittal axis, corresponding nearly with the line of sight ; 
 a frontal axis, extending horizontally from right to left ; and 
 a vertical axis. Theoretically all these intersect at right 
 angles in the Centre of Rotation of the eye. As a land- 
 mark from which to measure eye-movements, that position 
 (approximately) is taken which the eyes assume when the 
 head and body are erect and the eyes are directed forward 
 to a distant horizon. This is known as the Primary Posi- 
 tion of the eyes (or the lines of sight) ; any other is a 
 Secondary Position. The point on which the eyes are fixed 
 when in the primary position is the Primary Fixation 
 Point, or Principal Point of Regard. The Field of Vision 
 is the extent of space that can be seen with the eye at 
 rest. The Field of Regard is the extent of space that can 
 be seen when the eyes are moved. In the following experi- 
 ments the word Rotation, except in the expression " centre 
 of rotation," is reserved for turnings about the sagittal 
 axis. 
 
120 LABORATORY COURSE IN PSYCHOLOGY. 
 
 129. Reflex Movements of the Eye. Of the first impor- 
 tance among eye movements is the constant reflex tendency 
 of the eye to move in such a way as to bring any bright 
 image lying on a peripheral part of the retina, or any to 
 which attention is directed, into the area of clearest vision. 
 Many evidences of this tendency will be found in the 
 ordinary course of vision. By way of experiment, try to 
 study attentively a musca volitans or a negative after-image 
 that is just to one side of the direct line of sight. The 
 apparent motion of the object measures the energy of the 
 reflex. 
 
 130. Associated Movements of the Eyes. The two eyes 
 form a single visual instrument; and even when one eye 
 is closed, it follows to a considerable degree the movements 
 of its open companion. Movements upward or downward 
 in normal vision are always performed simultaneously by 
 the two eyes. 
 
 a. Close one eye, and, resting the finger-tip lightly on the 
 lid, feel the motions of that eye as the other looks from 
 point to point of the field of regard. 
 
 b. Get a monocular after-image, as in Ex. 127, and when 
 it seems visible to the open eye, notice that it accom- 
 panies the fixation point of that eye as it moves from point 
 to point of the field of regard. 
 
 Aubert, A, 651 ff. ; Bering, A, 519 ff. 
 
 131. Motions of the Eyes when the Lines of Sight are 
 Parallel. The movements here considered are somewhat 
 simplified for easier exposition. 
 
 a. Donders's Law ; the Law of ^Constant Orientation 
 (Helmholtz) ; the Law of Like Position with Like Direction 
 (Hering). It is evident that when the eye is fixed upon some 
 point of its field, e.g., ten degrees upward and fifteen degrees 
 to the right of the primary position, it is not thereby fixed 
 
THE MECHANISM OF THE EYE. 121 
 
 as regards its sagittal axis, but might conceivably assume 
 an indefinite number of positions by different degrees of rota- 
 tion about, that axis. It might also, if not entirely free in 
 its rotation, rotate now through one angle and now through 
 another, depending on the direction in which the line of 
 sight had moved to reach the position in which it is then 
 found. As a matter of fact, however, it does not assume 
 an indefinite number of positions, but one and only one, no 
 matter by what movements the line of sight has come to 
 that point. This is Donders's Law; and the fact that it 
 expresses is of importance for sure and easy recognition of 
 directions in the field of regard, and for deciding whether 
 or not objects in the field have moved when the eye itself 
 has been moved. The correctness of this law is easy to 
 demonstrate. 
 
 Cut in a sheet of black cardboard two slits an eighth of 
 an inch wide and four or five inches long, crossing at right 
 angles. Set the cardboard in the window or before some 
 other brightly lighted surface. Arrange a head-rest at a 
 considerable distance, and when the head is in position, get 
 a strong after-image of the cross, fixating its middle point. 
 Then, without moving the head, turn the eyes to different 
 parts of the walls and ceiling. The image will suffer 
 various distortions from the different surfaces upon which it 
 is projected, but each time the eye returns to the same point 
 the image will lie as before. If the wall does not offer fig- 
 ures by which this can be determined, have an assistant 
 mark the position of the image upon it. The after-image is 
 of course fixed on the retina and can move only as the eye 
 moves. 
 
 b. Listing's Law. This law goes beyond Donders's Law, 
 and asserts that the position is not only fixed, but that in 
 movements from the primary position there is no rotation 
 at all about the sagittal axis. In other words, the final posi- 
 
122 LABORATORY COURSE IN PSYCHOLOGY. 
 
 tion is such, as the eye would assume if it were moved from 
 its primary position to the position in question by turning 
 about a fixed axis standing perpendicular at the centre of 
 rotation to both the primary and the new position of the 
 line of sight. To show this requires a little more care than 
 the last experiment. 
 
 The observer must be placed at a distance of twenty-five 
 or thirty feet from an extensive wall space, with a suitable 
 head-rest as before. The lines of sight are, of course, not 
 strictly parallel at this distance, but the difference may be 
 neglected. On the wall stretch dark-colored strings as indi- 
 cated in the accompanying diagram. The cross at the lower 
 right hand corner should be approximately in the primary 
 position for the observer. The longer vertical and horizontal 
 strings should be twelve or fifteen feet long, the inclined 
 one eighteen or twenty feet. The angle that the last makes 
 with the others is not important so long as it is not too 
 
 small with either. Fix- 
 ation points of black 
 cardboard or some other 
 conspicuous substance 
 should be affixed as indi- 
 cated by the little circles. 
 The cross in the corner 
 may be made by pasting 
 strips of bright-colored 
 paper half an inch wide 
 and a foot long on a 
 disk of white c a r d- 
 board, or (better still) it 
 may be made by the line 
 of junction of four colored sectors, two red arid two blue, for 
 example. The disk in either case must be so arranged that 
 it can be turned about its centre and one of its diameters 
 
TEE MECHANISM OF THE EYE. 123 
 
 be made to coincide with the oblique string. When all has 
 been arranged make the following tests : 
 
 Exact determination of the primary position. For most 
 observers this is somewhat depressed below the horizontal 
 position. Let the observer fixate the centre of the disk 
 till he has secured a strong and clear-cut after-image of it 
 and then turn his eyes, taking care not to move his head, 
 to the fixation marks on the horizontal and vertical strings. 
 If the corresponding lines of the after-image coincide with 
 the strings, the head is in the required position. If not, the 
 head must be moved a little to right or left if the error 
 is with the vertical bar, and up or down if with the hori- 
 zontal. The primary position differs a little from observer 
 to observer, and even with the same observer at different 
 times. 
 
 Having found the primary position, have an assistant 
 turn the cross disk so that one of its diameters coincides 
 with the oblique string. Get a clear after-image of it, and 
 look at the fixation point on that string. Again the bar of 
 the cross will lie exactty upon the string, thus showing that 
 no rotation of the eye about the line of sight has taken 
 place. The same would be true for any other direction of 
 motion from the primary position, provided the movement 
 were not of extreme extent. There is then a set of lines, 
 radiating from the primary fixation point, along which the 
 eye can move, so as to bring all parts of the same line suc- 
 cessively on the same part of the retina. Direct examina- 
 tion of such a line and comparison of its parts is easy. 
 
 Restore the cross disk to its first position, incline the 
 head forward or backward, or turn it to right or left before 
 getting the after-image (thus bringing the eye into a sec- 
 ondary position), and repeat the experiments just made. 
 Notice that the bars do not now coincide with the strings, 
 showing that the eyes have suffered a certain amount of 
 
124 LABORATORY COURSE IN PSYCHOLOGY. 
 
 rotation. Such a rotation appears for all secondary posi- 
 tions (except when the fixation point both at starting and 
 ending lies in a straight line passing through the primary 
 fixation point) , but the extent of it is small in the ordinary 
 movements of the eyes, and extreme movements are usually 
 avoided by simultaneous movements of the head. 
 
 With the cross on the disk vertical as in the cut, get an 
 after-image and fixate the mark on the oblique string. In- 
 stead of being rectangular as before, the after-image cross 
 now appears somewhat distorted, like an oblique X. The 
 after-image on the retina of course remains rectangular. 
 The distortion of the image on the wall is the result of the 
 interpretation now placed upon it by the mind. The short 
 string cross at the same centre is known to be rectangular, 
 and if the after-image cross fails to agree with it, the only 
 harmonization of the two is that the latter is not really 
 rectangular. Oblique crosses in such a position in previous 
 experience have given rise to rectangular retinal images 
 so often that this interpretation is immediate, and seems 
 wholly a matter of sensation. 
 
 For a fuller account of Listing's Law see Appendix I. 
 
 Cf. Helmholtz, A, Fr. 601-609 (462-469), 621 (479) ff., 702 (548) ff.; 
 Aubert, A, 653 ff. ; Wundt, A, 3te. Aufl., II., 94 ff. ; Hering, J5, 248 ff. ; 
 Le Conte, 164-177. 
 
 132. Actual Movements of the Eyes. Wundt-Lamansky 
 Law. Rapid motions of the eyes when they move freely 
 and do not follow strongly marked lines in the field of re- 
 gard, are not executed exactly according to Listing's Law, 
 though that gives correctly the end positions reached. The 
 axis about which the eye turns is not always constant, and 
 the paths of the fixation point as it moves in the field of re- 
 gard are therefore not all straight. This is easy to observe 
 as follows. In a dark room turn down the gas till it burns 
 in a very small flame. Then using this as a distant point 
 
THE MECHANISM ^FjfjHB^YE. J 125 
 
 of departure in the primary position, look suddenly from it 
 to other points of fixation in various directions about it, and 
 notice the shape of the long positive after-images that result 
 from the' motion of the image of the flame over the retina. 
 These will probably have the 'shape of the radii in the left 
 hand figure below, the vertical and horizontal being nearly 
 straight, and the oblique curved. These, however, do not 
 show immediately the track of the fixation point. The 
 newest part of the after-image is that next the light, the 
 oldest, part is that next the fixation point at a in the 
 diagram. If the points of the after-image curve are now 
 interpreted in the order of time (taking the oblique curve to 
 
 the right and upward, for example), it appears that the eye 
 at first moved rather rapidly toward the right, but rather 
 slowly upward, while at last it moved rather slowly toward 
 the right and rapidly upward. Plotting a curve in accord- 
 ance with this interpretation, we get that given in B, which 
 shows the true track of the fixation point. By similar plot- 
 ting the other tracks may be found. 
 
 It is said that for some eyes the after-images, though 
 curved, do not coincide with those figured in A. 
 
 Wundt, B, 139 ff., 201-202; Bering, A, 450-451; Lamansky. 
 
 133. Convergent Movements of the Eyes. The laws of 
 Ex. 131 do not hold for convergent motions of the eyes. 
 
126 LABORATORY COURSE IN PSYCHOLOGY. 
 
 When the lines of sight converge in the primary position, 
 both eyes rotate outward ; as the lines of sight are elevated, 
 the convergence remaining the same, the outward rotation 
 increases ; as they are depressed, the rotation diminishes 
 and finally becomes zero. On' a sheet of cardboard draw a 
 series of equidistant parallel vertical lines one or two inches 
 apart and eight or ten inches long, drawing the left half of 
 the group in black ink, the right half in red. Cross both 
 sets midway from top to bottom by a horizontal line, red in 
 the red set, and black in the black set. Fasten the card- 
 board flat upon a vertical support, and arrange the head rest 
 in front of it. The horizontal line of the diagram should 
 be on a level with the eyes. 
 
 a. If the operator is unable to control the degree of con- 
 vergence voluntarily, he should fasten a bit of wire vertically 
 between his eyes and the diagram in such a way that it can 
 be moved to and from the eyes. If he is able to control the 
 convergence voluntarily, the wire is unnecessary. Bring 
 the head into position and converge the eyes, giving atten- 
 tion to the diagram. It will be seen that the red and black 
 lines are not quite parallel (or do not quite coincide), and 
 that they are less nearly so as the convergence is increased. 
 The red lines (seen by the left eye) seem to incline a little 
 toward the right, and the black lines (seeji by the right eye) 
 toward the left. When the convergence is great, the hori- 
 zontal lines also will show the rotation. This apparent 
 rotation of the lines is not, as in the case of the after-image, 
 a sign that the corresponding eye has rotated in the same 
 way, but that it has rotated in the opposite way. 
 
 b. Repeat this with the head much inclined forward 
 (the equivalent of elevating the eyes) and with it thrown 
 far back (equivalent of depressing the eyes), taking care 
 that the same degree of convergence is maintained. In the 
 first case the apparent rotation of the lines is increased, and 
 
THE MECHANISM OF THE EYE. 127 
 
 in the second decreased to zero, or even transformed into 
 rotation in the opposite direction. 
 
 Helmholtz, A, Fr. 609-610 (469-470); Le Conte, 177-191; Hering, 
 A, 496 ff. ; Aubert, A, 658 ff. 
 
 134. Involuntary Movements of the Eyes. Lay a small 
 scrap of red paper on a large piece of blue. Fixate some 
 point on the edge of the red. After a few seconds of steady 
 fixation, the color near the line of separation will be seen 
 to brighten, now in the red and now in the blue, thus be- 
 traying the small unintentional movements of the eyes. 
 
 Helmholtz, A, 539, Fr. 511 (389). 
 
 BIBLIOGRAPHY. 
 
 AUBERT: A. Grundziige der physiologischen Optik, Leipzig, 1876. 
 This work, though obtainable separately, forms a part of the 
 second volume of v. Graefe and Saemisch's Handbuch der 
 gesammten Augenheilkunde. It contains in its three hundred 
 pages a very large amount of matter stated with great brevity 
 and clearness, and is in every way excellent. 
 B. Physiologic der Netzhaut, Breslau, 1865. 
 
 VON BEZOLD: Ueber Zerstreuungsbilder auf der Netzhaut, v. 
 Graefe's Archiv, XIV., 1868, ii., 1-29. 
 
 BOWDITCH AND HALL: Optical Illusions of Motion, Journal of 
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 BUDDE: Ueber metakinetische Scheinbewegungen und iiber die 
 Wahrnehmung der Bewegung, Du Bois-Eeymond' 1 s Archiv, 
 1884, 127-152. 
 
 CHARPENTIEK : A. La lumiere et les couleurs, Paris, 1888. 
 
 B. Keaction oscillatoire de la retine sous F influence des excitations 
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 1891, 147-150, 217-219. See also Nature, XL VIII., 1893, 380. 
 
 DELABAERE : On the Seat of Optical After-images, American Jour- 
 nal of Psychology, II., 1888-89, 326-328. 
 
128 LABORATORY COURSE IN PSYCHOLOGY. 
 
 DVORAK: Versuche iiber die Nachbilder von Keizveranderungen, 
 Sitz.-ber. d. k. Akademie d. Wiss. i. Wien, math.-nat. Classe, 
 LXL, 1870, Abth., ii. 257-262. 
 
 EXNER : A. Das Yerschwinden der Nachbilder bei Augenbewegung- 
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 B. Einige Beobachtungen iiber Bewegungsnachbilder, Central- 
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 C. Ueber optische Bewegungsempfindungen, Biologisches Cen- 
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 D. Ueber die Functionsweise der Netzhautperipherie und den 
 Sitz der Nachbilder, v. Graefe's Archiv, XXXII., 1886, i., 233- 
 252. 
 
 E. Ueber den Sitz der Nachbilder im Centralnervensystem, Rep. 
 der Physik, XX., Protokoll d. chem. phys. Ges. in Wien, 18 
 Marz, 1884. 
 
 F. Ueber einige neue subjective Gesichtserscheinungen, Pfluger's 
 Archiv, I., 1868, 375-394. 
 
 FICK, A.: A. Dioptrik und Nebenapparate des Auges, Hermann's 
 
 Handbuch der Physiologic, III., i. 3-138. 
 B. Die Lehre von der Lichtempfindung. ibid., 139-234. 
 
 FICK, A. E., AND GURBER: Ueber Erholung der Netzhaut, v. Graefe's 
 Archiv, XXXVI., 1890, ii., 245-301. See also Bering's critique, 
 Fick's reply, Hering's rejoinder, and Fick's second reply. 
 ibid., XXXVII. and XXXVIII. 
 
 VON FLEISCHL: Physiologisch-optische Notizen (2te Mittheilung), 
 Sitz.-ber. d. k. Akademie d. Wiss. i. Wien, math.-nat. Classe, 
 LXXXVL, 1882, Abth. iii., 8-25. 
 
 HELMHOLTZ: A. Handbuch der physiologischen Optik, 2te. Aufl., 
 
 Hamburg und Leipzig, 1886-1892. 
 
 Of this second edition of Helmholtz's work but seven parts have 
 so far appeared. The latest complete edition is the French 
 translation by Javal and Klein (Optique physiologique, Paris, 
 1867) . The references following the experiments are given when 
 possible for both the second German edition and the transla- 
 tion. The figures in parentheses following those for the trans- 
 lation are the pages of the first German edition taken from the 
 double paging of the French version. Having been taken thus 
 at second hand, they may sometimes be in error by a page or 
 
THE MECHANISM OF THE EYE. 129 
 
 two, but it seemed better to run that risk than to omit them 
 altogether. It is hardly necessary to add that this work is 
 above all others the masterpiece of physiological and psycho- 
 logical optics. 
 
 B. Die Stoning der Wahrnehmung kleinster Helligkeitsunter- 
 schiede durch das Eigenlicht der Netzhaut, Zeitschrift fur 
 Psychologic, L, 1890, 5-17. 
 
 HERING : A. Der Raumsinn und die Bewegungen des Auges, 
 Hermann's Handbuch der Physiologic, III., Th. i., 343-601. 
 
 B. Beitrage zur Physiologic, Leipzig, 1861-64. 
 
 C. Ueber den Einfluss der Macula lutea auf spectrale Farben- 
 gleichungen, Pfluger's Archiv, LIV., 1893, 277-318. 
 
 HESS: Untersuchungen iiber die nach kurzdauernder Reizung des 
 
 Sehorgans auftretenden Nachbilder, Pfluger's Archiv, XLIX., 
 
 1891, 190-208. 
 HEUSE : Zwei kleinere Mittheilungen aus dem Gebiete der physio- 
 
 logischen Optik, v. Graefe's Archiv, XXXI Y., 1888, ii., 127- 
 
 134. 
 
 HILLEBRAND: Ueber die specifische Helligkeit der Farben (mit 
 Vorbemerkungen von E. Hering). Sitz.-ber. d. k. Akademie d. 
 Wiss.i. Wien,math.-nat. Classe, XCVIII., 1889, Abth. iii., 70- 
 120. 
 
 LAMANSKY: Bestimmung der Winkelgeschwindigkeit der Blick- 
 bewegung, respective Augenbewegung, Pfluger's Archiv, II., 
 1869, 418-422. 
 
 LAQUEUR: Ueber pseudentoptische Gesichtswahrnehmungen, v. 
 
 Graefe's Archiv, XXXYI., 1890, i., 62-82. Contains historical 
 
 references. 
 
 LE CONTE: Sight, New York, 1881. 
 MACH: A. and 5., works cited with same letters in bibliography 
 
 of Chap. II. 
 
 MAXWELL: On Color-vision at Different Points of the Retina, 
 Report of the British Association, 1870; or Maxwell's Scientific 
 Papers, Cambridge, 1890, Yol. II., 230. 
 
 ROOD : On a probable means of rendering visible the Circulation 
 in the Eye, American Journal of Science, 2d Ser., XXX., 
 1860, 264. Additional observations on the Circulation in the 
 Eye, ibid., 385. 
 
130 LABORATORY COURSE IN PSYCHOLOGY. 
 
 SACHS: Ueber die specifische Lichtabsorption des gelben Fleckes 
 der Netzhaut, Pfluger's Archiv, L., 1891, 574-586. 
 
 SCHWAKZ: Ueber die Wirkung des constanten Stroms auf das 
 normale Auge, Archivfilr Psychiatric, XXI., 1890, 588-617. 
 
 TREITEL: Ueber das Yerhalten der normalen Adaptation, v.Graefe's 
 Archiv, XXXIII., 1887, ii., 73-112. 
 
 TSCHERNING: Beitrage zur Dioptrik des Auges, Zeitschrift fur 
 Psychologic, III., 1892, 429^92. 
 
 TUMLTRZ: Ueber ein einfacbes Verfahren, die Farbenzerstreuung 
 des Auges direkt zu sehen, Pfluger's Archiv, XL., 1887, 394. 
 
 UHTHOFF: Ueber die kleinsten wahrnehmbaren Gesichtswinkel in 
 den verscbiedenen Teilen des Spektrums, Zeitschrift fur Psy- 
 chologic, I., 1890, 155-160. Contains bibliographical notices 
 on minimum visibile. 
 
 WOLF : Ueber die Farbenzerstreuung im Auge, Wicdemanri's An- 
 nalen, XXXIII., 1888, 548-554. 
 
 WUNDT : A. Work cited in bibliography of Chapter I. 
 
 B. Beitrage zur Theorie der Sinneswahrnehmung, Leipzig, 1862. 
 
 ZEHFUSS: Ueber Bewegungsnachbilder, Wiedemanri 1 s Annalen, IX., 
 
 1880, 672-676. 
 The works of Helmholtz and Aubert mentioned above contain 
 
 full bibliographies for the earlier literature of all the subjects con- 
 sidered in this and the next two chapters. 
 
SENSATIONS OF LIGHT AND COLOR. 131 
 
 CHAPTER VI. 
 Sensations of Light and Color. 
 
 THE aim of the following experiments is not to settle 
 conflicting color theories, but rather to present the most im- 
 portant experimental facts which all color theories nmst 
 take into account. 1 Authoritative statements of theories 
 may be found as follows : Young-Helmholtz theory ; Helm- 
 holtz, A, 344-350, Fr. 380-387, 424-425, 484 (290-294, 
 320-321, 367) ; B, 249-256. Bering's theory ; Bering, A, 
 70-141 ; M, 76-79. Hering has not yet made a general state- 
 ment of his theory in its later developments, and his present 
 views must be gathered in more or less fragmentary con- 
 dition from his numerous special articles. The theories of 
 Helmholtz and Hering are the most prominent of current 
 theories ; and something on them, especially on the first, 
 will be found in the physiologies generally, and in some 
 works on color in the arts. Of other theories there are a 
 considerable number ; see, for some of them, von Kries ; 
 Wundt, A, and B; Bonders, A and B\ Christine Ladd 
 Franklin, A and B ; Ebbinghaus, A. 
 
 Most color theories attempt to simplify the multiplicity 
 of ordinary color sensations by considering them as com- 
 pounds of a small number of simple or primary sensations. 
 The number of primary colors is different in different 
 theories ; red, green, and violet (or blue) are selected by 
 
 1 For concise statements of these facts, see Wundt, A, 3te Aufl., I., 487, 501, 
 4te Aufl., I., 529; and Christine Ladd Franklin, A. 
 
132 LABORATORY COURSE IN PSYCHOLOGY. 
 
 the supporters of the Young-Helmholtz theory ; red, green, 
 yellow, and blue by Hering, Mach, and others; while 
 Wundt is indisposed to make any particular colors more 
 original for sensation than the rest. The selection has 
 generally been dictated by considerations of physics, or the 
 results of introspective analysis of the sensations ; but 
 efforts have lately been made to settle the question by 
 careful examination of the color-blind, and by calcula- 
 tions based upon careful experiments. On the first, see 
 the literature on color-blindness below ; on the second, see 
 Helmholtz, A, 456 if., D, and Konig und Dieterici, A. 
 White is unquestionably a sensation, and Helmholtz and 
 Hering agree in holding the same with reference to black ; 
 though Fick and some others disagree, regarding it rather 
 as the absence of sensation. 
 
 A given color sensation may be changed in three ways : 
 in color-tone, in saturation, and in intensity, or, to use 
 Maxwell's terms, in hue, tint, and shade. Changes in color- 
 tone are such as are experienced when the eye runs through 
 the successive colors of the spectrum. Changes in satura- 
 tion are such as are produced by the addition or subtrac- 
 tion of white ; when much white light is added, the color 
 is a little saturated. Changes in intensity are changes in 
 the brightness of the color. Changes in saturation and in 
 intensity, if excessive, involve some change of color-tone 
 also. Hering' s theory does not admit changes in the inten- 
 sity of light and color sensations in any ordinary sense of 
 the word. Colors that by others are said to be of low in- 
 tensity are regarded by Hering and his school as mixed 
 with a large proportion of black ; similarly those of high 
 intensity are mixed with much white. In Hering's theory 
 the possible changes are then reduced to two ; changes in 
 color-tone and in saturation" the latter including admixtures 
 of both white and black (Hillebrand; Hering, A, 51 if.). 
 
SENSATIONS OF LIGHT AND COLOR. 133 
 
 In this group of experiments it has seemed best to follow 
 the better known terminology, though Hering's conception 
 of the matter ought not to be disregarded. 
 
 LIGHT AND COLOR IN GENERAL. 
 
 135. Color-Blindness, Holmgren's Method, a. Spread 
 the worsteds on a white cloth in good daylight. Pick out a 
 pale green (i. e., a little saturated green) that leans neither 
 toward the blue nor the yellow ; lay it by itself and require 
 the person under examination to pick out and lay beside it 
 all other skeins that are colored like it, not confining him- 
 self, however, to exact matches, but taking somewhat darker 
 and lighter shades also, so long as the difference is only in 
 brightness and not in color-tone. Do not tell him to pick 
 out "the greens" nor require him to use or understand 
 color words in any way ; simply require the sorting. If 
 he makes errors, putting grays, light browns, salmons, or 
 straws l with the green, he is color-blind ; if he hesitates 
 over the erroneous colors and has considerable difficulty, his 
 color- vision is probably defective, but in a less degree. 
 
 b. If the experimentee makes errors, try him further to 
 discover whether he is " red-blind " or " green-blind " by 
 asking him to select the colors, including darker and lighter 
 shades, that resemble a purple (magenta) skein. If he is 
 red-blind, he will err by selecting blues or violets, or both ; 
 if he is green-blind, he will select green or gray, or both, 
 and if he chooses any blues and violets, they will be the 
 brightest shades. If he makes no errors in this case, after 
 having made them in the previous case, his color-blindness is 
 incomplete. Violet-blindness is rare. See also ExI 141 b. 
 
 Complete certainty in the use of even such a simple 
 
 l It is difficult to give the tints accurately in words. The experimenter should 
 consult the colored charts given in the works of Jeffries mentioned in the bibli- 
 ography, and in Rayleigh, B. 
 
134 LABORATORY COURSE IN PSYCHOLOGY. 
 
 method as this is not to be expected without a full study of 
 it and experience in its application. Helmholtz, Hering, 
 Konig, Kirschmann, and others give exact methods for 
 determining the particular colors that are lacking in the 
 vision of the color-blind. 
 
 On color-blindness and methods of testing for it, see Helmholtz, 
 A, 357-372, 456-462; Fr. 388-399, (294-300, 847-848); Holmgren; 
 Jeffries, A and B; Rayleigh, A and B\ Bering, H, I, N y Hess, B; 
 Abney, A- Abney and Festing; Konig, B and C; Brodhun, A and 
 J3; Konig und Brodhun; Konig und Dieterici, A\ Schuster; Preyer; 
 Donders, (7; Kirschmann, A' Pole. 
 
 136. Vision with Peripheral Portions of the Eetina: 
 Perception of Light. A very faint light often appears 
 brighter when its image lies not in the fovea, but a few 
 degrees away from it. If no increase of brightness is ob- 
 served, it is at least difficult to trace any decrease in bright- 
 ness till the image is many degrees from the fovea. This 
 experiment is most easily made at night with faint stars. 
 In the laboratory it may be made with the dark box. On 
 the rear wall of the box place in a horizontal line three 
 bits of white paper of equal size, at such distances that the 
 line of sight moves through an angle of ten degrees in turn- 
 ing from the middle one to either of the outer ones. Make 
 a pin-hole above and below the middle piece, distant from it 
 about an inch, and cover the holes on the outside with 
 paper till the holes are barely visible after the eye has been 
 some time adapted. These bright points serve to steady 
 the eye. The eye should not, however, 'be directly fixed 
 upon them, but at a point midway between them. Reduce 
 the illumination of the box to a minimum (e. g., to the 
 amount of light that would enter through a pin-hole cov- 
 ered with one or more pieces of porcelain or translucent 
 cards), wrap the head and the end of the box in an opaque 
 cloth, and allow the eyes to become adapted to the darkness, 
 
SENSATIONS OF LIGHT AND COLOR. 135 
 
 looking from time to time for the shimmer of the papers at 
 the back of the box. Full adaptation requires a long time, 
 but fifteen minutes is sufficient in this case. By degrees, if 
 the illumination is of the right intensity, the papers will be 
 seen very faintly. If the eye is turned directly towards 
 one of them, it often disappears in the retinal light while 
 the others brighten. Fixate each of them successively, and 
 compare its brightness with the others ; fixate also other 
 points in the field so as to bring the images upon different 
 quadrants of the retina. Close the eyes from time to time 
 to renew the adaptation, and avoid observations when the 
 retinal light is strongly concentrated in the centre of the 
 field. 
 
 On the results of such experiments as this, and on the explana- 
 tion of the phenomenon observed, experimenters are somewhat at 
 variance, but see Helmholtz, A, 268; Aubert, A, 495, B, 89 ff.; A. 
 E. Fick, B\ Kirschmann, B; Treitel, and the literature cited by 
 them. 
 
 137. Vision with Peripheral Portions of the Retina: 
 Perception of Color. The distribution of the sensibility of 
 the retina for color is unlike that for light. At the very 
 centre the pigment of the yellow spot itself interferes some- 
 what with the correct perception of mixed colors (see Ex. 
 115). In a zone immediately surrounding this all colors 
 can be recognized. Outside of this again is a second zone 
 in which blue and yellow alone can be distinguished, and 
 at the outermost parts not even these, all colors appearing 
 black, white, or gray. The zones are not sharply bounded, 
 but blend into one another, their limits depending on the 
 intensity and area of the colors used. The fixing of the 
 boundaries of the zones of sensibility is known as perimetry 
 or campimetry. 
 
 a. With the apparatus at hand, find at what angles from 
 the centre of vision on the vertical and horizontal meridians 
 
136 LABORATORY COURSE IN PSYCHOLOGY. 
 
 of the eye the four principal colors, red, yellow, green, and 
 blue, can be recognized; try white also. Keep the eye 
 steadily fixed on the fixation mark of the instrument, and 
 have an assistant slide the color (say a bit of colored paper 
 5 mm. square pasted near the end of a strip of black card- 
 board an inch wide) slowly into the field from the outside. 
 It will be well to move the paper slowly to and fro at right 
 angles to the meridian on which the test is made, so as to 
 avoid retinal fatigue. Take a record of the point at which 
 the color can first be recognized with certainty. Repeat 
 several times and average the results. The size of the 
 colored spot shown should be constant for the different 
 colors, and the background (preferably black) against which 
 the colors are seen should remain the same in all the 
 experiments. 
 
 b. Repeat the tests with colored squares 20 mm. on the 
 side, and notice the earlier recognition of their color as they 
 approach from the periphery. 
 
 c. Try bringing slowly into the field (best from the nasal 
 side) bits of paper of various colors, especially violet, pur- 
 
 . pie, orange, greenish yellow, and greenish blue ; or better, 
 hold the bit of paper somewhat on the nasal side of the 
 field and turn the eye slowly toward it, beginning at a con- 
 siderable angle from it. If the paper is held before a back- 
 ground containing a line along which the eye can approach 
 the paper, the eye will be assisted in making the approach 
 gradual ; the apparatus used in Ex. 113 b can easily be 
 adapted for this purpose. Observe that on the outer parts 
 of the retina these colors first get their yellow or blue com- 
 ponents, and only later the red or green. If the range of 
 choice is sufficiently large, it may be possible to find a red 
 (inclined toward red-purple) and a green (inclined toward 
 the blue), which, like pure blue and yellow, change only in 
 saturation and not at all in color-tone as they move inward 
 
SENSATIONS OF LIGHT AND COLOR. 137 
 
 toward the centre of the field. These four colors are the 
 Urfarben or primary colors of Hering. 
 
 Helmholtz, A, 372-374, Fr. 399-400; Hess, A; Hering, <2, ; 
 
 A. Fick, A,^B, 206 ff. ; A. E. Fick, J5, 479 ff. ; Aubert, A, 539-546, B, 
 116 ff. ; Kirschmann, C. 
 
 138. Changes in Color-Tone. In the spectrum, change 
 of wave-length, if not too small, is accompanied by change 
 of color-tone. The change is most rapid in the yellow- 
 green and blue-green regions of the spectrum, less rapid 
 toward the ends, and at the extreme ends the only changes 
 are those in brightness. With the spectroscope and day- 
 light find the characteristic Fraunhofer lines D, E, F, G, and 
 ff. The D line lies in the golden yellow, F in the greenish 
 blue, and H at the end of the violet. Between D and F the 
 wave-length changes from 589.2 to 486.1 pp. (from 5.092 X 
 10 14 to 6.172 X 1C 14 vibrations per second), and the color runs 
 through yellow and green to blue, while from F to ff with 
 the nearly proportional change in wave-length from 486.1 
 to 393.3 fjLft, (from 6.172 X 10 14 to 7.628 X 10 14 vibrations 
 per second) the change is only from greenish blue to violet. 
 Notice the region from near the line G to the end of the 
 spectrum which shows little change in color-tone and a simi- 
 lar region of uniform color-tone at the red end. Notice 
 also the tendency of the succession of spectral colors to return 
 upon itself, shown in the resemblance of the violet and red. 
 
 Helmholtz,^, 289,320, Fr. 319 (237); Wundt, 3te Aufl., L, 449 f., 
 4te Aufl., I., 485 f. ; A. Fick, B ; Aubert, A, 530 f. On just observ- 
 able changes in color-tone, see B. O. Peirce, Jr., Konigund Dieterici, 
 
 B, Brodhun, -4, and the literature there cited. 
 
 139. Changes in Saturation. These are easily shown on 
 the color-mixer. Make a succession of mixtures of red and 
 white, beginning with a proportion of white that just 
 changes the red, and increase the proportion till no effect 
 of red remains. At first use a small disk of red laid on 
 
138 LABORATOET COURSE IN PSYCHOLOGY. 
 
 over the larger disks as a sample with which to compare the 
 mixtures. Toward the end of the experiment exchange the 
 red for a small white disk. Notice the changes of color- 
 tone that are to be observed, especially when the amount of 
 color is small. Try similarly with the other chief colors. 
 According to Rood, who worked with the color-mixer, yellow- 
 green and violet are unchanged; Helmholtz's results with 
 spectral colors are somewhat different. 
 
 Changes in saturation can also be made by adding gray 
 of any shade instead of white. The whole range of mix- 
 tures can be shown on a single disk, like that in Ex. 141, by 
 painting the star upon a white or gray ground, or by past- 
 ing a star of colored paper on such a ground. With white, 
 however, the rays of the star must be given a leaf shape, or 
 the color will fall off too rapidly from the centre. 
 
 Helmholtz, A, 322, 470-471, Fr. 369 (281); Aubert, A, 531-532; 
 Kood, A, 39-40, 194-201; Nichols, A. 
 
 140. Changes in Intensity : Black and White. Black and 
 white are the extremes of intensity in the series of grays. 
 The ordinary black and white of conversation are, however, 
 considerably short of these extremes. 
 
 a. Compare a bit of black velvet or of black cardboard 
 with a still deeper black by holding it in front of the open- 
 ing in the dark box. Compare, also, ordinary white paper 
 in diffused light with the same in direct sunlight, or with a 
 brightly illuminated white cloud. 
 
 b. Just observable differences with medium intensities. 
 Prepare a disk like that shown in the accompanying cut by 
 drawing along a radius of a white disk a succession of short 
 black lines of equal breadth. Let the breadth of the line 
 correspond to about one degree on the edge of the disk. 
 Since the breadth of the line is everywhere the same, it 
 will occupy a relatively greater angle as it nears the centre. 
 
SENSATIONS OF LIGHT AND COLOR. 139 
 
 When the disk is set in rapid rotation, each short line 
 will give a faint gray ring, those at the outer edge being 
 very faint, those nearer the 
 centre, darker. Find which 
 is the faintest ring that can 
 be seen, and calculate the 
 proportions of black and 
 white in it. 1 The ratio of 
 black to white measures 
 approximately the just ob- 
 servable decrease in in- 
 tensity below the general 
 brightness of the- disk. 
 The results of Helmholtz 
 and Aubert are respec- 
 tively : Helmholtz, 1 : 117 to 1 : 167, Aubert, 1 : 102 to 1 : 186, 
 the differences depending on the intensity of the general 
 illumination of the disk. Some wandering of the eyes is 
 helpful, but too rapid motions which tend to break up the 
 even gray of the rings must be avoided. It is absolutely 
 essential that the rotation be very rapid and perfectly free 
 from vibration so rapid that with moderate motions of 
 the eyes the uniform gray of the rings is not disturbed. If 
 great rapidity is impossible, replace the single black line 
 by two of proportionately less breadth on opposite sides of 
 the disk, or by four at 90. 
 
 c. With these very faint rings a disappearance and reap- 
 pearance is to be observed somewhat like that found for 
 
 1 The formula for the amount of black, assuming that the radial line is abso* 
 lutely black, and taking some arbitrary point, e.g., the middle, for calculation, is of 
 
 course - . where b is the breadth of the radial line, and r the distance of the 
 
 27r r 
 
 chosen point from the centre of the disk. The black of the lines is not quite abso- 
 lute, even when the blackest black paint is used. The differences in sensation 
 are therefore smaller than those shown by the calculation. 
 
140 LABORATORY COURSE IN PSYCHOLOGY. 
 
 just audible sounds in Ex. 61 b. The observation is most 
 conveniently made, according to Pace, on a disk of the fol- 
 lowing dimensions : diameter of disk, 20 cm., width of radial 
 line, 5 mm., length of the short lines, 5 mm., spaces between 
 the short lines, 8 mm., distance of innermost short line from 
 the centre of the disk, 17 mm. 
 
 Helmholtz, A, 384-393; Fr. 411-419 (310-316); Aubert, A, 487- 
 492 ; on c, Pace. For references on the just observable difference of 
 intensity with different standard intensities, see the chapter on 
 Weber's Law below. 
 
 141. Changes in Intensity : Colors. At their maximum 
 intensity all colors tend toward white or yellowish white. 
 Red, however, hardly gets beyond the yellow ; green be- 
 comes first yellow, then white, while blue and violet easily 
 reach it. At their minimum intensity all colors appear 
 gray or black. 
 
 a. The maximum intensity may be observed with spec- 
 tral colors, though not entirely homogeneous ones, with a 
 prism placed in the sunlight so that it throws an extended 
 spectrum on the wall. Hold a card, pierced with a pin-hole, 
 before the eye, and bring the eye successively into the dif- 
 ferent colors, looking meanwhile at the prism. Something 
 of the same kind may be seen by looking through pieces of 
 colored glass at the disk of the sun behind a cloud (in which 
 case the portions of the cloud seen at the sides of the glass 
 afford a means of comparison), or at the image of the sun 
 reflected from an unsilvered glass plate, or by concentrating 
 light from colored glass on white paper with a convex lens. 
 
 b. The minimum intensity with spectral colors may be 
 observed with a spectroscope. Adjust the instrument so 
 that the chief Fraunhofer lines can be seen, and then place, 
 as a source of light, at a little distance from the slit of the 
 instrument, a screen covered with dark gray paper or black 
 velvet. Though no color remains, a little light can be made 
 
SENSATIONS OF LIGHT AND COLOR. 141 
 
 out brightest in the region before occupied by the green. 
 The observer must envelop his head and the ocular of the 
 instrument in an opaque cloth, and allow time for the adap- 
 tation of his eye. This colorless spectrum probably repre- 
 sents what is seen by a totally color-blind eye. 
 
 Von Bezold, with whom this experiment originates, ob- 
 served with gradually decreasing intensity a falling out of 
 the yellows and blues before the final stage of colorlessness 
 was reached. Konig doubts whether the red ever loses 
 its color entirely. 
 
 With pigment colors a convenient way is to paste equal 
 squares of colored papers upon a piece of cardboard, and 
 then to place the whole in 
 the dark box, and gradually 
 reduce the illumination, or 
 starting with the illumina- 
 tion at zero, gradually in- 
 crease it. Try with both 
 black and white cardboard 
 as background. For dem- 
 onstrational purposes a disk 
 like that in the accompany- 
 ing cut (in which the shaded 
 part stands for color, and 
 the solid black for black) may be used and the whole series 
 of intensities shown at once. 1 
 
 Helmholtz, A, 402-444 ; A. Fick, B, 200-202 ; Aubert, ^1,532-536 ; 
 Rood, A, 181-194 ; C. S. Peirce. On cr, Helmholtz, A, 284-285, 
 465-466, Fr. 315 (234); Brodhun, B. On 6, Helmholtz, A, 469, 471- 
 
 1 Since the black of the disk is really a very dark gray, and would thus make a 
 change in saturation, this is not an absolutely pure experiment, but is sufficiently 
 exact for showing the general effect of darkening. If a practically perfect black 
 is desired, it may be had, following Rood, by making the colored star rotate 
 before an opening into a dark room or a suitable dark box. 
 
142 LABORATORY COURSE IN PSYCHOLOGY. 
 
 472; von Bezold, A] Ebert; Abney and Testing; Konig, A, 354 ff., 
 where other literature is cited. 
 
 For measurements of the just observable difference of intensity 
 for different colors, see Helmholtz, -4, 402-415; Aubert, A, 531; 
 A. Fick, A, 177; and the references given by them. 
 
 142. Purkinje's Phenomenon. In a light of moderate 
 brightness choose a bit of red paper and a bit of blue paper 
 that are of about equal intensity and saturation, carry both 
 into full sunlight and notice which appears brightest ; carry 
 both into a darkened room, or place them in the dark box 
 and compare them again. If a dark room or box is not at 
 hand, observe them through a fine pin-hole in a card, or 
 even with nearly closed eyes. 
 
 Helmholtz, A, 428-430, 443-444, Fr. 420-425 (317-321); Hillebrand; 
 Konig, A-, Charpentier, A, 227 ff., 335 ff.; Hood, A, 189 ff. 
 
 143. Size of the Colored Field. When the retinal area 
 stimulated is very small, colored surfaces appear colorless, 
 with ordinary intensities of illumination. When somewhat 
 larger they may appear colored, but not necessarily in their 
 true color-tone. The background against which they are 
 placed is also important. 
 
 a. On pieces of black and white cardboard, paste small 
 squares of several kinds of colored paper, one series 5 mm. 
 square, one 2 mm. square, and one 1 mm. square. Walk 
 backward from them and notice their loss of color. Ob- 
 serve also the changes in color-tone. 
 
 b. A number of retinal impressions, even when not con- 
 tiguous, are mutually supportive in color effect. This is 
 conveniently shown in the indirect field. In a two-inch 
 square of black cardboard, punch sixteen holes arranged in 
 the form of a square, four rows of four holes each. The 
 holes should be an eighth or three-sixteenths of an inch in 
 diameter, and be separated by spaces of the same extent. 
 Paste upon the back of the square a piece of red paper of 
 
SENSATIONS OF LIGHT AND COLOR. 143 
 
 sufficient size to cover the holes, thus making of them six- 
 teen little red circles. Prepare also another piece of black 
 cardboard of such shape that it may be laid over the square 
 and cover all the holes except one of the corner ones, and 
 again when necessary may easily be removed. 
 
 With the apparatus used in Ex. 137, find the point on the 
 nasal half of the retinal horizon where the single red circle 
 can just no longer be seen in its true color. In making this 
 determination, the square should be so held that the diag- 
 onal to which the uncovered circle belongs is horizontal. 
 When the point has been found, uncover the remaining 
 fifteen circles (all farther toward the periphery), and notice 
 that the color of the group can be seen distinctly. Fatigue 
 in fixing the limit at which the circle can be seen should be 
 avoided. 
 
 On a, Helmholtz, A, 374-375, Fr. 399-400 (300); Aubert, A, 536- 
 539; Bering, R, 18. On 6, A. E. Tick, A and B (especially 451-452). 
 
 144. Duration of Illumination. Fechner's Colors. The 
 retinal inertia is different for different colors. In the ex- 
 periments on after-images (Ex. 125 eZ), it was observed that 
 the after-image of a white surface faded away through a 
 succession of colors ; a succession of colors appears also to 
 result from a very brief vision of a white surface. This 
 can be seen upon almost any slowly rotating disk of black 
 and white ; those used in Exs. 128 b and 145 c show the 
 colors well, and that in Ex. 145 a shows something of the 
 dependence of particular colors upon particular rates of 
 recurrence. Rotate any of these disks with less rapidity 
 than that required for a uniform gray, and, keeping the 
 eyes steadily fixed upon some point of its surface, notice 
 both the advancing and the retreating edges of the white 
 portions of the disk. The colors may not appear instantly, 
 but are not difficult to get with attentive gazing. 
 
 Very striking and beautiful effects can be obtained by 
 
144 LABORATORY COURSE IN PSYCHOLOGY. 
 
 substituting for the black and white disk a black one from 
 which narrow sectors have been removed. This pierced 
 disk is rotated before a brightly lighted background, e. g., 
 a sheet of white cardboard in full sunlight, a bright cloud, 
 or the clear sky, and the eye is brought very close to the 
 disk. 
 
 Helmholtz, A, 530-533, Fr. 500-504 (380-383) ; Fechner, A ; Briicke ; 
 Exner; Aubert,^!, 560; Kood, A, 92 ff., B\ Nichols, jB; Charpentier, 
 B and C. 
 
 145. Rate of Rotation Required for a Uniform Blending 
 of Black and White. All blending of colors by rotation 
 depends on the phenomenon of positive after-images (Ex. 
 125). A disturbance once set up in the retina does not at 
 once subside, but continues an instant after the removal of 
 the stimulus. If stimuli follow in sufficiently rapid succes- 
 sion the disturbances fuse, and the result is the same as if 
 the stimuli had been mixed before reaching the retina. A 
 rough determination of the rate required for uniform blend- 
 ing may be made with the color-mixer and a metronome. 
 a. Place the color-mixer in such a position that the disk 
 (like that in the margin) shall 
 be illuminated by diffused 
 daylight only. Turn the 
 driving-wheel slowly and 
 ascertain, by counting, how 
 many turns of the disk 
 correspond to one turn of 
 the driving-wheel. Start 
 the metronome, and turn 
 the driving-wheel in time 
 to its beats, making a turn 
 every one. two or four beats. 
 Notice which of the rings, if any, is just blended into a 
 uniform gray. If none is just blended, change the rate of 
 
SENSATIONS OF LIGHT AND COLOR. 145 
 
 the metronome a little, and repeat the trial till such a one 
 is found. From the rate of the metronome, the number of 
 turns of the driving-wheel, and the number of white sectors 
 in the just blended ring, find the number of stimuli per 
 second required. The experiment is easier when two 
 observers work together, one giving his attention to the 
 regular driving of the color-mixer, and the other to watch- 
 ing the disk. The driving-belt of the instrument must 
 be tight enough not to slip, and the metronome should 
 be kept well wound up. Its scale should also be verified 
 by counting with a watch. The observer must of course 
 avoid eye motions which break up the uniformity of the 
 gray. 
 
 b. Eepeat the determination with the disk in direct sun- 
 light ; also in a partially darkened room or at twilight. 
 
 c. A disk like that in the margin shows mixtures of 
 several different proportions of 
 
 black and white at once. If 
 
 such a disk is brought slowly 
 
 to the rate just neces- 
 
 sary to give a uniform 
 
 gray at the centre, a 
 
 little flickering can still 
 
 be traced in the outer 
 
 rings. Care should be 
 
 taken not to fixate the 
 
 middle of the disk ex- 
 
 clusively, for with mod- 
 
 erate illumination the per- 
 
 iphery of the retina requires a 
 
 little greater speed for uniform 
 
 blending than the centre. Helmholtz states that little dif- 
 
 ference is to be observed in the rate at which the flickering 
 
 ceases with the somewhat similar disk shown at the left in 
 
 OF THE 
 
 UNIVERSITY )) 
 
 JJ 
 
146 LABORATORY COURSE IN PSYCHOLOGY. 
 
 Ex. 152 d, but with that given here, it is believed that 
 careful observation will not fail to show a difference. 
 
 Helmholtz, A, 488 ff., Fr. 453 (344) ff.; Aubert, A, 517; A. Fick, 
 J3, 211-222; Nichols, J5; Bellarminow and the literature cited by 
 him. 
 
 146. The Talbot-Plateau Law. This law may be stated 
 as follows : When once the rate of rotation is sufficient to 
 give a uniform sensation, the color and brightness of any 
 given concentric ring of the disk are the same that they 
 would be if all the light reflected from it were evenly dis- 
 tributed over its surface, and no further increase in rapidity 
 produces any effect upon its appearance. Rotate the disk 
 used in Ex. 145 a, and increase the rapidity till the inner- 
 most portion gives a uniform gray. When this appears, the 
 rate of recurrence in the outermost ring is 32 times more 
 rapid than in the innermost, and yet no difference in shade 
 is to be seen. To show that the gray is actually of the 
 same brightness that would come from an even distribution 
 of the light reflected from the whole surface of the ring, 
 prepare a disk with many equal black and white sectors 
 32 or more of each. Place the disk on the color-mixer, and 
 look at it when at rest through a double convex lens of 
 short focus (e.g., 1 in.), held at such a distance from the eye 
 and disk that no distinct image is formed, but the field of 
 the lens appears an even blur of gray. Now put the disk 
 in rapid rotation and notice that the gray remains un- 
 changed. 
 
 The result of these experiments would be the same were 
 other colors substituted for black and white. 
 
 Helmholtz, A, 482-485, Fr. 446-450 (338-341); Aubert, A, 515-516; 
 Talbot; Plateau. 
 
 147. Brticke's Experiment. When the rate of rotation 
 is insufficient to produce an even blending, the brightness 
 
SENSATIONS OF LIGHT AND COLOR. 147 
 
 of the disk is influenced by the rate. Set the disk used in 
 Ex. 145 a in rapid enough rotation to blend the innermost 
 ring, and then let it gradually come to rest. As it turns 
 more and more slowly, there will be observed in one ring 
 after another, beginning with the innermost, just as it loses 
 its uniform character, a notable brightening. The white 
 sectors now have opportunity to produce their full effect 
 upon the retina before they are succeeded and their impres- 
 sion cut off by the black sectors. 
 
 Helmholtz, A, Fr. 455-456: Exner; Aubert, A 9 510. 
 
 COLOR MIXING. 
 
 148. Mixed Colors. Experiments upon this subject can- 
 not be regarded as entirely satisfactory except when made 
 with pure (homogeneous) spectral colors. The colored 
 papers with which the following experiments are made 
 show anything but homogeneous colors, as can easily be seen 
 by looking at scraps of them on a dark background through 
 a prism. They produce the same mixture effects, however, 
 that spectral colors of the same tone, intensity, and satura- 
 tion would produce ; and the great facility of their manipu- 
 lation on the color-mixer recommends them for preliminary 
 experiments and for illustrative purposes. 
 
 Three colors properly selected serve to produce by their 
 mixtures all the intermediate colors (though in most cases 
 in less saturation) with purple and white (i. e., gray) in addi- 
 tion. The colors generally selected are red, green, and blue 
 or violet. Green cannot be mixed from colors that them- 
 selves do not resemble it ; i.e., it can be mixed from yellow- 
 green and blue-green, but not from yellow and blue, and not 
 in anything like full saturation. 
 
 The general facts of color mixing, together with the 
 method of representing them in a two dimensional diagram, 
 were first discovered by Newton, and are sometimes desig- 
 
148 LABORATORY COURSE IN PSYCHOLOGY. 
 
 nated by the general term of Newton's Law. For the 
 methods of constructing such diagrams, see, among others, 
 Helmholtz, A, 334 ff., Aubert, A, 524 ff., and Rood, A, 218 
 ff., 224 ff. 
 
 a. Mix a yellow from red and green on the color-mixer. 
 The yellow produced will be dark, and, as a test of its 
 hue, should be matched with a mixture of yellow and 
 black made with smaller disks set on above the first. In 
 the same way mix a blue from green and violet that shall 
 match a mixture of blue and black (or blue, black, and 
 white). 
 
 b. From red and violet or blue, mix several purples be- 
 tween violet arid red. 
 
 c. From red, green, and violet, mix a gray that shall 
 match a mixture of black and white on the small disk. In 
 such a case as this it is highly probable that the gray 
 appears, because the combined colors furnish among them 
 light of all wave-lengths in about the proportions in which 
 they occur in ordinary white light. With the homogeneous 
 red, green, and violet of the spectrum, the case would of 
 course be different. To avoid troublesome after-images, the 
 adjustment of the disks should be left to an assistant, or 
 the observer should wear dark glasses, except when the 
 disks are in revolution at full speed. 
 
 If the colored disks used in these experiments are not 
 opaque, several should be used at once instead of a single 
 one. 
 
 ^ For demonstrational purposes mixtures of two colors in 
 different proportions can be shown on a single disk of the 
 star form (see Ex. 141) by painting the star in one color 
 and the ground of the disk in another (or by pasting colored 
 papers instead of painting), but in either case some trial 
 will be necessary to determine the proper shape for the 
 rays. 
 
SENSATIONS OF LIGHT AND COLOR. 149 
 
 Helmholtz, A, 311-316, 320-322, 325-333, 375, 376-473, 485, Fr. 
 359-365, 367-369, 450 (272-277, 279-281, 341); Aubert, A, 521-524- 
 Bering, Jf; Maxwell, A and #; Kood, A, 124 ff. 
 
 149. Complementary Colors. The combination of* red, 
 green, and violet mentioned in the last experiment is not 
 the only combination that gives white or gray. For every 
 color there is another or complementary color, which, mixed 
 with it, gives a colorless combination. Some of these pairs 
 are red and blue-green, yellow and indigo-blue, green and 
 purple, blue and orange, violet and yellow-green. 
 
 a. Try several of these pairs upon the color-mixer, match- 
 ing the resultant gray with a mixture of black and white 
 on the small disk. It will probably be found in some cases 
 that no possible proportions of the colored papers at hand 
 will give a pure gray. In that case a little of the color 
 complementary to that remaining in the gray must be 
 added. Suppose the red and blue-green papers, when com- 
 bined, give gray with a tinge of brown (i.e., dark orange) ; a 
 certain amount of blue must then be added to compensate. 
 For example, with certain papers 180 of blue-green -f- 36 
 indigo-blue + 144 red make a gray that matches 90 white 
 + 270 black. To see the true complement of the red used, 
 it is then necessary to prepare a disk carrying green and 
 indigo in the proportions of 180 and 36; i.e., 300 blue- 
 green, 60 indigo. In the same way the complement of the" 
 blue-green used is a bluer red than that of the red paper, 
 and may be seen by itself by mixing 288 red with 72 
 indigo. It is very important here, and in all cases where 
 a resultant white or gray is to be observed, to have some 
 undoubted white or gray in the field to prevent mistake in 
 very faint tinges of color. 
 
 The criticism made upon Ex. 148 c applies here with 
 equal force. To be conclusive, the experiment must be 
 made with far simpler colors than those of colored papers. 
 
150 LABORATORY COURSE IN PSYCHOLOGY. 
 
 b. Negative after-images, when projected on a white sur- 
 face, are seen in colors approximately complementary to 
 those that give rise to the after-images. Compare comple- 
 mentary colors found in this way with those found on the 
 color-mixer. 
 
 Helmholtz, A, 316-319, Fr. 365-367 (277-278) ; Aubert, A, 521- 
 524; Konig und Dieterici, A, 284 ff. ; Kood, A, 161 ff. 
 
 150. Other Methods of Mixing Colored Lights, a. Lam- 
 bert's Method. The Reflection Color-Mixer. This is the 
 
 simplest of all the methods. 
 The colors to be mixed are 
 placed on a suitable back- 
 ground (e. g., a smooth sur- 
 face of black velvet), on op- 
 posite sides of a vertical 
 glass plate. The eye is 
 brought into such a position 
 that the reflected image of the color on one side appears to 
 overlie that seen by transmission on the other side. The 
 glass must of course be of good quality and clean. The 
 relative intensity of the colors can be varied by varying 
 their distance from the glass. Bringing the colors near the 
 glass, or raising the eye, strengthens the reflected and weak- 
 ens the transmitted light. Strips of colored paper placed 
 with their ends next the glass, provided the illumination is 
 equal, will show an even blending of the colors through a 
 considerable range of intensities, one color predominating 
 at one end of the combined image, the other at the other 
 end. 
 
 By substituting a bit of glass on a black background for 
 one of the colors, and then placing the instrument so that a 
 portion of clear sky may be reflected in the glass, it is possi- 
 ble to mix sky-blue with its complement, or with any other 
 color. 
 
SENSATIONS OF LIGHT AND COLOR. 
 
 151 
 
 To mix two colors in equal proportions, arrange them 
 with black and white, as in the diagram below. Adjust 
 the glass (or the position of the eye) till the grays made 
 by the black and white at the ends exactly match ; the 
 colors will then be mixed in equal proportions. 
 
 b. Mixture by Double Kefraction. Colored areas placed 
 side by side appear mixed when regarded through a double 
 refracting prism. The prism doubles both fields, and causes 
 a partial overlapping. In the overlapped portion the colors 
 are mixed, each color being present in the mixture at ap- 
 proximately half its original brightness. The prism should 
 be achromatic. 
 
 c. Mixture of Spectral Colors. Fine mixtures may be ob- 
 tained with a prism and Figs. 1, 2, and 3 of Plate I. ; or, still 
 better, from figures shaped like these, but in white upon a 
 black ground. Since a prism refracts different kinds of light 
 in different degrees, it produces a multitude of partially over- 
 lapping images of a bright object, which appear to the eye 
 as colored fringes. (Observe through a prism held horizon- 
 tally, an inch square of white paper on a black background.) 
 These overlapping images may be illustrated by the follow- 
 ing diagram, in which the horizontal lines stand for the 
 
152 LABORATORY COURSE IN PSYCHOLOGY. 
 
 images, and the capital letters for the colors of the light 
 producing them. 
 
 a b 
 
 rr TT" 
 
 d c 
 
 In the area a b c d all the images overlap and the white 
 of the paper is still seen. Toward the left from a, however, 
 the different kinds of light gradually fail, beginning with 
 the red. The successive colors from greenish blue to violet 
 result from the mixture of what remains. At the other 
 end a similar falling away of the colors gives the succession 
 from greenish yellow to red. In Fig. 1, the spectra seen on 
 the upper and lower edges of the inch square of white 
 paper are brought side by side ; on one side red, orange, 
 and yellow, and on the other greenish blue, blue, and violet. 
 The colors that stand side by side are complementary pairs, 
 both in tone, intensity, and saturation; for the greenish 
 blue is the white of the paper less the red, and the blue the 
 same less the red, orange, and yellow, and so with the rest ; 
 and if the two spectra be exactly superposed, as can be 
 done with an adaptation of the method, of b above, they 
 will make precisely the white from which they originated. 
 
 If a very narrow strip of white upon a black ground is 
 looked at through the prism, the images overlap less and 
 another color appears ; namely, green, as may be seen in Fig. 
 2 on the narrow white band between the black bars. WJien, 
 on the other hand, a narrow black band on a white ground 
 is taken, the spectrum of the white surface above and of 
 that below partially overlap, and give another set of mix- 
 tures. If the diagram is held near the prism at first, and 
 
SENSATIONS OF LIGHT AND COLOR. 153 
 
 then gradually withdrawn from it, the advance and mixing 
 of the spectra can easily be followed. Besides the greenish 
 yellow at one end and the greenish blue at the other, there 
 are a rich, purple, complementary to the green beside it, and 
 a white between the purple and the greenish yellow. The 
 last is a white produced by the mixture of the blue of one 
 spectrum with the complementary orange-yellow of the 
 other. 
 
 Fig. 3 shows a number of color mixtures with different 
 proportions of the constituents. In the spectra from the 
 white triangle appear mixtures of each color in the spectrum 
 seen on the white band in Fig. 2, with every other color 
 found there. Upon the black triangle the spectra from the 
 white edges above and below show mixtures similar to 
 those on the black band in Fig. 2. The diagram should be 
 placed at such a distance that a little of the white and black 
 triangles can still be seen. 
 
 Helmholtz, A, 350-357, 485, 491-493, Fr. 402-407, 450, 458-461 
 (303-306, 341, 347-349); Aubert, .4,521-524 ; Maxwell, A\ Rood, A, 
 108 ff., 124 ff.; Hering, O; von Bezold, B, 77 ff. On a and c, Ben- 
 son. On refined methods of mixing spectral colors, see especially 
 the first reference to Helmholtz. 
 
 CONTRAST. 
 
 The effect of one color on another, when not mixed with 
 it, but presented to the eye successively, or simultaneously 
 in adjacent fields, is known as contrast. Two kinds are 
 distinguished, Successive contrast and Simultaneous contrast. 
 The color that is changed or caused to appear upon a color- 
 less surface, is known as the induced color ; the color that 
 causes the change is called the inducing color. Successive con- 
 trast is largely a matter of negative after-images, and their 
 projection upon different backgrounds, and is universally 
 regarded as a matter of physiology. Simultaneous contrast, 
 on the contrary, has been regarded by Helmholtz and his 
 
154 LABORATORY COURSE IN PSYCHOLOGY. 
 
 supporters as a matter of psychology, as a sort of mis- 
 judgment. The studies of the last few years, however, 
 chiefly those of Hering, have demonstrated that simultane- 
 ous contrast also in most, and probably in all cases, is 
 physiological, a phenomenon of the retina (and its central 
 connections), not of mistaken inference. 
 
 151. Successive Contrast, a. Prepare a set of colored 
 fields of the principal colors, including white, black, and 
 gray, say 3x5 inches in size, and some small bits of the 
 same colors, say 1 cm. square. Lay a small square on the 
 black field, get a strong negative after-image, and project 
 it first on the white and then on the other fields. Notice 
 that the color of the after-image spot is that of the field on 
 which it is projected, minus the color that produced the 
 spot ; e. g., the after-image of red projected on violet looks 
 blue, and on orange looks yellow. Or, to say the same 
 thing in other words, the color of the spot is a mixture of 
 the color of the after-image with the color of the ground 
 upon which it is projected. Thus a blue-green after-image 
 when projected on violet, gives blue ; when projected on 
 orange, gives yellow. Notice that when the image is pro- 
 jected on a field of the inducing color it causes the spot on 
 which it rests to look dull and faded ; but when it is pro- 
 jected upon a field of complementary color, it makes the 
 spot richer and more saturated. Indeed, it is only by first 
 fatiguing the eye for one color and then looking at its com- 
 plement that the most saturated color sensations can be 
 produced. In general, colors that are complementary, or 
 nearly so, are helped in appearance by contrast ; those that 
 resemble each other more nearly are injured. 
 
 b. These effects, in even greater brilliancy, can be seen 
 by laying the small square of color directly on the larger 
 colored surface, staring at it a few seconds, and then sud- 
 denly puffing it away with the breath. See also Ex. 134. 
 
SENSATIONS OF LIGHT AND COLOR. 155 
 
 c. This contrast effect may be so strong as actually to 
 overcome a moderately strong objective color. Place a 
 small piece of opaque orange paper in the middle of a pane 
 of red glass and look through the glass at a clear sky or 
 bright cloud. The strength of the induced blue-green will 
 be sufficient to make the orange seem blue. See also Ex. 
 124 d. 
 
 Helmholtz, A, 537-542, Fr. 510-515 (388-392); Hess, C ; Rood, A, 
 235 ff. 
 
 152. Mixed Contrasts. When special precautions are 
 not taken to exclude successive contrast, both successive 
 and simultaneous co-operate in the general effect. Some of 
 the results are striking and beautiful. 
 
 a. Colored Shadows. Arrange two lights so that they 
 shall cast a double shadow of a pencil or small rod upon a 
 white surface. The daylight will answer for one light if it 
 is not too strong, but it must not be forgotten that unless 
 the light comes from an overcast sky it will be blue. In- 
 troduce different colored glasses one after another before 
 one of the lights, and notice the beautiful complementary 
 color that immediately appears in the shadow belonging to 
 that light. The brightness of the two lights should be so 
 regulated that the shadows shall be about equally dark 
 when the colored glass is introduced before one of the 
 lights. See also Ex. 155. 
 
 Use a blue glass, and adjust the relative intensities of the 
 lights so that the yellow shadow appears at its brightest, 
 and notice that it seems as bright as the surrounding blue, 
 or even brighter. As a matter of fact, however, it receives 
 less light than the surrounding portions ; for in order to be 
 a shadow, it must be a portion of the field from which the 
 light is partly cut off. 
 
 b. Mirror Contrasts. Eagona Scina's Experiment. Place 
 upon the horizontal and vertical surfaces of the instrument 
 
156 LABORATORY COURSE IN PSYCHOLOGY. 
 
 white cards carrying black diagrams. 1 The diagrams being 
 in place, hold between the two at an angle of 45 a pane of 
 colored glass, say green, and observe that the black of the 
 horizontal diagram seems tinged with the complementary 
 color, that is, purple. This contrast color may often be im- 
 proved by slightly altering the inclination of the glass, or 
 by changing the relative illumination of the diagrams by 
 interposing a colorless screen between one or the other of 
 them and the source of light, or by shifting the whole in- 
 strument. This experiment will be readily understood after 
 
 a consideration of the accompany- 
 ing cut. The glass plate is repre- 
 sented by C D, the black portion 
 of the vertical diagram by the 
 projection opposite A, that of the 
 horizontal diagram by the projec- 
 tion at B. The light reaching the 
 eye from the white portion of the 
 horizontal diagram is colored green 
 by the glass ; that from the white 
 portion of the vertical diagram is reflected from the upper 
 surface of the plate, and is therefore uncolored. 2 The mix- 
 ture of the two gives a light green field. For simplicity, 
 we may assume that no light comes from the black portions 
 of the diagram. Then in the portion of the light green 
 
 1 Any black spot will answer. For this experiment diagrams made up of sets 
 of heavy concentric black rings, lines a quarter of an inch wide, separated by 
 white rings of triple width, give an excellent effect. The diameters should be so 
 chosen that a black ring on the horizontal diagram shall correspond to a white 
 one on the vertical and vice versa, and shall appear to lie in the midst of the 
 white when the diagrams are combined in the way described above. A pair of 
 diagrams made up of parallel black bars, a quarter of an inch wide, separated 
 by quarter inch spaces, and so placed in the instrument that they give a checker- 
 board pattern when combined, are useful for keeping in the field a true black 
 with which the changed colors can be compared. 
 
 1 As a matter of fact, a small portion is also reflected from the lower surface 
 of the glass, and contributes a minute amount of green. 
 
SENSATIONS OF LIGHT AND COLOR. 157 
 
 field corresponding to the black of tlie vertical diagram, the 
 white component will be wanting and the green will appear 
 undiluted ; in the portion corresponding to the black of the 
 horizontal diagram, the green component will be wanting 
 and the faint white (i. e., gray) should appear by itself. 
 It does not, however, because of the contrast color induced 
 upon it. As a matter of fact, the black portions are not 
 absolutely black ; the small amount of light that comes 
 from them tends on one hand to make the green image (im- 
 age of the black of the vertical diagram) a little whiter, and 
 on the other hand to counteract the contrast in the purple 
 image by adding to it a little green. Try the experiment 
 with other glasses than green. 
 
 Another form of the mirror contrast experiment is as 
 follows. Place a mirror where the sky or a white surface 
 of some kind will be seen reflected in it. Lay upon its sur- 
 face a plate of colored glass (green for example), and hold 
 a little way above it a narrow strip of black cardboard or a 
 pencil. Two images will be seen : one a vivid green, the 
 other a complementary purple. The green image belongs 
 to the surface reflection of the colored glass, as may be 
 proved by observing that when the strip of cardboard 
 touches the surface, the green image touches it also. The 
 purple image belongs to the reflection from the back of 
 the mirror. It is easy, by substituting a gray strip for the 
 black, to show that contrast can suppress a weaker objective 
 color actually present. 1 
 
 c. Meyer's Experiment. Lay on a large colored field a 
 small piece of gray or even black paper (e.g., 1 cm. wide by 
 2 cm. long), and cover the whole with a piece of semi- 
 transparent white paper of the same size as the colored 
 field. The contrast color will appear on the gray paper. 
 
 1 For fuller explanation with diagram, see American Journal of Psychology t 
 V., 1892-93, 407, and von Bezold, 154 f. 
 
158 LABORATORY COURSE IN PSYCHOLOGY. 
 
 If thin tissue paper is used, more than one thickness may 
 be needed for the best result. Paper mats, woven one way 
 of gray paper and the other of colored, show this contrast 
 beautifully. They may easily be made from kindergarten 
 materials. 
 
 d. Mixed Contrasts with the Color-mixer. Disks made on 
 the pattern of the cut at the left show beautiful contrasting 
 
 grays. The disk used in Ex. 145 c shows a longer series, 
 but requires a more rapid rate of rotation. The same can 
 be shown also by laying a number of small sheets of tissue 
 paper over one another in such a way that they partially 
 overlap, making a portion where there is but a single thick- 
 ness, and next it a portion where there are two thicknesses, 
 and next that again one of three thicknesses, and so on. 
 When the whole is held up to the light, the contrasts of 
 adjacent portions are very easily seen. 
 
 Contrast colors can be shown finely with disks like that 
 in the cut at the right, in which the shaded portions repre- 
 sent color, the black portions, black, and the white, white. 
 A little care is necessary in fixing the proportions of the 
 color to white and black in the disks, but in general the 
 
SENSATIONS OF LIGHT AND COLOR. 159 
 
 brightness of the gray should be about that of the color. 
 When the contrast color has been satisfactorily obtained, 
 bring near it a piece of white cardboard (e.g., 3 x 5 in.), so 
 held with reference to the source of light that it appears 
 about as bright as the contrast ring. Hold the card so that 
 its shadow does not fall on the disk, or at least is out of 
 sight. Notice the retreat of the contrast color from its 
 edges. On such experiments as this much stress is laid by 
 Helrnholtz and the supporters of the psychological explana- 
 tion of contrast. 
 
 Contrasts with two colors at once can be shown by mak- 
 ing the inner portion of the colored sectors of one color, the 
 outer portion of another. A temporary disk for showing 
 contrast effects may be arranged by putting on the spindle 
 of the color-mixer first a large colored disk (e.g., 20 cm. 
 . in diameter), then smaller combined disks of black and 
 white (e.g., 12 cm. in diameter), and finally a still smaller 
 colored disk (e.g., 10 cm. in diameter). 
 
 Helmholtz, A, 542 ff., Fr. 515-546 (392-417); Hering, j; Aubert, 
 496 if., 546 ff.; von Bezold, 144-171; Rood, 241-272; Mayer. 
 
 For particular experiments, see the following: on a (second part), 
 von Bezold, B, 153-154; on b (second part), Dove; on c, Meyer. 
 
 For quantitative measurements of contrast in grays, see Ebbing- 
 haus, B ; Lehmann; and Kirschmann, D. 
 
 153. Some of the Conditions that Influence Contrast. 
 
 a. Contrasts are stronger when the colors are near to- 
 gether. Lay a bit of white paper on a black surface, e.g., 
 a piece of black velvet, and notice that the paper is whiter 
 and the velvet blacker near the margin of the paper than 
 elsewhere, notwithstanding that the eye moves about freely. 
 This has received the name of " Marginal contrast " (Rand- 
 contrast). 
 
 On a piece of gray paper, the size of a letter-sheet, lay two 
 strips of colored paper close side by side (e.g., pieces of 
 
160 LABORATORY COURSE IN PSYCHOLOGY. 
 
 red and yellow or of green and blue, 1 cm. wide by 4 cm. 
 long). Below them to the right and left, as far apart as the 
 paper will permit, lay two other strips of the same size and 
 color, red on the red side of the former pair, yellow on the 
 yellow side. Notice the effect of the difference in distance 
 on the contrasting pairs. Contrast of this sort is at a maxi- 
 mum when one color entirely surrounds the other. 
 
 b. Effect of size. When the area of the inducing color is 
 large and that of the induced color is small, the contrast 
 is shown chiefly on the latter ; when the two areas are of 
 about equal size, as in a above, the effect is mutual. Try 
 with large and small bits of paper upon a colored field. 
 
 c. Borders and lines of demarcation that separate the 
 contrasting areas tend to lessen the effect by excluding mar- 
 ginal contrast ; and (since the eye tends to move along 
 rather than across strongly marked lines), by hindering such 
 motions of the eye as would bring about successive contrast. 
 Eepeat Ex. 152 c, using two slips of gray paper 5 mm. wide 
 by 2 cm. long, and substituting a piece of moderately trans- 
 parent letter-paper for the tissue paper. When the contrast 
 color has been observed, trace the outline of one of the 
 slips with a fine ink line upon the paper that covers it, and 
 notice that the color nearly or quite vanishes. A disk like 
 that in the cut accompanying Ex. 152 d, when provided with 
 a second contrast ring, marked off on both its edges with a 
 firm black line, shows a weakening of the induced color in 
 the bordered ring. 
 
 This experiment and others like it play an important part 
 in the psychological, as opposed to the physiological, expla- 
 nation of simultaneous contrast ; see Helmholtz, A, 543 ff., 
 559 f., Fr. 533 f., 539, 542, (406 f., 411, 414). Such a black 
 border will, however, also make a weak objective color 
 invisible. 
 
 d. Saturation. Contrast effects are generally most strik- 
 
SENSATIONS OF LIGHT AND COLOR. 161 
 
 ing with little saturated colors. Compare the effect of 
 increasing, decreasing, and extinguishing the second non- 
 colored light in the colored shadow experiments. It is 
 necessary, however, to see to it that reflected light from the 
 walls and surrounding objects does not complicate the ex- 
 periment. 
 
 Compare the intensity of the contrasts in Meyer's experi- 
 ment (Ex. 152 c) before and after the application of the 
 tissue paper. Notice also the part played by the white 
 light mixed with the colored light in the mirror contrast 
 experiments above. Try the effect of introducing white or 
 black or both into the largest and smallest disks in the 
 arrangement mentioned at the end of Ex. 152. Powerful 
 contrasts with the most saturated colors can be observed, 
 however, when the proper conditions are fulfilled. 
 
 e. Colors, induced upon gray fields are stronger when the. 
 gray has about the same brightness as the inducing color. 
 Eepeat Meyer's experiment, using white paper instead of 
 the gray or black. With the three disk arrangement try 
 the effect of making the intermediate disk all white and all 
 black. Eood finds that grays slightly darker than the 
 inducing color are advantageous when the inducing color 
 is red, orange, or yellow, and slightly lighter when the 
 inducing color is green, blue, violet, or purple. 
 
 On conditions in general, see Helmholtz, A, 540-541, Fr. 513-514, 
 (390-391), Kirschmann, D. In Hering, E, will also be found much on 
 the effect of various conditions. On ft, Exner, B. On c, Helmholtz, 
 A, 546-547, Fr. 539-542 (411-414). On d, Helmholtz, A, Fr. 523-524 
 (399-400). On e, Hood, A, 261. 
 
 154. The Halo or Lichthof of Hering. Contrast is often 
 to be seen in negative after-images. That observed in after- 
 images of white objects on a dark ground has been adduced 
 by Hering as an argument against the psychological expla- 
 nation of contrast. Some of the simpler experiments are 
 
162 LABORATORY COURSE IN PSYCHOLOGY. 
 
 as follows ; for his development of them consult Hering, A. 
 
 a. Lay a half inch square of white paper on a large sheet 
 of black cardboard (or better of black velvet), and put a 
 small dot at its centre. Stare with unmoved eyes at the 
 dot for from 15 to 30 seconds or more, then close and cover 
 the eyes. There will then be seen, neglecting incidental 
 color effects, the dark after-image of the paper surrounded 
 by a halo of light, brightest next the paper and gradually 
 falling off in brilliancy toward the periphery. This is ex- 
 plained on the psychological theory as due to contrast with 
 the deep black of the after-image of the square. When, 
 however, the converse of the experiment is properly made 
 (a black square on a white ground), the dark halo which 
 would be expected by contrast is not found, though the 
 after-image of the black square is very bright. 
 
 b. Lay two white squares side by side two or three milli- 
 meters apart on the dark ground and between them a 
 minute clipping of paper for a fixation point. Secure the 
 after-images as before. . The halos qf the two squares 
 coincide in the narrow space between and give a much 
 brighter band in the after-image. Under favorable circum- 
 stances this bright band may remain visible while the after- 
 images of the squares themselves are temporarily invisible. 
 In both these experiments it is better to use both eyes than a 
 single one. The explanation of the halo as a matter of false 
 judgment, especially in the last mentioned case, is not easy. 
 
 Hering, A. 
 
 155. Simultaneous Contrast with Colored Shadows. The 
 effects of simultaneous contrast are almost always lost in the 
 more powerful ones of successive contrast. The first requi- 
 site, therefore, of an experiment on the first, is the exclusion 
 of the second. This is not difficult for colored shadows. 
 
 a. Place a good-sized piece of white paper on a table in 
 such a position that it may be illuminated at the same 
 
SENSATIONS OF LIGHT AND COLOR. 163 
 
 time from a window (if the day is overcast) and from a gas- 
 jet. Set upon it a small block or other object (about 5 cm. 
 by 10 cm. in size) ; something black in color is best. Light 
 the gas and observe the two shadows, one cast by the light 
 from the window, the other by the gas. The first will 
 appear yellowish, the second clearly blue. 1 Adjust the dis- 
 tance and position of the block with reference to the light 
 so that the shadows shall appear about equally dark, and 
 the blue shadow shall be as sharply bounded as possible, 
 and for that purpose it is well to have the shadow cast by 
 the edge rather than the flat side of the flame. The color 
 of the yellowish shadow is objective and due to the yellow 
 of the gas-flame, that of the blue is due to the contrast, but 
 largely, as yet, to successive contrast. Put a dot in the 
 centre of the blue shadow, to serve as a fixation-point, and 
 another on the edge. Fasten a paper tube (preferably 
 blackened inside) so that it can easily be shifted from one 
 dot to the other. Cut off the gas-light by holding a card 
 between it and the block ; adjust the tube so that the dot in 
 the middle of the shadow may be fixated without any of 
 the field outside of the shadow being seen. Wait until all 
 of the blue has disappeared from the shadow, and then, 
 still looking through the tube, remove the card. The field 
 remains entirely unchanged and appears, as before, a color- 
 less gray. The former blue color is thus shown to be sub- 
 jective and due to contrast with the yellow lighted area in 
 which it lies. 
 
 1 This setting of the experiment succeeds best when the daylight is weak, as, 
 for example, just before the lights are usually lighted in the evening. If the ex- 
 periment is to be made in broad day, the light must be reduced by curtains or 
 otherwise; if at night, there must be two lights, one corresponding to the win- 
 dow and one to the gas, and the latter must shine through a pane of colored glass. 
 If yellow glass is used, the colors will be the same as those in this experiment, the 
 free flame taking the place of the daylight. If the sky is clear, its light is itself 
 blue, and would complicate the experiment somewhat. Its light may, however, 
 be passed through colored glass or gelatine, but then the orange color of the 
 gas-light must be regarded. 
 
164 LABOEATOllY COURSE IN PSYCHOLOGY. 
 
 b. Cut off the gas-light again and adjust the tube so that 
 the dot in the edge of the shadow may be fixated. Taking 
 great care not to move the eye, withdraw the card. The 
 part of the field of the tube filled by the shadow will ap- 
 pear bluish, that of the remainder reddish yellow. After 
 a little time of steady fixation, cut off the gas-light once 
 more and observe the instant reversal of the colors. The 
 shadow now appears in reddish yellow, the rest of the field 
 blue. The color of the shadow, both before and after the 
 final interposition of the card, is due to simultaneous con- 
 trast, in the first case with the reddish yellow light, and in 
 the second with its after-image. 
 
 Helmholtz and his supporters explain all cases of simul- 
 taneous contrast as errors of judgment ; in the case of the 
 colored shadow, for example, we mistake the yellow of the 
 gas-lighted field for white, and consequently find the shadow 
 which is really gray to be bluish. In the case of this par- 
 ticular experiment, Hering and Delabarre have shown this 
 psychological explanation unnecessary and a physiological 
 one all sufficient, and Hering has done the same for other 
 forms of experiments. 
 
 On simultaneous contrast in general, see Helmholtz, J., 542 ff., 
 Fr. 515-547 (392-418) ; Hering, A and E. On colored shadows see 
 Helmholtz, A, 551-553, Fr. 517-519 (394-396) ; Hering, E ; Delabarre. 
 
 On Helmholtz's theory see Helmholtz, J., 543 ff., Fr. 516, 533-538 
 (392, 407-411); Hering, E ; Rood, A, 252 ff.; von Bezold, B, 146 ff. 
 
 For quantitative measurements of simultaneous contrast under 
 various conditions, see Kirschmann, D. 
 
 156. Simultaneous Contrast. Hering's Binocular Method. 
 
 a. Set a red glass in the right frame of the binocular 
 color-mixer, a blue glass in the left. Look fixedly through 
 the colored glasses at the cork ball below, bringing the eyes 
 close to the glasses and the nose between them. Adjust the 
 side screens till the white ground below appears in a uni- 
 
SENSATIONS OF LIGHT AND COLOR. 165 
 
 form light violet from the binocular mixture of the red and 
 blue (see Ex. 167). The narrow strip of black paper on the 
 white is seen double, the right hand image bluish, the left 
 yellowish. 
 
 b. The possibility of successive contrast, however, is not 
 yet excluded. Lay a sheet of black paper over the whole 
 of the white field and its black strip ; rest the eyes ; and 
 finally, when everything is in readiness, and the eyes again 
 fixed on the ball, swiftly draw away the black paper, keep- 
 ing the eyes motionless. The contrast colors are seen on 
 the instant, before any motions of the eyes that might intro- 
 duce successive contrast have been made. 
 
 Hering argues that this experiment is conclusive against 
 the psychological explanation of simultaneous contrast, 
 unless a separate unconscious judgment is to be made for 
 each eye ; for that which is seen is a light violet field, 
 and the contrast color to that should be a greenish yellow, 
 and both images of the strip should be alike, whereas, 
 actually, the images appear in different colors, neither of 
 which is the color required. 
 
 Hering, J. 
 
 157. Induction of a Like Color. An effect the reverse 
 of the ordinary contrast effects sometimes appears, the in- 
 ducing color reappearing in the induced field. 
 
 a. Place close side by side a large piece of black paper 
 and an equal sized piece of white. Make a dot as a fixation 
 point at the middle of their line of junction, and stare 
 fixedly at it for half a minute. After a few seconds the 
 white will appear decidedly darker and the black decidedly 
 lighter, the effect becoming more marked as fixation is 
 continued. See also Ex. 122. 
 
 b. A darkening or brightening of a colored ground is often 
 to be observed when a figure in black or white is placed 
 
166 LABORATORY COURSE IN PSYCHOLOGY. 
 
 upon it. This is a method of obtaining shades and tints 
 often used in polychromatic decoration. Observe the effect 
 in Fig. 4 of Plate I. The same may be observed occasion- 
 ally in plaid fabrics, and is shown very satisfactorily in 
 kindergarten mats woven in checker-board pattern of col- 
 ored and gray papers. If a set of graded grays is used so 
 that the strips may range evenly from a black at one side 
 to a white at the other, the corresponding shading of the 
 colored paper is striking. 
 
 On a, Helmholtz, A, 554 ff., Fr. 527 ff. (401 ff.); Hering, A, 36 ff. 
 On &, von Bezold, I?, 182-183 and Plate Y. For what is perhaps a 
 related phenomenon, see Briicke, 424 ff . ; Helmholtz, A, 549, Fr. 
 520 (396); Aubert, A, 549 f. 
 
 158. Influence of Experience in Visual Perception. While 
 in the previous experiments a physiological explanation 
 seems sufficient for the facts, psychical action is not ex- 
 cluded, even by Hering, from a considerable share in sense 
 perception. In the following experiments experience co- 
 operates in the result. 
 
 a. Place upon the color-mixer a short-pointed star of 
 white cardboard, or even a square ; when in sufficiently 
 rapid rotation, it appears as a white central circle sur- 
 rounded by a more or less transparent ring. While in this 
 condition bring behind it a broad strip of black cardboard 
 of somewhat greater length than the diameter of the star 
 from point to point. As the edge of the card advances, it 
 can be seen not only behind the transparent ring, but, appar- 
 ently, also behind the opaque central circle, and the portions 
 of the latter in front of the black card seem darkened by 
 its presence. The illusion holds, though with a lightening 
 instead of a darkening effect, when a white card is moved 
 behind a black star. The illusion fails by degrees if the 
 card is kept motionless, but may be observed to a certain 
 extent when the star is at rest, or even on a square of card- 
 
SENSATIONS OF LIGHT AND COLOR. 167 
 
 board held in the hand while another is moved to and fro 
 behind it. In all cases the latter card should often be 
 wholly withdrawn, so that its edge can be clearly seen. 
 
 b. Cover a piece of black cardboard smoothly with tissue 
 paper, and notice that it seems at first blacker (because its 
 color is well known) than it afterwards proves to be on com- 
 parison with other grays. 
 
 c. In mixing colors by reflection (Ex. 150 a), notice the 
 tendency to see one color through the other, instead of see- 
 ing the mixture of the two. This tendency may be so 
 strong at first as to interfere, to a certain extent, with the 
 success of the experiment. See also Ex. 164. 
 
 Helmholtz, A, 312, 323 f., Fr. 360 (273); Kirschmann, E. On the 
 difficulty of judging small differences in the color of surfaces that 
 present other small unlikenesses, see Hering, E. 
 
 SOME PHENOMENA OP KOTATING DISKS. 
 
 159. The Munsterberg-Jastrow Phenomenon, a. Set a 
 black and white disk, e.g., that used in Ex. 145 a, in rapid 
 enough rotation to give a uniform gray; pass rapidly before 
 it a thin wooden rod or thick wire, and notice the multitude 
 of shadowy images of the rod that appear on the disk. The 
 number of images is greatest in the portion of the disk 
 having the most frequent interchange of black and white. 
 
 b. Replace the disk by one carrying two or more colors. 
 Notice the repetition of the phenomenon, and that the 
 colors of the images are the colors (otherwise completely 
 blended) which the disk actually carries. The explanation 
 of the phenomenon is not altogether clear, but the sudden 
 changes of the background against which the rod is seen 
 seem to have an effect not unlike that of a stroboscopic disk 
 or of intermittent illumination, and thus show the rod at 
 rest in its successive positions. 
 
 Jastrow. 
 
168 LABORATORY COURSE IN PSYCHOLOGY. 
 
 160. Eetinal Oscillation. Prepare a disk of black card- 
 board 25-30 cm. in diameter, and paste upon it a sector of 
 white of 90 extent. Put the disk in slow rotation (one turn 
 a second), fixate the middle of the disk, and notice that the 
 retreating edge of the black is always followed by a narrow 
 shadowy sector in the white. Under favorable conditions 
 more than one may be seen. The retina on first being stimu- 
 lated with white, apparently reacts in the direction of black 
 (see Ex. 125), then swings again toward white, and so on. 
 
 Charpentier, B. 
 
 161. Perception of Flicker with Different Parts of the 
 Eetina. Place upon the color-mixer a black and white disk 
 in which the sectors are complete from centre to circumfer- 
 ence ; those used in Ex. 145 will not answer here. Eotate 
 the disk at such a rate as to give a lively flicker, fixate its 
 centre and slowly increase the rate. With care a point will 
 be found where the sectors are blended for the central parts 
 of the retina, but still flicker for the periphery. Try also 
 looking at one edge of the disk while giving attention to the 
 centre or opposite edge. This is in accord with the general 
 principle that peripheral after-images are of shorter duration 
 than those of the retinal centre. Too bright illumination 
 should be avoided, for with intense light the difference be- 
 tween the centre and periphery is less, or even quite reversed. 
 
 Bellarminow. On rotating disks and their phenomena in general, 
 see Helmholtz, A, 480-501, Fr. 445-471 (337-357). 
 
 BINOCULAR PHENOMENA OF LIGHT AND COLOR.* 
 
 162. In general the two eyes co-operate to bring about 
 a single visual result, but the union of the impressions upon 
 the two retinae is influenced by a number of circumstances. 
 
 1 The experiments that follow can all be made with the stereoscope, but prac- 
 tice will enable the experimenter to combine the diagrams with free eyes, either 
 by crossing the lines of sight (fixating a point nearer than the diagram), or by 
 making them parallel or nearly so (fixating a point beyond the diagram). This 
 
SENSATIONS OF LIGHT AND COLOR. 169 
 
 a. If the stimulus to one eye is considerably stronger 
 than that to the other, the sensation in the latter is in most 
 cases totally suppressed. Close one eye and look at a sheet 
 of white paper with the other, letting the open eye move 
 about ffeely. There is no tendency for the darkened field 
 of the closed eye to assert itself. 
 
 b. When, however, the effect of the stimulus in the open 
 eye is somewhat weakened by steady fixation, such a ten- 
 dency is to be observed, and the whole of the field of the 
 open eye, except a small area about the point fixated, may 
 be suppressed from time to time by the dark field of the 
 closed eye. A slight motion will, however, instantly re- 
 store the first. See also Ex. 127. 
 
 c. A field that contains sharply marked objects or con- 
 tours will generally triumph over one that does not. Try 
 combining the letters below in such a way that the B's are 
 superposed. In this diagram the white field of either eye, 
 which corresponds to A or C in the other eye, will generally 
 not triumph over the letter. 
 
 AB BC 
 
 Helmholtz, A, Fr. 964 ff. (767 ff.); Hering, P, 380-385; Aubert, 
 A, 550-553; Wundt, A, 3te Aufl., II., 183 ff., 4te Aufl., II., 209 ff. 
 
 163. Fechner's Paradoxical Experiment. Hold close be- 
 fore one eye a dark glass, such as is used in protecting the 
 eyes, or a piece of ordinary glass moderately smoked over, 
 or even a black card with a good-sized pin-hole in it, allow- 
 ing the other eye to remain free. It is easy to see that the 
 
 skill the experimenter should try to acquire. In these experiments it is impor- 
 tant that the eyes should be of approximately equal power; and if the poorer eye 
 cannot be helped with lenses, the vision of the other must be somewhat reduced 
 by the interposition of a sufficient number of plates of ordinary glass. 
 
170 LABORATORY COURSE IN PSYCHOLOGY. 
 
 binocular field is darkened by the interposition of the dark 
 glass. If, however, the eye behind the glass is closed, or 
 the light wholly cut off from it by holding a black card in 
 front of the glass, the field appears decidedly brighter ; that 
 is to say, cutting off a portion of the stimulus received by 
 the total visual apparatus, has caused an increased intensity 
 of sensation. The experiment fails for very dark and very 
 light glasses. Several explanations have been given, but 
 that of Aubert (according to which the sensations of the 
 two retinae blend in a sort of average result when the dif- 
 ference is not too great, but one wholly suppresses the other 
 when the difference is very great) seems to be the most 
 satisfactory. 
 
 Fechner, B, 416 ff. ; Helmholtz, A, Fr. 993-994 (790-791) ; Bering, 
 Q, 311 f.; Aubert, A, 499-503. 
 
 164. Rivalry. When the two retinae are stimulated at 
 the same time separately with strong light of different 
 colors, or are confronted with otherwise incongruous fields, 
 i.e., fields that cannot be given a unitary interpretation, 
 there results a peculiar instability and irregular alternation 
 of the colors over part or the whole of the combined fields 
 of vision. This apparent struggle of the fields is known 
 as Retinal Rivalry. Hold close before one eye a piece of 
 blue glass, before the other a piece of red glass, and look 
 toward the sky or a brightly lighted uniform wall. The 
 struggle of colors will at once begin. The same may be 
 observed with a stereoscope when the usual paired photo- 
 graphs are replaced by colored fields, or even with no ap- 
 paratus at all, when both eyes are closed and turned toward 
 a bright sky and one of them is covered with the hand. 
 Lcng looking generally tends to quiet the rivalry. Rivalry 
 has been explained as due to fluctuations of attention, and 
 some observers find that it can be more or less controlled 
 by attention (Helmholtz). Fechner discusses the attention 
 
OF THR 
 
 UNIVERSITY 
 
 SENSATIONS OF LIGHT 
 
 theory, and finds it insufficient. Von Bezold thinks rivalry 
 associated with changes in accommodation which follow 
 attention. Hering and others regard the changes as of 
 more purely jphysiological .origin. See also Ex. 165 b. 
 
 Helmholtz, A, Fr. 964 ff. (767 ff.), 974 ff. (775 ff.); Hering, P, 
 380-385, Q, 308 ff.; Aubert, A, 550 ff.; Wundt, A, 3te Aufl., II. , 
 185 ff., 4te Aufl., II., 211 ff.; Chauveau, C. 
 
 165. Prevalence and Rivalry of Contours. By contours 
 is here meant lines of separation where fields of one color 
 border upon fields of another color. 
 
 a. Combine stereoscopically the two bars below, and notice 
 that it is the contours that suppress the solid parts of both 
 the black and white. This figure gives excellent results 
 also when colors are substituted for the black and white. 
 
 Notice a similar triumph of the contours of the cross in 
 the left-hand figure below, or, better still, in an enlargement 
 of it. 
 
 b. Notice the rivalry of the contours in all of these 
 figures. 
 
172 LABORATORY COURSE IN PSYCHOLOGY. 
 
 c. The last two pairs of diagrams are suitable for the 
 study of the part played by attention in rivalry. While 
 it is doubtful whether mere attention to one field or the 
 other can cause it to predominate, it yet seems possible by 
 indirect application of attention to cause it to do so. If 
 attention is given to an examination of the lines and small 
 squares in the left-hand figure, or if one of the sets of 
 lines in the right-hand figure is counted, both will appear 
 to be somewhat assisted in their struggle with the cross or 
 the other set of lines. 
 
 d. A printed page has a decided advantage. Try a dia- 
 gram in which a printed page is put in rivalry with a field 
 of heavy cross lines. The lines will be found- .to yield -to* 
 the print, at least at the point at which the reader is look- 
 ing at the instant. Two printed pages, however, become 
 hopelessly mixed ; and it is hard to say how much of the 
 advantage, when a single one is used, is due to its superior 
 power as a holcler^ of ^attention, and how much to its excel- 
 lence as a set of contours. A portion of the power of 
 contours is probably to be explained by the mutual intensi- 
 fication of both the black and the white by contrast j but a 
 part is perhaps due to a strong tendency, observable in 
 other cases also, for the eyes (and attention) to follow lines, 
 and especially outlines. 
 
 Helmholtz, A, Fr. 964 ff. (767 ff.); Bering, P, 380-385, Q, 314: 
 Wundt, A, 3te Aufl., II., 183 ff., 4te Aufl., II., 209 ff. 
 
 166. Luster. Sheen. When one of the rival fields is 
 white and the other colored (especially when one is white 
 and the other is black), there results, besides the rivalry, 
 a curious illusion of shine or polish, known as binocular 
 lustre. 
 
 a. Examine in the stereoscope a diagram made like the 
 accompanying cut, and notice the graphite-like shine of the 
 
SENSATIONS OF LIGHT AND COLOR. 173 
 
 pyramid. The explanation seems to be that polished sur- 
 faces, which at some angles reflect light enough to look 
 
 white, and at others appear in their true color, have often 
 in previous experience given rise to such differences of sen- 
 sation in the two eyes, and from this difference it is inferred 
 that the object seen in the diagram is shiny. 
 
 b. A species of monocular lustre (or transparence) is to 
 be observed when black or white or colors are combined by 
 means of the reflection color-mixer, especially when the 
 inclination of the plate is so changed that one color ap- 
 pears to be reflected in the surface of the other, or to be 
 seen through and behind it. The experiment works well 
 when real objects are reflected in the surface of the glass, 
 the reflecting power of the latter appearing to be trans- 
 ferred to the horizontal surface on the opposite side. 
 
 Helmholtz, A, Fr. 983 ff. (782 ff.); Bering, P, 576-577; Aubert, 
 A, 550 ff.; Wundt, A, 3te Aufl., II., 177 ff., 183 ff., 4te Aufl., II., 
 204 ff., 209 ff. 
 
 167. Binocular Color Mixing. The result of simultane- 
 ous presentation of different colors to the two eyes is not 
 always rivalry or lustre. If the colors are not too bright 
 and saturated, and the fields are without fleck or spot to 
 
174 LABORATORY COURSE IN PSYCHOLOGY. 
 
 give one the predominance, a veritable, though somewhat 
 unsteady, mixture of the colors may result. 
 
 a. Place a red and a blue glass of equal transparency 
 in the binocular color-mixer, and adjust the side screens till 
 the proper amount of white light is mixed in with that 
 transmitted from below. The mixture will then be seen on 
 the white field below. Try also with other combinations of 
 glasses. Mixtures obtained in this way are not always the 
 same in appearance as the monocular mixtures studied 
 above, and some observers have great difficulty in getting 
 them satisfactorily. Long^juid-steady -gazing, which inter- 
 feres with rivalry, favors binocular color mixing. 
 
 b. The same effect may be conveniently obtained with a 
 stereoscope, from which the middle partition has been 
 removed. Try with equal areas of dull colors of little satu- 
 ration. Hering recommends two squares of red and two of 
 blue, set at equal distances in a horizontal line, the two reds 
 on one side, the two blues on the other. When the middle 
 pair are combined stereoscopically, they show a mixed color, 
 while the unmixed colors can be seen for comparison beside 
 them. He also suggests the use of lenses to prevent sharp 
 focusing of the eyes upon the contours, which interferes 
 with the mixture. Complementary colors are said to be 
 more difficult to fuse than those standing nearer in the 
 color scale. The same is true of colors differing greatly in 
 brightness ; see Ex. 163. 
 
 Helmholtz, A, Fr. 976 ff. (776 ff.); Hering, P, 591-600; von 
 Bezold, C; Chauveau, A; Aubert, A, 550 ff.; Wundt, A, 3te Aufl., 
 II., 183 ff., 4te Aufl., 209 ff. 
 
 168. Binocular Contrast. The Side- Window Experiment. 
 Stand so that the light from the window falls sidewise into 
 one eye, but not at all into the other. Place in a convenient 
 position for observation a strip of white paper on a black 
 surface. The paper when looked at with both eyes appears 
 
SENSATIONS OF LIGHT AND COLOR. 175 
 
 perfectly colorless. On looking now at a point nearer than 
 the strip of paper (e.g., at the finger held up before the face), 
 double images of the strip will be seen. The two images will 
 be different in brightness and slightly tinged with comple- 
 mentary colors. The image belonging to the eye next the 
 window (which may be recognized by its disappearance 
 when that eye is closed) will appear tinged with a faint 
 blue or blue-green color, the other with a very faint red or 
 yellow. The light that enters the eye through the sclerotic 
 is tinged reddish yellow, and makes the eye less responsive 
 to that color ; the white of the paper strip therefore appears 
 bluish. It appears darker partly for a similar reason, and 
 perhaps also, as Fechner suggests, because it lies in a field 
 which, for the eye in question, is generally bright. The 
 reddish color of the other eye's image of the strip is ex- 
 plained as due to contrast with the first , but whether this 
 contrast color is a psychical matter, or whether it is to be 
 explained by the action of the stimulus in the first eye 
 upon the second, as there seems some reason to think, is as 
 yet uncertain. Its greater brightness is probably due to 
 the fresher condition of the eye to which it belongs, and to 
 contrast with its less brilliant field. The same thing is 
 often to be noticed when reading with the lamp at one side, 
 or even when one eye has been closed for a short time 
 while the other has been open. The double images are 
 in no wise essential ; simple alternate winking will show 
 decided differences in the condition of the two eyes. 
 
 Fechner, B, 511 ff.; Brucke, 420 ff.; Bering, P, 600-601; Helm- 
 holtz, A, Fr. 987 ff. (785 ff.); Chauveau, B\ Titchener; Wundt, A, 
 3te Aufl., II., 183 ff., 4te Aufl., II., 209 ff. 
 
 169. Binocular After-images. Lay a bit of orange-colored 
 paper on a dark ground, and provide two white cards. Hold 
 one of the cards close to the left eye, but a little to one 
 side, so as not to hide the bit of paper. Hold the other 
 
176 LABORATORY COURSE IN PSYCHOLOGY. 
 
 eight or ten inches from the right eye in such a way as to 
 hide the paper. Look at the paper for a few seconds with 
 the left eye, then bring the card before it. A faint, washy, 
 orange-colored positive after-image will appear on the card 
 before the right eye. The image is by no means easy to 
 observe. It is supposed to belong to the right eye's half 
 of the visual apparatus, possibly to the central, i.e., cerebral, 
 part. 
 
 Ebbinghaus, C ; Chauveau, B ; Titchener. 
 
 BIBLIOGRAPHY. 
 
 ABNEY: A. On the Examination for Colour of Cases of Tobacco 
 Scotoma and of Abnormal Colour Blindness, Proc. Roy. Soc., 
 XLIX., 1891, 491-508. 
 
 B. On the Limit of Visibility of the different Rays of the Spec- 
 trum, ibid., XLIX., 1891, 509-518. 
 
 C. The Sensitiveness of the Eye to Light and Colour. Nature, 
 XL VII., 1892-93, 538-542. 
 
 ABNEY AND FESTING: Colour Photometry, iii., Phil. Trans., 
 
 CLXXXIIL, 1892, A, 531-565. 
 ALBERT : Ueber die Aenderung des Farbentones von Spectralf arben 
 
 und Pigmenten bei abnehmen der Lichtstarke, Wiedemanrfs 
 
 Annalen, XVI., 1882, 129-160. 
 AUBERT: A and B. Works cited with same letters in bibliography 
 
 of Chap. V. 
 BELLARMINOW: Ueber intermittirende Netzhautreizung, von Graefe's 
 
 Archiv, XXXV., 1889, L, 25-49. 
 
 BENSON: Manual of the Science of Colour, London, 1871. 
 VON BEZOLD: A. Ueber das Gesetz der Farbenmischung und die 
 
 physiologischen Grundfarben, Poggendorff's Annalen, CL., 
 
 1873, 71-93, 221-247. 
 B. The Theory of Color in its Relation to Art and Art Industry, 
 
 Boston, 1876. 
 
SENSATIONS OF LIGHT AND COLOR. 177 
 
 C. Ueber binoculare Farbenmischung, Poggendorff's Annalen, 
 Jubelband [1874], 585-590. Cites earlier literature. 
 
 BRODHUN: A. Ueber die Empfindlichkeit des griinblinden und des 
 normalen Auges gegen Farbenanderung im Spektrum, Zeit- 
 SQhriftfur Psychologie, III., 1892, 97-107. 
 
 B. Die Gliltigkeit des Newton' schen Farbenmischungsgesetzes 
 bei dem sog. griinblinden Farbensystem, ibid., V., 1893, 323- 
 334. Cites literature. 
 
 BRUCKE: Untersuchungen iiber subjective Farben, Poggendorff's 
 Annalen, LXXXIV., 1851, 418-447. 
 
 CHARPENTIER : A and B. Work cited with same letters in bibliog- 
 raphy of Chap. V. 
 
 C. Sur le retard dans la perception des divers rayons spectraux, 
 Comptes rendus, CXIV., 1892, 1423-1426. 
 
 CHAUVEAU: A. Sur la fusion des sensations chromatiques per- 
 cues isolement par chacun des deux yeux, Comptes rendus, 
 CXIII., 1891, 358-362. 
 
 B. Sur les sensations chromatiques exercitees dans 1'un des deux 
 yeux par la lumiere coloree qui eclaire la re tine de 1'autre ceil, 
 ibid., 394-398. 
 
 C. Sur la theorie de 1'antagonisme des champs visuels, ibid., 439- 
 442. 
 
 CHEVREUL: The Principles of Harmony and Contrast of Colours, 
 London, 1859. 
 
 DELABARRE: Colored Shadows, American Journal of Psychology, 
 II., 1888-89, 636-643. 
 
 DODDERS: A. Ueber Farbensysteme, von Graefe's Archiv, XXVII., 
 1881, i., 155-223. 
 
 B. Noch einmal die Farbensysteme, ibid., XXX., 1884, i., 15-90. 
 
 C. Farbengleichungen, DuBois-Rey monads Archiv, 1884, 518-552. 
 DOVE: Versuche iiber subjective Complementarfarben, Poggen- 
 
 dorff's Annalen, XLV., 1838, 158-162. 
 
 EBBINGHAUS: A. Theorie des Farbensehens, Zeitschrift fur Psy- 
 chologie, V., 1893, 145-238. Full statement of matter pre- 
 sented in outline before the Psychological Congress in London, 
 1892, Proceedings, 101-103. 
 
178 LABORATORY COURSE IN 
 
 B. Die Gesetzmassigkeit des Helligkeitseontrastes, Sitz.-ber. der 
 Akademie zu Berlin, 1., Dec., 1887. 
 
 C. Ueber Nachbilder im binocularen Sehen und die binocularen 
 Farbenerscheinungen uberhaupt, Pfluger's Archiv, XLVI., 
 1890, 498-508. 
 
 EBEBT: Ueber den Einfluss der Schwellenwerthe der Lichtempfin- 
 
 dung auf den Charakter der Spectra, Wiedemanrts Annalen, 
 
 XXXIII., 1888, 136-155. 
 EXNER: A. Bemerkungen iiber intermittirende Netzhautreizung, 
 
 Pfliiger's Archiv, III., 1870, 214-240. 
 B. Ueber eine neue Urtheilstauschung im Gebiete des Gesichts- 
 
 sinnes, ibid., XXXVII., 1885,520-522, (this part also in Biol. 
 
 Centralbl, VI.) ; XL., 1887, 323-330. 
 
 FECHNER: A. Ueber eine Scheibe zur Erzeugung subjectiver Far- 
 ben, Poggendorff's Annalen, XLV., 1838, 227-232. 
 B. Ueber einige Verhaltnisse des Binocularen Sehens, Abhl. d. 
 k. sacks. Ges. d. Wiss., VII., 1860, 339-564. 
 
 FICK, A. : A. Zur Theorie des Farbensinnes bei indirektem Sehen, 
 
 Pfluger's Archiv, XL VII., 1890, 274-285. 
 B. Work cited with same letter in Chap. V. 
 
 FICK, A. E. : A. Eine Notiz iiber Farbenempfindung, Pfluger' l s 
 
 Archiv, XVII., 1878, 152-153. 
 
 B. Studien iiber Licht- und Farbenempfindung, ibid., XLIII., 
 1888, 441-501. 
 
 FRANKLIN, Christine Ladd: A. Eine neue Theorie der Lichtem- 
 pfindung, Zeitschriftfur Psychologic, IV., 1892, 211-221. Ab- 
 stracts of this paper may be found in Proc. Congr. Exper. Psy., 
 London, 1892; Johns Hopkins University Circulars, XII., No. 
 106, June, 1893, 108-110; Science, XXII., July 14, 1893, 18-19. 
 B. On Theories of Light Sensation, Mind, Ser. 2, II., 1893, 473- 
 489. 
 
 HELMHOLTZ: A. Work cited with same letter in bibliography of 
 Chap. V. 
 
 B. Popular Scientific Lectures, First Series, New York, 1885. 
 
 C. Versuch einer erweiterten Anwendung des Fechnerschen Ge- 
 setzes im Farbensystem, Zeitschrift fur Psychologic, II., 1892, 
 1-30. 
 
SENSATIONS OF LIGHT AND COLOR. 179 
 
 _D. Yersuch das psychophysische Gesetz auf die Farbenunter- 
 schiede trichromatischer Augen anzuwenden, ibid., III., 1891, 
 1-20. 
 
 E. Kiirzeste Linien im Farbensystem, ibid., 108-122. An extract 
 
 from Sitz.-ber. der Akademie zu Berlin, 17, December, 1891. 
 HERiNG: 1 A. Zur Lehre vom Lichtsinne, Wien, 1878. Keprint of 
 six communications to the Vienna Academy, 1872-74. For an 
 extended abstract of this work, made by Dr. William Pole, see 
 Nature, XX., 1879, 611-613, 637-639; XXI., 1879-80, 14-17. 
 
 B. Zur Erklarung der Farbenblindheit aus der Theorie der Gegen- 
 farben, Prag, 1880. Keprint from Lotos, Neue Folge, I., 1880. 
 
 C. Ueber individuelle Yerschiedenheiten des Farbensinns, Lotos, 
 Neue Folge, VI., 1885. 
 
 D. Beleuchtung eines Angriffes auf die Theorie der Gegenf arben, 
 Pfluger's Archiv, XLL, 1887, 29-46. 
 
 E. Ueber die Theorie des simultanen Contrastes von Helmholtz, 
 ibid., XL., 1886-87, 172-191 (Die farbigen Schatten); XLL, 
 1887, 1-29 (Der Contrastversuch von H. Meyer und die Yer- 
 suche am Farbenkreisel) ; 358-367 (Der Spiegelcontrastversuch) ; 
 XLIIL, 1888, 1-21 (Die subjective Trennung des Lichtes in 
 zwei complementare Portionen "). 
 
 F. Ueber die von v. Kries wider die Theorie der Gegenfarben 
 erhobenen Einwande, ibid., XLIL, 1888, 488-506; XLIIL, 1888, 
 264-288, 329-346. 
 
 G. Ueber die Hypothesen zur Erklarung der peripheren Farben- 
 blindheit, v. Graefe's Archiv, XXXV., 1889, iv., 63-83; XXXVL, 
 1890, L, 264. 
 
 H. Zur Diagnostik der Farbenblindheit, ibid., XXX VI., 1890, i., 
 217-233. 
 
 J. Die Untersuchung einseitiger Storungen des Farbensinnes mit- 
 tels binocularer Farbengleichungen, ibid., XXXVI. , 1890, iii., 
 1-23. 
 
 J. Beitrag zur Lehre vom Simultankontrast, Zeitschrift fur Psy- 
 chologic, L, 1890, 18-28. 
 
 1 Herlng's work upon color has not yet been gathered into one consecutive 
 whole. It has seemed well, therefore, to insert here, in addition to the titles of 
 papers bearing directly on the experiments of Chap. VI., such other titles on 
 light and color as came to hand. 
 
180 LABORATORY COURSE IN PSYCHOLOGY. 
 
 IT.. Eine Methode zur Beobachtung des Simultancontrastes, 
 
 Pfluger's Archiv, XLYII., 1890, 236-242. 
 L. Priifung der sogenannten Farbendreiecke mit Hiilfe des Farben- 
 
 sinns excentrischer Netzhautstellen, ibid., XLYII., 1890, 417- 
 
 438. 
 M. Ueber Newton's Gesetz der Farbenmischung, Prag, 1887. 
 
 Reprint from Lotos, VII., 1887. 
 N. Untersuchung eines total Farbenblinden, Pfluger's Archiv, 
 
 XLIX., 1891, 563-608. 
 O. Eine Vorrichtung zur Farbenmischung, zur Diagnose der Far- 
 
 benblindheit und zur Untersuchung der Contrasterscheinungen, 
 
 Pfluger's Archiv, XLII., 1888, 119-144. 
 P. Work cited with reference letter A in the bibliography of 
 
 Chap. V. 
 Q. Work cited with reference letter B in the bibliography of 
 
 Chap. V. 
 R. Ueber Holmgren's vermeintlichen Nachweis der Elemen- 
 
 tarempfindungen des Gesichtssinns, Pjluger^s Archiv, XL., 
 
 1887, 1-20. 
 S. Kritik einer Abhandlung von Bonders, Lotos, Neue Folge, II., 
 
 Prag, 1882. 
 T. Ueber Sigmund Exner's neue Urtheilstauschung auf dem 
 
 Gebiete des Gesichtsinnes, Pfliiger's Archiv, XXXIX., 1886, 
 
 159-170. 
 U. Ueber den Begriff Urtheilstauschung " in der physiologi- 
 
 schen Optik und iiber die Wahrnehmung simultaner und suc- 
 
 cessiver Helligkeitsunterschiede, ibid., XLL, 1887, 91-106. 
 
 HESS: A. Ueber den Farbensinn bei indirectem Sehen, v. Graefe's 
 Archiv, XXXV., 1889, iv., 1-62. 
 
 B. Untersuchung eines Falles von halbseitiger Farbensinnsstorung 
 am linken Auge, ibid., XXXVI., 1890, iii., 24-36. 
 
 C. Ueber die Tonanderungen der Spektralfarben durch Ermiidung 
 der Netzhaut mit homogenem Lichte, ibid., XXXVI., 1890, i., 
 1-32. 
 
 HILLEBRAND : Work cited in bibliography of Chap. V. 
 
 HOLMGREN : Color-blindness in its Relation to Accidents by Rail and 
 Sea. Translation by M. L. Duncan, Smithsonian Report, 1877, 
 131-195. 
 
SENSATIONS OF LIGHT AND COLOR. 181 
 
 JASTBOW: A Novel Optical Illusion, American Journal of Psy- 
 chology, IV., 1891-92, 201-208. 
 
 JEFFRIES: A. Color-blindness, its Dangers and its Detection, Bos- 
 ton, 1879. This work contains a seventeen-page bibliography 
 on .color-blindness and kindred topics. 
 
 B. Color-blindness, Article in the Keference Handbook of the 
 Medical Sciences, New York, 1886, II., 241. 
 
 KIRSCHMANN: A. Beitrage zur Kenntniss der Farbenblindheit, 
 WundVs Philos. Studien, VIII., 1892-93, 173-230, 407-430. 
 
 B. Ueber die Helligkeitsempfindung im indirecten Sehen, ibid., 
 V., 1889, 447-497. 
 
 C. Die Farbenempfindung im indirecten Sehen, Erste Mittheilung, 
 ibid., VIII., 1892-93, 592-614. 
 
 D. Ueber die quantitativen Verhaltnisse des simultanen Hellig- 
 keits- und Farben-Contrastes, ibid., VI., 1890, 417-491. 
 
 E. Some Effects of Contrast, American Journal of Psychology, 
 IV., 1892, 542-557. 
 
 KONIG: A. Ueber den Helligkeitswert der Spektralfarben bei 
 verschiedener absoluter Intensitat, Beitrage zur Psychologic 
 und Physiologic der Sinnesorgane (Helmholtz Festgruss), Ham- 
 burg und Leipzig, 1891, 311-388. 
 
 B. The Modern Development of Thomas Young's Theory of 
 Colour Vision, Report of British Association, Birmingham 
 Meeting, 1886, 431-439. 
 
 C. Zur Kenntniss dichromatischer Farbensysteme, Wiedemann's 
 Annalen, XXII., 1884, 567-578. 
 
 KONIG UND DIETERICI: A. Die Grundempfindungen in normalen 
 und anomalen Farbensystemen und ihre Intensitatsverteilung 
 im Spektrum, Zeitschrift fur Psychologie, IV., 1892, 241-347. 
 B. Ueber die Empfindlichkeit des normalen Auges fur Wellen- 
 langenunterschiede des Lichtes, Wiedemann's Annalen, XXII., 
 1884, 579-589. 
 
 VON KRIES : Die Gesichtsempfindungen und ihre Analyse, Du Bois- 
 Reymond's Archiv, 1882, Supplement-Band, 1-178. A care- 
 ful summary and discussion of the whole subject. 
 LEHMANN : Ueber die Anwendung der Methode der mittleren Abstu- 
 fungen auf den Lichtsinn; die quantitative Bestimmung des 
 Lichtcontrastes, WundVs Philos. Studien, III., 1886, 516-528. 
 
182 LABORATORY COURSE IN PSYCHOLOGY 
 
 MAXWELL : A. On the Theory of Compound Colours, and the Relation 
 
 of the Colours of the Spectrum, Phil. Trans., CL., 1860, 57-84. 
 B. On Colour Vision, Proc. Royal Institution of Great Britain, VI. 
 
 These two papers are also to be found in Maxwell's Scientific 
 
 Papers, Cambridge, 1890, I., 410-440, II., 267-280. 
 MAYER: Studies of the phenomena of Simultaneous Contrast-Color; 
 
 and on a Photometer for measuring the intensities of Lights of 
 
 different colors, American Journal of Science, Ser. 3, XLVL, 
 
 1893, 1-22; also Phil. Mag., Ser. 5, XXXVI., 1893, 153-175. 
 MEYER: Ueber Contrast- oder Complementarfarben, Poggendorff's 
 
 Annalen, XCV., 1855, 170-171; also Phil. Mag., Ser. 4, IX., 
 
 Jan.-June, 1855, 547. 
 NICHOLS: A. On the Sensitiveness of the Eye to Colors of a Low 
 
 Degree of Saturation, American Journal of Science, Ser. 3, 
 
 XXX., 1885, 37-41. 
 B. Duration of Color Impressions upon the Retina, American 
 
 Journal of Science, Ser. 3, XXVIII. , 1884, 243-252. 
 PACE: Zur Frage der Schwankungen der Aufmerksamkeit nach 
 
 Versuchen mit der Masson'schen Scheibe, Wundfs PMlos. 
 
 Studien, VIII., 1892-93, 388-402. 
 
 PEIRCE, B. O., JR. : On the Sensitiveness of the Eye to Slight Differ- 
 ences of Color, American Journal of Science, Ser. 3, XXVI. , 
 1883, 299-302. 
 PEIRCE, C. S. : Note on the Sensation of Color, American Journal 
 
 of Science, Ser. 3, XIIL, 1877, 247-251. 
 
 PLATEAU: Betrachtungen iiber ein von Hrn. Talbot vorgeschla- 
 genes photometrisches Pr'mcip,Poggendorff's Annalen, XXXV., 
 1835, 457-468. 
 
 POLE: Further Data on Colour-Blindness, P/i^.Jfa<?., Ser. 5,XXXIV., 
 1892, 100-114, 439-443, XXXV., 1893, 52-62, XXXVI., 1893, 
 188-195. 
 
 PREYER: Work cited in bibliography of Chap. I. 
 RAYLEIGH: A. Experiments on Colour, Nature, XXV., 1881-82, 
 
 64-66. 
 
 B. Rayleigh and others: Report of the [Royal Society's] Commit- 
 tee on Colour- Vision, Proc. Roy. Soc., LI., No. 311, July 19, 
 1892, 281-396. 
 
SENSATIONS OF LIGHT AND COLOR. 183 
 
 ROOD: A. Students' Textbook of Color, New York, 1881. 
 
 B. On a new Theory of Light, proposed by John Smith, M.A., 
 
 American Journal of Science, Ser. 2, XXX., 1860, 182-186. 
 SCHUSTER: Experiments with Lord Rayleigh's Colour Box, Proc. 
 
 RO& Soc., XL VIII., 1890, 140-149. 
 TALBOT: Experiments on Light, Phil. Mag., Ser. 3, V., July-Dec., 
 
 1834, 321-334, especially, 327-334. 
 TITCHENER: Ueber binoculare Wirkungen monocularer Reize, 
 
 WundV s Philos. Studien, VIII., 1892-93, 231-310. Cites liters 
 
 ture. 
 WUNDT: A. Work cited with same letter in the bibliography of 
 
 Chap. V. 
 B. Die Empfindung des Lichts und der Farben, Wundfs Philos. 
 
 Studien, IV., 1888, 311-389. 
 
 For further bibliographical references, see the works of Helmholtz 
 and Aubert and the following by Plateau : Bibliographic analytique 
 des principaux phenomenes subjectifs de la vision, depuis les temps 
 anciens jusqu'a la fin du XVIII. siecle; suivie d'une bibliographic 
 simple pour la partie ecoulee du siecle actuel. Mem. cour. de 1'Acad. 
 R. de Belgique; Bruxelles, 1876-77. 
 
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