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PLATE I.

III

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Fig. 4

COURSE

EXPERIMENTAL PSYCHOLOGY

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.

BIOLOGY

LIBRARY

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.

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.

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,

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.

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.

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
Physiology, III., 1881-82, 297-307.

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
lumineuses, Archives de Physiologic, Ser. 5, IV., 1892, 541-553,
629-639. Essential points of the same, Comptes rendus, CXIIL,
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-
en, Zeitschriftfur Psychologic, I., 1890, 47-51.

B. Einige Beobachtungen iiber Bewegungsnachbilder, Central-
blattfur Physiologic, L, 1887, No. 6, 135-140.

C. Ueber optische Bewegungsempfindungen, Biologisches Cen-
tralblatt, YIII., 1888, No. 14, 437-448.

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 ))

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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