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 INSTRUCTIONS 
 
 FOR 
 
 OBSERVING THE TRANSIT OF VENUS, 
 
 DECEMBER 0, 1882, 
 
 PREPARED BY 
 
 THE COMMISSION AUTHORIZED BY CONGRESS, 
 
 AND 
 
 PRINTED FOR THE USE OF THE OBSERVING PARTIES BY AUTHORITY 
 OF THE HON. SECRETARY OF THE NAVY. 
 
 DOCIJ 
 
 oc 
 
 WENT 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICE. 
 1882. 
 
INSTRUCTIONS 
 
 FOK 
 
 OBSERVING THE TRANSIT OF VENUS, 
 
 K (5 , 1 8 8 2 , 
 
 PREPARED BY 
 
 THE COMMISSION AUTHORIZED BY CONGRESS, 
 
 PRINTED FOR THE USE OF THE OBSERVING 1'ARTIES BY AUTHORITY 
 OF THE HON. SECRETARY OF THE NAVY. 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICE. 
 
 1882. 
 
[GENERAL ORDEK.] 
 
 NAVY DEPARTMENT, Washington, August u, 1882.. 
 
 The parties organized by the Secretary of the Navy to observe the Transit of 
 Venus in December, 1882, under the authority of an act of Congress approved August 
 7, 1882, are organizations invested with naval character, and subject to naval rules, 
 regulations, and discipline. The command of each party is assigned to the Chief 
 Astronomer, to whose authority all others will be obedient. 
 
 The following is the order of rank and authority in each party, viz: 
 
 1. Chief Astronomer. 
 
 2. Assistant Astronomer. 
 
 3. Chief Photographer. 
 
 4. Assistant Photographer. 
 
 This order will be respected and followed as the order of rank and succession in 
 each party under all circumstances, including the contingencies growing out of any 
 separation of the p^rty, or the happening of any vacancy. 
 
 The party destined for the coast of Patagonia, about to embark in the U. S. 
 steamer Brooklyn, will be personally subject to the authority of the commanding offi- 
 cer while on board that ship, and to its discipline, police authorities, and regulations. 
 
 The gentlemen engaged in this interesting and important service will readily and 
 fully understand that discipline, harmony, and co-operation are essential to its satisfac- 
 tory performance as well as to their own safety and convenience, and that for this 
 reason they have been required to assume the obligations, and are subjected to the rules 
 
 of the naval service. 
 
 WM. E. CHANDLER, 
 
 Secretary of the Navy. 
 
 799 
 
TABLE OF CONTENTS. 
 
 Order of the Honorable Secretary of the Navy respecting these instructions 7 
 
 I. Introductory note 9 
 
 II. Selection of station 9 
 
 III. Laying oft' ground and setting np instruments 10 
 
 IV. Memoranda respecting the photoheliograph 13 
 
 V. Adjustments of the, photoheliograph 14 
 
 VI. The heliostat 16 
 
 VII. The exposing slide 17 
 
 V1IL The tube 17 
 
 IX. The plate-holder plumb line 18 
 
 X. Batteries and electrical connections 18 
 
 XI. The measuring-rod . 19 
 
 XII. Instrumental errors . 20 
 
 XIII. Fittings of the photographic house 21 
 
 X IV. Care of the sensitive emulsion 21 
 
 XV. Selecting and marking glass 22 
 
 XVI. Cleaning and albumenizing glass . ... 22 
 
 XVII. Coating plates . . . . : 24 
 
 XV111. Exposure of plates in the photoheliograph 26 
 
 XIX. The development ., 27 
 
 XX. Fixing and varnishing 29 
 
 XXI. Blistering and lifting of the film 29 
 
 XXII. Spots and other defects 29 
 
 XXIII. Preparatory practice 29 
 
 XXIV. Preparation for the transit 30 
 
 XXV. Photographing the transit 31 
 
 XXVI. Development and packing of transit plates 33 
 
 XXVII. Wet emulsion plates 33 
 
 XXVIII. Bath wet plates 33 
 
 X X IX. General precautions 34 
 
 XXX. The chronograph 35 
 
 XXXI. Time observations and chronometer comparisons 35 
 
 XXXIL Exchange of time with other parties 36 
 
 X XXII I. Latitude and longitude of station 36 
 
 XXXIV. The equatorial telescope 36 
 
 X XXV. Occultations 37 
 
 XXXVI. General instructions respecting observations of contacts 37 
 
 X X XVII. Methods of recording contacts 44 
 
 XXXVIII. Diameter of Venus 45 
 
 XX XIX. Data required 46 
 
 XL. Records of observations and operations 49 
 
 XLI. Transmission and publication of observations 50 
 
NAVY DEPARTMENT, Washington, August u, 1882. 
 
 The following instructions, prepared by the Commission on the Transit of Venus, 
 are issued, under the authority of this Department, for the guidance and government 
 of the parties charged with the duty of making the observations of the Transit. Every 
 person engaged in the scientific operations is enjoined to study them carefully and to 
 conform to them strictly. 
 
 WM. E. CHANDLER, 
 
 Secretary of the Navy. 
 
INSTRUCTIONS 
 
 FOR 
 
 OBSERVING THE TRANSIT OF VENUS. 
 
 DECEMBER 6, 1883. 
 
 I. INTRODUCTORY NOTE. 
 
 The following instructions were prepared primarily for the use of the parties organ- 
 ized by the U. S. Transit of Venus Commission. But as the transit will be visible in 
 this country, they have also been adapted to the use of amateur observers who desire 
 to be made acquainted with the methods by which they may make observations of 
 
 value. 
 
 II. SELECTION OF STATION. 
 
 In choosing ground for a station, it is most important to obtain good foundations 
 for the instruments. Gravel is best, but a sandy soil is unobjectionable, if the sand is 
 not dry. 
 
 A nearly level spot, measuring at least sixty feet from north to south, is necessary 
 for setting up the transit instrument and photoheliograph. The floor of the photo- 
 graphic house must be eight and a half inches lower than that of the transit house, and 
 it will be advantageous if the natural slope of the ground gives that amount of fall ; 
 but if it does not, one or other of the houses must be elevated to obtain the proper 
 difference of level between them. For the sake of dryness, the sills of the houses 
 should be raised three or four inches above the surface of the soil. In many situa- 
 tions this can be convenietly done by driving stakes about four inches square into the 
 ground, setting the houses upon them, and nailing them fast. In other places it may 
 be necessary to use small piers of stone or brick. 
 
 Shelter from the prevailing winds is to be sought for; but great care must be 
 taken that the sun be visible from the point occupied by the photographic objective, 
 and by the equatorial telescope, during the whole time of the transit, and, indeed, a 
 little longer. For this and other purposes, the observers must make sure of the exact 
 local times of all the contacts. 
 
 After the site for the station is selected, a survey of the surrounding region must 
 be made to determine the location of the station relatively to neighboring permanent 
 objects. If a large scale map of the region can be obtained, the position of the 
 station may be marked upon it; but if no such map is available, one must be made. 
 
 9 
 
 2 INS 
 
10 INSTRUCTIONS FOR OBSERVING THE 
 
 A carefully- written description of the position of the station must also accompany 
 tin- map. The object of these records is to provide means for relocating the station 
 within one or two hundred yards at any future time. 
 
 III. LAYING OFF GROUND, AND SETTING UP INSTRUMENTS. 
 
 The approximate position of the meridian-line, on which are to be placed the 
 three piers for supporting the transit, the photographic objective, and the plate-holder, 
 being determined by compass or otherwise, the next two operations, which may be 
 carried on together, will be the laying down of an exact meridian-line and the erection 
 of a pier for the transit. 
 
 As soon as the position of the transit pier is decided, a point should be selected 
 about seven feet east or west of its center, from which to lay off a meridian-line by 
 the theodolite. The latter instrument being placed over this point, a stake should be 
 driven nearly south or north of it, and at a distance of not less than one hundred 
 feet; and then the azimuth of the line joining the instrument and the stake should be 
 found from observations of the sun or stars made on both sides of the meridian. 
 From this line of known azimuth a true meridian-line should next be laid down, and 
 that can be done either with the theodolite, or by setting off a proper offset from the 
 stake. Perhaps the latter is the better method. 
 
 The foundation of the transit pier should be three or four feet below the surface 
 of the ground, and, in northern stations, below the reach of frost, if practicable. The 
 last earth at the bottom of the excavation should be carefully removed with a spade 
 in horizontal slices, so as to produce a clean, hard, surface for the footing course to rest 
 upon. It is desirable to prevent the surface earth pressing against the sides of the 
 pier, and for that reason a vacant space one or two feet deep should be left around 
 it a curb being used to keep back the earth, if necessary. The sides of the pier must 
 face the cardinal points accurately- Above ground, it should measure twenty-one 
 inches from north to south, and twenty-five inches from east to west; below, it may 
 be from three to three and a half feet square. The cap-stone must be twenty-four by 
 twenty-eight inches, and three inches thick. The transit house must be built around 
 the pier after the latter is erected, the floor of the house 1 icing placed twenty-nine 
 inches below the top of the pier. 
 
 In the northern hemisphere the photographic house will be south, and in the 
 southern hemisphere north, of the transit house. The positions and depths of the holes 
 for the iron piers of the photoheliograph can be readily determined by means of the 
 meridian line and the numbers in TAHLK I. The holes may be dug at any time, but 
 the piers cannot be set until the transit instrument has been brought into the meridian 
 and its errors made as small as possible. The lower ends of the piers have thin edges, 
 and to prevent them settling into the ground they should be placed upon large Hat 
 stones in the bottoms of the holes. The conditions to he fulfilled in erecting the piers 
 an- as follows: 
 
 1. The flanged ends of the piers must he uppermost. 
 
 2. The larger of tile two piers carries the photographic objective, and goes nearest 
 the transit house. Its center must be about fourteen feet from the center of the transit 
 pier. 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 11 
 
 3. The distance between the centers of the two piers must be as stated in TABLE I. 
 
 4. The height of the upper surface of the flange of each pier, or, in other words, 
 the height of its top, must be as stated in TABLE I. 
 
 5. The centers of the two piers must be in the plane of the true meridian pass- 
 ing through the transit instrument. 
 
 6. The piers must stand perfectly plumb. 
 
 7. The screw-holes in the flanges of the piers must be so placed that in each pier 
 the hole nearest the transit house is in the plane of the meridian that is, in the plane 
 passing through the centers of the piers. The aperture in the side of the plate- 
 holder pier must also be in the meridian, but must face away from the transit house. 
 
 The simultaneous fulfillment of so many conditions is troublesome. A convenient 
 way of proceeding will be to tack a lath across the slit in the transit house at a point 
 four feet above the floor, and from it to stretch a cord, perhaps sixty feet long, in the 
 meridian of the transit instrument and horizontally. A strip of wood must also be 
 jammed into the top of each iron pier in such a way that a five-penny nail can be driven 
 into it, and be left projecting an inch, to mark the center of the pier. Then, by stick- 
 ing pins through the line at the points beneath which the centers of the piers must be 
 located, the latter can be brought very approximately into position. In making the 
 final adjustments, an engineer's level and a theodolite will be required. The engi- 
 neer's level should be set up about sixty feet from the transit pier, and in such a posi- 
 tion that the latter can be seen through the door of the transit house. The theodolite 
 must be mounted behind the transit house, and in the meridian of, but a little higher 
 than, the transit instrument, so that the tops of the iron piers can be seen over the 
 latter. By moving the transit instrument in altitude only, and the theodolite in both 
 altitude and azimuth, the two instruments must be pointed on each other, and the im- 
 age of the central wires of the transit must be brought into coincidence with the cent- 
 ral wires of the theodolite, and then the line of collimation of the latter will describe 
 a plane parallel to the meridian of the former. This suffices for our purpose ; but if 
 it is desired to bring the meridians of the two instruments into absolute coincidence, it 
 can be done by shifting the theodolite towards the east or west until the images of 
 the two objectives given by the theodolite eye-piece are seen to be concentric. For 
 that purpose a magnifier is necessaiy, and care must be taken that the coincidence of 
 wires of the two instruments is left perfect. Thus adjusted, the theodolite will show 
 with great precision when the centers of the piers are in the meridian of the transit 
 instrument ; and the engineer's level will show equally exactly when their tops are at 
 the proper height. 
 
 Instead of using a theodolite to place the piers in the meridian, the transit 
 instrument itself may be employed, if its objective is covered by a cap having a dia- 
 metral slit one-fifth of an inch wide. This slit must be placed truly vertical, and then 
 it will be possible to obtain somewhat indistinct vision of any vertical line situated in 
 the meridian of the instrument and having a diameter not less than the width of the 
 slit. If the centers of the piers are marked by twenty-penny nails, instead, of five- 
 penny ones, they can be seen through the transit. To guard against error, after a 
 pier has been apparently brought into the meridian, the cap should be twisted through 
 
12 INSTRUCTIONS FOR OBSERVING THE 
 
 half a revolution, and if that produces any change in the position of the image in the 
 transit, the mean of the two positions will be the true one, and the center of the pier 
 must be adjusted accordingly. 
 
 The pier for the photographic objective should first be brought accurately into 
 position, and afterwards the same thing should be done for the plate-holder pier. In 
 adjusting the distance between the two piers a steel tape-line must be used, not a linen 
 one. Nails answer well as wedges for making small changes in the heights of the 
 piers. When all is ready, the piers must be finally fixed by filling the holes around 
 them with masonry laid in cement, or with cement concrete earth does not give suffi- 
 cient firmness; and in doing this special care must be taken that they are not accident- 
 ally shifted from their true positions. Their interiors should also be filled with concrete 
 to the level of the ground. 
 
 The photographic house must be built around the plate-holder pier, the floor of 
 the house being placed exactly three feet ten and three-quarter inches below the upper 
 surface of the flange of the pier. 
 
 The cast-iron plates which support the photographic objective and plate-holder 
 are next to be fixed in position. The coffin-shaped plate is to be bolted to the larger 
 pier, its long end being turned toward the photographic house, and at least one washer 
 being placed around each bolt, between the plate and the pier, so as to give the former 
 a solid bearing. The coffin plates are not all alike. The older ones have pipes at the 
 end, to receive the prongs of the piece which supports the objective. These pipes must 
 be turned downward. The newer ones have a planed seat, upon which the bottom of 
 the support for the .objective is bolted. This seat must face upward. When the 
 objective-carrier is first mounted upon the coffin plate, the screw, or screws, which 
 hold it should be only lightly set up; but afterwards, when all has been found right, 
 they must be firmly turned home, and the pipes, if there are any, may be filled with 
 plaster or cement. The round cast-iron disc which carries the plate-holder is to be 
 secured to the pier in the photographic house by the proper supporting and binding 
 screws, care being taken that it is turned in the right direction. 
 
 To mount the plate-holder, the brass cross must be screwed to the cast-iron disc 
 on top of the plate-holder pier, the tube of the cross passing down through the hole 
 in the disc. The vertical axis of the plate-holder is to be set in this tube, and when 
 the plate-holder is rotated its ends should just graze the raised ends of the cross below 
 it. Milled-headed screws, passing through slots in pieces at the ends of the arms of 
 the cross, are provided for fixing tU,e plate-holder in position, with the reticule plate 
 at right angles to the optical axis of the objective 
 
 The structure for carrying the iron measuring-rod and the tube of the photohelio- 
 graph should next be erected; and the measuring-rod, but not the tube, must be mounted. 
 The last frame of the structure should be two feet distant from the nearest edge of 
 the coffin-plate, and the position of the measuring-rod must be parallel to, but nine 
 inches above, the optical axis of the photographic objective. For further details, con- 
 sult section XI. 
 
TEANSIT OF VENUS, DECEMBEE 6, 1882. 
 
 13 
 
 IV. MEMOBANDA EESPECTING THE PHOTOHELIOGEAPH. 
 
 For convenience of reference the measurements required in erecting the horizontal 
 photoheliograph are here recapitulated. 
 
 The houses may be set up on stakes 4" square, driven firmly into the ground. 
 The upper surfaces of the floors should be 7" or 8" above the surface of the ground. 
 The distance from the upper surfaces of the floors to the bottoms of the sills is about 3 ". 
 
 Size of transit pier: below ground, 3' or 3' 6" square; above ground, 2i"x2$". 
 Capstone for ditto, 24" x 28" x 3". Top of pier above floor, 29". Eye-piece of transit 
 above floor, 3' 9^". 
 
 The iron piers for the photoheliograph are 8' o" long. The larger of the two 
 carries the objective. 
 
 Distance from center of transit pier to center of pier for photographic objective, 
 14' o". 
 
 Photographic house: Floor 8 J" lower than floor of transit house. Distance from 
 inner side of wall of house to center of plate-holder pier, 1 2". The top of this pier 
 is 9^" above top of transit pier, and 3' lof" above floor of photographic house. Cen- 
 ter of plate-holder above floor, 4' 6". 
 
 In TABLE I, the quantities on each line refer to a single photoheliograph. The 
 number of the objective is given in column A. The distance between the back sur- 
 face of the objective and the sensitive surface of the photographic plate is given in 
 column B; it being assumed that the thickness of the reticule plate is 0.25 of an inch. 
 The distance of the back surface of the objective from its second principal point is 
 given in column C. The distance between the centers of the piers which carry the 
 objective and plate-holder is given in column D. The height of the top of the objective 
 pier above the top of the transit pier is given in column E. The number of the meas- 
 uring-rod to be used with each objective is given in column F. The length of each 
 measuring-rod, at 62 Fahrenheit, is given in column Gr. 
 
 TABLE I. 
 
 A. 
 
 B. 
 
 C. 
 
 D. 
 
 E. 
 
 F. 
 
 G. 
 
 I 
 
 Inches. 
 
 462.51 
 
 Inch. 
 0.711 
 
 Inches. 
 477.6 
 
 Inches. 
 10.4 
 
 VIII 
 
 Inches. 
 
 45I-49' 
 
 2 
 
 465.08 
 
 .682 
 
 
 . 
 
 VII 
 
 453-49S 
 
 3 
 
 462.27 
 
 .674 
 
 476.9 
 
 9.4 II 
 
 450-437 
 
 4 
 
 463.33 
 
 .718 
 
 478.8 
 
 10.4 VI 
 
 451-946 
 
 5 
 
 464.79 
 
 538 
 
 479-9 
 
 10.4 
 
 III 
 
 453.488 
 
 6 
 
 472.90 
 
 .708 
 
 488.5 
 
 10.4 
 
 I 
 
 461.425 
 
 7 
 
 461.20 
 
 .644 
 
 476.4 
 
 10.4 
 
 v 
 
 449-485 
 
 8 
 
 461.30 
 
 0.708 
 
 476.0 
 
 9.4 
 
 IV 
 
 450.357 
 
 The expansion of the measuring-rods may be taken as 0.0000070 of their length 
 for one degree Fahrenheit. 
 
14 INSTRUCTIONS FOR OBSERVING Till' 
 
 V. ADJUSTMENTS OF THE mOTOITELIOGRAPH. 
 
 The photoheliograph must fulfill the following conditions: 
 
 1. The sensitive surface of the photographic plate must be at the focus of the 
 objective. 
 
 2. The line joining the optical center of the object-glass and the cross-lines in the 
 middle of the reticule plate must be in the true meridian, within a fraction of a minute 
 of arc. 
 
 3. The same line must be horizontal, within the same limits. 
 
 4. The optical axis of the objective must be directed toward the center of the 
 reticule plate. 
 
 5. The reticule plate must be perpendicular to the line joining its center and the 
 center of the objective. 
 
 6. To let the plumb line hang freely, the sides of the plate-holder must be verti- 
 cal; and that will be attained by making its top level. 
 
 These adjustments are made as follows: 
 
 1. By the aid of the measuring-rod, set the coffin-plate so that the distance between 
 the back surface of the objective and the sensitive surface of the photographic plate is 
 as stated in column B of TABLE I. To guard against errors, after the adjustments i 
 to 4 have been made, remove the brass plate-holder and use the wooden one to take a 
 number of photographs of the sun at different distances inside and outside the focus. 
 The points at which the small spots on the sun begin to disappear when the plate is too 
 far in, and again when it is too far out, must be noted. The mean of the two positions 
 is the true focus. If, after repeated trials, it differs more than one-quarter of an inch 
 from the point found by measurement, the position of the objective must be changed 
 accordingly. 
 
 2. When the photographic objective is in position, point the telescope of the 
 transit instrument at it, and set a bull's-eye lantern behind the center of the reticule- 
 plate. The lines upon the latter will then be visible through the transit, but not very 
 distinctly, because the photographic focus of the photoheliograph differs considerably 
 from its visual focus. If there is any difficulty in seeing and identifying the intersection 
 of the central lines, gum a small triangular bit of paper upon the reticule-plate with 
 one of its angles at the point in question; or rule a little cross with ink, making its 
 lines not more than half an inch long, and taking care that they coincide exactly with 
 the etched lines of the plate. If the a/hmith and collimation of the transit are quite 
 right, its middle vertical wire should be on the middle vertical line of the reticule- 
 plate. If it is not found so, the error must be corrected by moving the brass cross 
 which carries the plate-holder. 
 
 3. Point the transit so that its middle horizontal wire accurately coincides with 
 the image of the middle horizontal line of the reticule-plate, and clamp it firmly in that 
 position. Then .-H ii] a carefully adjusted engineer's level between the transit house 
 ;unl the photographic objective, point it into the latter, and bring its horizontal wire 
 into accurate coincidence with the image of the middle horizontal line of the reticule- 
 plate. Read the bubble of the level, and if it is within six or eight divisions of the 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 15 
 
 middle of its scale the height of the plate-holder is probably satisfactory. To make 
 sure of this, point the level at the transit, bring its horizontal wire into exact coinci- 
 dence with the middle horizontal wire of the latter, and again read its bubble. Half 
 the distance traveled by the bubble between the two readings will be the error of level 
 of the center of the reticule-plate. 
 
 Probably at the first trial the bubble will run all the way to one end or the other 
 of its tube. If, at each pointing, it runs to the end nearest the photographic house, 
 the reticule-plate is too high; but if to the end nearest the transit house, the reticule- 
 plate is too low. In either case the error must be corrected by changing the elevation 
 of the plate-holder, or by changing the elevation of the coffin-plate, or by both. The 
 height of the plate-holder is controlled by the adjusting-screws of the iron disk sup- 
 porting it, and the altitude of the coffin-plate may be modified by increasing or djmin- 
 ishing the number of washers under it. 
 
 If instead of an engineer's level a level of precision is employed, its telescope 
 must be pointed at the center of the reticule-plate, and its bubble must be read, 
 reversed, and read again. Let the difference of these two readings be A. Then the 
 telescope must be rotated aboiit its optical axis through half a revolution, once more 
 pointed at the center of the reticule-plate, and, as before, the bubble must be read, 
 reversed, and read again. Calling the difference of this last pair of readings B, the 
 error of level of the center of^the reticule-plate will be \ (A+B). The object in 
 rotating the telescope through half a revolution is to eliminate its collimation-error. 
 
 4. After the plate-holder is fixed in its true position, adjust the objective by its 
 three supporting-screws so that if a candle in the photographic house is held in the 
 line passing through the centers of the objective and reticule-plate, its three reflections 
 from the objective will also lie in the same line. Instead of a candle, it is sometimes 
 convenient to use a reflector consisting of a card-board disc two or three inches in 
 diameter, witli a hole one-quarter of an inch in diameter through its center. In that 
 case, the three images of the hole reflected from the objective must lie in the line in 
 question. 
 
 5. Adjust the reticule-plate so that if a candle in the photographic house is held 
 as far as possible from it, and in the line passing through its center and the center of 
 the objective, the reflection of the candle from the reticule-plate will also lie in the 
 same line. Here, again, a card-board reflector may be used instead of a candle. The 
 adjustment is made in altitude by the screws supporting the iron disc which carries 
 the plate-holder, and in azimuth by turning the latter on its vertical axis. When all 
 is right, the screws confining the plate-holder must be firmly turned home. 
 
 If this adjustment is correctly made the surfaces of the reticule-plate will be 
 vertical. This may be independently tested in two ways: 
 
 (). Set the engineer's level outside the house, at the height of, and near the cen- 
 tral line joining the objective and plate-holder, in either direction from the latter, and 
 at such distance from it, not less than 1 5 or 20 feet, as will be favorable for the obser- 
 vation. Point the level at the plate-holder, turn the latter so that a horizontal line 
 'upon the surface of the reticule shall be perpendicular to the line from the level, and 
 adjust the focus of the level so that its objective can be seen by reflection from the 
 
1C, IN8BUGTION8 FOR OBSERVING THE 
 
 reticule-plate. If the plate is vertical, and the level properly adjusted, the horizontal 
 wire of the level will bisect the reflected image of the objective. The latter should 
 return to its position when the plate-holder is turned half-way round on its axis, so 
 that the reflection takes place from the other surface. 
 
 (ft). Adjust the base of the plate-holder so that the bubble of a level set upon 
 its top shall not vary greatly in position when the holder is turned on its axis. 
 
 If these two tests cannot be both satisfied within one or two minutes of arc by 
 the same adjustment, the chief should endeavor to ascertain what is wrong, though it 
 may not be advisable for him to try to correct it. 
 
 6. The verticality of the sides of the plate-holder is controlled by the supporting 
 screws of the iron disk which carries it. An ordinary carpenter's level suffices to show 
 whei^ the adjustment is correct. 
 
 The adjustments from i to 6 are necessarily made consecutively, and they must 
 be gone over a second time to make sure that the later ones have not disturbed those 
 first established. 
 
 VI. TIIE HELIOSTAT. 
 
 As the heliostat will presently be needed, it should now be set in front of the 
 objective, upon the coffin-shaped plate, and the three adjustments which it requires 
 should be made. They are as follows: i. Its main^ixis must be brought into the 
 plane of the meridian; 2. Its main axis must be set at that inclination which will 
 keep the sun's image most nearly at a constant height upon the reticule plate; 3. The 
 driving-clock must be arranged to rotate the main axis at the proper speed. Extreme 
 accuracy in these adjustments is superfluous, because, in order to obtain freedom from 
 vibration, a form of heliostat has been adop'ted which can throw the sun's rays only 
 approximately in a constant direction. A convenient way of proceeding, and one 
 which will probably give as good results as any, will be to make the first adjustment 
 by estimation, using a ruler laid against the cube of the main axis to aid the judgment; 
 and then to effect the second and third adjustments by trial. 
 
 If more exactness is desired, the first adjustment may be made by setting the 
 mirror at right angles to the main axis of the heliostat, and then setting the latter so 
 that the mirror is also at right angles to the axis of the photographic telescope. The 
 setting of the mirror is accomplished when the direction of a ray reflected from its first 
 surface is not affected by rotating the main axis; but in applying this test care must 
 be taken not to mistake the ray reflected from the second surface for that from the 
 first. Then the main axis is brought into the plane of the meridian, and made hori- 
 zontal, by setting the heliostat so that it will reflect back upon itself the light from a 
 candle held at the center of the reticule-plate. The proper inclination for the main 
 axis may be computed, and the axis can he set by means of a clinometer; but the 
 rating of the driving-clock can only be effected by trial. 
 
 Whenever tin- a/imuth and level of the center of the reticule-plate are determined, 
 the heliostat will have to lie removed from the coffin plate: and to save trouble in t 
 returning it to its proper place, the points where its feet rest should be marked. 
 
TRANSIT OP VENUS, DECEMBER 6, 1882. 17 
 
 The driving-clock is provided with three pulleys, whose time of revolution may 
 be varied from 37.8 seconds to 42.3 seconds by raising or lowering the pendulum bob. 
 The screw of the heliostat carries two wheels, either of which can be connected by a 
 leather band to any one of the driving-clock pulleys; and by making suitable combi- 
 nations, the screw can be driven at any desired speed between the limits 58.8 seconds 
 and 115.0 seconds per revolution. This suffices for all localities. Sometimes the 
 pendulum of the driving-clock takes a wabbling motion, moving in an ellipse instead 
 of a circle. When this happens it is generally occasioned by friction at the point of 
 suspension, and a little oil will remedy the difficulty. 
 
 VII. THE EXPOSING SLIDE. 
 
 The frame carrying the exposing slide must be screwed to the inner surface of 
 the wall of the photographic house in such a position that the line joining the centers 
 of the objective and reticule-plate passes through the center of the opening in the 
 frame. Upon each end of that surface of the slide which is nearest the objective 
 a target is painted, and whenever the slide is brought into contact with the pieces 
 which limit its motion, one other of these targets covers the aperture in the frame, 
 arid is visible to a person standing at the objective. If the image of the sun given by 
 the latter is then centered upon the target, it is intended that it shall also be found 
 centered upon the reticule-plate when the slide is moved across the opening in the 
 frame. To secure this result, tfie adjustments of the photoheliograph should be com- 
 pleted before the frame is put up, and special pains should be taken to fix it exactly 
 in its right position. 
 
 By means of the six milled-headed screws upon the slide, the brass plates can be 
 set so as to give any desired width of slit; but in doing this care must be taken to 
 keep the center of the slit coincident with the center of the opening in the slide, 
 because the automatic key for recording the instant of exposure upon the chrono- 
 graph is arranged to break when the centers of the openings in the slit and its frame 
 coincide with each other. It is sometimes desirable to see the entire image of the 
 sun upon the reticule-plate, and the slit is arranged to open wide enough for that. 
 
 VIII. THE TUBE. 
 
 Experience has shown that for a photoheliograph of thirty-eight and a half feet 
 focus twelve feet of tube is sufficient. For convenience of transportation, that fur- 
 nished to the parties is slightly conical, and' in two-feet lengths packed inside each 
 other. In mounting the tube, four points require attention, namely: i. The largest 
 end of the tube should pass snugly through the wall of the photographic house and 
 rest against the back of the exposing-slide frame. 2. The centers of the openings in 
 all the diaphragms should be in the straight line joining the centers of the objective 
 and reticule-plate. 3. From the reticule plate nothing but black surfaces should 
 be visible. The presence of white light risks fogging the photographic plates, and 
 therefore this condition is imperative. To fulfill it, that one of the frames carrying the 
 measuring-rod which is nearest the objective must have its upper part boarded over 
 so as to shut out extraneous light from the tube. Both surfaces of this screen should 
 3 INS 
 
18 INSTRUCTIONS FOR OBSERVING THE 
 
 be colored dead black, and a bole must be cut through it just large enough to permit 
 the free passage of rays from all parts of the objective. 4. It is absolutely necessary 
 that both the tube and the measuring-rod be thoroughly protected from the sun's rays. 
 As there must be an air-space of from six to twelve inches around the tube, the root' 
 or awning covering it should have a depth not less th;in three and one-quarter feet. 
 Beyond the termination of the tube, the covering for the meairaring-rod may be formed 
 of three boards, each eight inches wide, put together so MS to form a kind of inverted 
 trough. 
 
 IX. THE PLATE-HOLDER PLUMB-LINK. 
 
 Both the chief of party and the chief photographer must give special attention 
 to the plate-holder plumb-line. The wire employed is of gilded brass, having a length 
 of about three feet and a diameter of 0.0025 f an inch. It must be without any 
 bends or kinks, must pass perfectly freely through the axis of the plate-holder, and 
 must be loaded with one-fifth of its breaking weight To secure steadiness, the weight 
 must hang in a vessel of water within the pier, care being taken that the vessel 
 has sufficient size and is so placed as to avoid any risk of the weight resting against 
 its bottom or sides. The upper extremity of the plumb-line is wedged into a brass 
 piece which fits into a socket in the top of the plate-holder, and is provided with an 
 arm for rotating the wire through half a revolution, so as to eliminate the effect of any 
 undetected kinks or bends. Aside from this motion of rotation, the plumb-line should 
 be disturbed as little as possible. 
 
 The chief of party must satisfy himself from time to time that the plumb-line 
 does not come into contact with anything in its passage through the opening in the 
 axis of the plate-holder. < >nc way of testing this will be to let the bob swing through 
 a minute arc, and see that the swinging motion of the suspending wire across the face 
 of the reticule plate, as viewed with a magnifying glass, is perfectly regular. 
 
 X. BATTERIES AND ELECTRICAL CONNECTIONS. 
 
 The batteries furnished to the parties are of the Daniell's gravity form. To set 
 up a cell, unfold the copper element; place it, together with about two pounds of 
 coarsely-powdered copper sulphate, in the bottom of the glass jar; and lead the insu- 
 lated copper terminal out over the top of the jar. If the cell is desired for immediate 
 use, fill it to within one and a half inches of the top with water in which a little zinc 
 sulphate has been dissolved; and suspend the zinc element in this solution by hooking 
 it to the top of the jar. If the cell is not required immediately, it is better to fill it 
 with pure water, and then to connect the copper terminal with the /.inc. In a few 
 hours it will be in good working order. When using these cells care must be taken 
 that the zinc sulphate solution does not become too strong, that it covers the zinc ele- 
 ment, and that there are always somouopper sulphate crystals in the bottom of the jar. 
 
 No stronger cm-rent than that from a single Daniell's cell should ever be passed 
 through a break-circuit chronometer; and as that will not usuallv suffice for working 
 a chronograph, the chronometer must be joined up in circuit with one cell of battery, 
 a single-point switch, and a repeater of about six ohms resistance. The break-circuit 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 19 
 
 points in the chronometer are very delicate, and to diminish the spark at them the lid 
 of the chronometer box contains a condenser which should be included in the circuit. 
 The chronograph must be joined up in a second circuit, passing through the points of 
 the repeater, and including the observing key at the transit instrument, the automatic 
 key of the exposing slide in the photographic house, and such a number of battery 
 cells as may be necessary. Thus the chronometer circuit will control the chronograph 
 circuit, and by opening the switch in the former, both circuits will be opened. It will 
 be well to arrange the wires in such a way that the automatic key in the photographic 
 house can be cut out when not in use. As a rule, only break-circuit signals will be 
 used, but the observing keys furnished to the parties have a screw at the back, by 
 shifting which they can be converted into make-circuit keys if desired. 
 
 XL THE MEASURING ROD. 
 
 The most convenient way of supporting the measuring-rod will be to pass it 
 through holes one and one-quarter inches in diameter in boards nailed across the tops 
 of A -shaped frames placed not more than six feet apart. Whatever mode of support 
 is adopted, special care must be taken to see that the rod lies perfectly straight, and 
 that it is parallel to the optical axis of the photographic telescope. 
 
 The rod is of wrought-iron gas-pipe, eight-tenths of an inch diameter, in sections 
 five feet long, with Arabic numbers at the joints to show how they go together. The 
 Roman numbers in the middle of each section are the number of the rod. The ends 
 of each section must be carefully cleaned with an oily rag before they are united, 
 and in screwing them together their shoulders must be made to meet snugly. Marks 
 at each joint show approximately the point of stopping. If these marks fail to come 
 together, there is dirt in the joint ; but if they pass each other slightly, it only indicates 
 that the screw is worn, which is of no consequence, because the length of the rod 
 depends upon the condition of the shoulders and not upon that^of the screws. To 
 avoid straining the rod in putting it together, the first two sections should be united 
 and passed on to the supporting frames nearest the objective; then the third section 
 should be added, and the rod should be pushed nearer the photographic house; and 
 so on till it is all together, and in place. The end inside the photographic house 
 should be at the same distance from the front surface of the reticule plate that the 
 end outside is from the back surface of the objective. After being mounted, the rod 
 should not again be disturbed till the party is about to leave the station. Its outer 
 extremity should be slightly greased to prevent it from rusting, and must be protected 
 from'the weather by the tin cover furnished for that purpose. 
 
 The measuring-rod is used in connection with the jaw-micrometer for determining 
 the interval between the back surface of the objective and the front surface of the 
 reticule plate. To do this, three thermometers must first b.e placed 'upon* the rod to 
 ascertain its temperature, one; being at its center and one near each end. Then a 
 plumb-line, consisting of a brass wire 0.0032 of an inch in diameter, must be hung 
 over the outer end of the rod, the bob of the line hanging in ;i vessel of water, and 
 being protected from wind by complete immersion in the fluid. Care must be taken 
 that the wire bends sharply over the end of the rod, and is in actual contact with its 
 
20 INSTRUCTIONS FOR OBSERVING THE 
 
 front surface. When the plumb-line has come to rest, press the arms of the jaw- 
 micrometer against the margin of the object-glass, taking great care to hold the 
 micrometer horizontally, and by the two rack motions bring the end of the central 
 arm gently into contact with the back surface of the objective, and the jaws into such 
 a position that the plumb-line is between them and in the line joining the centers of the 
 pin-holes. In adjusting the position of the pin-holes relatively to the plumb-line, it 
 will be advantageous to use a magnifier of low power. When all the adjustments arc 
 correct, read and record the vernier of the micrometer. After making several such 
 measurements at the objective, pass to the plate-holder and make similar measure- 
 ments from the front surface of the reticule plate. Each measure should be repeated 
 a number of times by different observers, and each separate result should be recorded 
 with the name of the observer, the temperature of the rod, and any other necessary 
 particulars. 
 
 Should the rod be too long, file a vertical notch on each side near one end, loop 
 the plumb-line, and let it hang in these notches. 
 
 The distance from the front surface of the reticule plate to the position of the 
 collodion film must also be measured as accurately as possible. 
 
 XII. INSTRUMENTAL ERRORS. 
 
 When the adjustments of the photoheliograph designated 1,4, 5, and 6, in sec- 
 tion V, have once been made, it is expected they will remain sufficiently exact, unless 
 purposely disturbed. If, by any chance, either 4 or 5 is found wrong, it must be cor- 
 rected. The errors of i, 2, and 3 must be determined at least twice a week, but 
 unless they become large they need not be corrected. 
 
 To find the error of the first adjustment, the distance from the back surface of 
 the objective to the front surface of the reticule plate, nmsr be measured with the 
 greatest care at times when the temperature is not varying rapidly. Directions tin- 
 doing this are given in the preceding section. 
 
 The error of the second adjustment must be found as follows : K very evening 
 before beginning work with the transit instrument, place its eye-piece at the same side 
 of the stand as the azimuths! adjusting screws, and by means of the latter, bring the 
 middle vertical wire of the transit into exact coincidence with the image of the middle 
 vertical line of the reticule plate. After this, the a/imuthal adjustment must remain 
 undisturbed during the night's work. Then make a set of time observations; that is, 
 observe two azimuth stars above the pole, two below the pole, and six or eight time 
 stars, one half the observations of each class being made with clamp east and the 
 other half with clamp west. At the close of the night's work again bring the eye- 
 piece to the same side of the stand with the a/imuthal screws, and examine the posi- 
 tion of the middle wire relatively to the image of the middle vertical line of the 
 reticule plate. If there is* any deviation, its amount must be estimated and recorded, 
 but the a/hmith of the transit must not be disturbed. 
 
 l'|)"" reducing the time observations the ax.imuth and collimation constants of 
 the tran-it will become known. Let them be <i and r, and let the azimuth of the cen- 
 ter of the reticule plate, counted fn.in the meridian toward the left, be A. Then 
 
 A = - + r) 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 21 
 
 There will be two values of c, one for clamp east and the other for clamp west. 
 The one to be used is that corresponding to the position of the instrument when its 
 middle wire was set upon the center of the reticule plate. If A exceeds one second 
 of time for several nights, the plate-holder must be adjusted by moving it one-thirtieth 
 of an inch for each second of error. 
 
 The method of determining the error of the third adjustment has been already 
 described in section V. 
 
 XIII. FITTINGS OF THE PHOTOGRAPHIC HOUSE. 
 
 The photographers will examine the photographic house as soon as it is erected, 
 and see that all white light is perfectly excluded; A single crack in the wall, or even 
 an unprotected keyhole may cause irreparable mischief. Such openings are readily 
 detected from the inside, and when found may be stopped up or covered with yellow 
 paper or other suitable material. In full daylight the orange-glass window will ad- 
 mit too much light, and must be covered with orange envelope paper. 
 
 The emulsion chest, drying box, and plate boxes are also to be examined, and 
 any cracks or other openings closed up. The drying box may be screwed to battens 
 fixed against the side of the house, or, if more convenient, it may be mounted on legs. 
 To facilitate cleaning, the top of the box lifts off, and the bottom may be removed by 
 taking out a couple of screws. 
 
 Close to the left-hand side of the pier carrying the plate-holder a stand large 
 enough to support a chronometer and the blank form for recording the exposure of 
 plates will be required. A shelf long enough to hold two plate-boxes, side by side, 
 will be fixed to the wall of the dark room close to the right-hand side of the pier, and 
 at a convenient height for taking plates from, and returning them to, the boxes. 
 
 A screen should be interposed between the plate-holder and the boxes on the shelf, 
 so that if the slide were moved while the boxes were open (which ought never to hap- 
 pen) no light could fall on the plates. That part of the opposite wall which receives 
 the sunlight passing through the plate-holder should be blackened, or covered with 
 dark-colored cloth, so as not to reflect light about the room. 
 
 If external objects illuminated by direct sunlight are visible from any part of the 
 plate-holder in any position of the slide, foggy plates and blurred images may be ex- 
 pected, and under circumstances requiring long exposures any light-colored surface, 
 exposed to radiation from the sky may produce similar effects. 
 
 An abundant supply of good water is indispensable. For developing dry plates 
 almost any well, spring, or river water may be used, even if it contains rather large 
 quantities of some salts and is not entirely free from organic matter. If it should be 
 necessary to prepare silver baths, water must be distilled if it cannot be otherwise 
 obtained of sufficient purity. 
 
 XIV. CARE OF THE SENSITIVE EMULSION. 
 
 The sensitive collodio-bromiile emulsion is contained in bottles of orange glass, 
 only partially filled to facilitate efficient shaking. After standing undisturbed for a 
 long time the silver bromide partly subsides to the bottom of the bottle, but it may be 
 perfectly re-emulsified by agitation. By inverting the bottle and looking through its 
 
22 INSTRUCTIONS FOR OBSERVING THE 
 
 bottom toward the dark-room window, a more or less abundant deposit will be seen, 
 Mini the shaking should be persevered in until this is entirely broken up and washed 
 away. When this has once been thoroughly effected the bromide will be easily kept 
 in suspension by a little shaking at intervals of a day or two. Collodion vials con- 
 taining emulsion should always be well shaken and then allowed to stand a few minutes 
 
 before coating plates. As issued it contains a certain amount of coarse sediment 
 
 ' * 
 
 which must be removed by filtration. 
 
 The emulsion must not be exposed to white light, for although the bottles con- 
 taining it are of colored glass they cannot be absolutely relied on to protect it from 
 injury. It is true that some collodion emulsions may be submitted for a slfort time 
 to the action of weak daylight without material deterioration, but in other cases only 
 foggy images can be obtained after such treatment. Additional caution is requisite 
 with emulsion that has been transferred to collodion vials of colorless glass. The 
 greatest care must also be taken not to contaminate the emulsion by flowing it upon 
 glass that is not perfectly clean, or by putting it into bottles that have been exposed 
 to light with traces of emulsion adhering to them. 
 
 XV.- SELECTING AND MARKING GLASS. 
 
 The photographic operations will be begun by examining the stock of glass and 
 rejecting all plates that are broken or cracked, not sufficiently flat, with a rough or 
 uneven surface, too large to enter the grooves of the plate-boxes easily, or having any 
 corner so short that it will not rest securely on the pins in the plate-holder of the 
 photoheliograph. A sufficient quantity of the best glass, not less than 204 plates, is 
 next to be selected for the transit plates, and after choosing the best surface for the 
 front, or film side, each plate is to be marked in one corner of the back with a number, 
 beginning with nnity and proceeding consecutively upward. This is to be done 
 neatlv and legibly with a writing diamond, and to facilitate* reference to the finished 
 photographs when stored in plate-boxes the numerals should be so placed as to be 
 upright in the right-hand upper corner of the plate. 
 
 xvi. CLEANING AND ALIUTMENIZING GLASS. 
 
 Remove the sharp edges with a whetstone, and clean the glass from any gross 
 impurity that mav lie adhering to it. Old films are best got rid of by soaking for a 
 few hours in a moderately strong solution of concentrated lye, alter which they can 
 generally be washed away without much labor; but the surface of the glass may be 
 injured by leaving it too long in such a solution, and especially by allowing the latter 
 to dry upon the plate. 
 
 The chromic solution recommended bv Mr. M.('. Lea is best adapted to the 
 circumstances of the transit of Venus photographers, as it rapidly oxidi/.es organic^ 
 impurities and gives off no vapor injurious to instruments. 
 
 Solltlioit fur Cli/llilil/l (i/tlsN. 
 
 liichromate of potash .) ounces. 
 
 Sulphuric acid 6 fluid ounces. 
 
 Water - 50 fluid ounces. 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 23 
 
 Put the bichromate of potash into a two-quart bottle and pour in the water. 
 Then add (out of doors) the sulphuric acid, a little at a time, shaking the bottle Veil 
 and allowing it to stand a few minutes after each addition. It Avill become very hot 
 and give off a little corrosive vapor, which, however, soon disappears. If too much 
 acid is added at once the bottle will probably be broken by the heat. When the 
 bichromate of potash is all dissolved, and the solution has cooled, it is ready for use. 
 Pour it into a large rubber pan, and immerse, one at a time, as many plates as the 
 solution will cover. If any bubbles are allowed to remain between the plates they will 
 prevent the solution from acting on the surfaces in contact with them. The glass 
 should remain in the chromic solution through one night at least, and a still longer 
 time may be desirable, but it ought not to exceed a week, or thereabout. 
 
 Remove the plates from the cleaning solution and put them into a pan or bucket 
 of clean water. The solution may be repeatedly used ; it will not injure the skin 
 beyond causing a slight stain arid a rather disagreeable odor, but it is very destructive 
 to clothing. Renew the water in the bucket, and separate the plates from each other 
 until the yellow fluid adhering to them is entirely washed away; then refill the bucket 
 so as to leave them entirely covered with clean water. 
 
 The plates are now to be taken one at a time from the bucket and rubbed 
 with the fingers, or with a clean cloth, on both sides and around all the edges while 
 the water from the tap flows on them. After a final rinse they will be ready for albu- 
 ineriizing. 
 
 Albumen Solution 
 
 White of egg i fluid ounce. 
 
 Water 16 fluid ounces. 
 
 Strong ammonia 15 minims. 
 
 Put the white of egg into a clean bottle of convenient size together with a few 
 pieces of broken glass, shake it vigorously until the albumen is thoroughly " beaten," 
 and allow it to stand undisturbed for several hours. Then add the water and ammonia, 
 and shake just enough to mix the contents of the bottle, after which it should again 
 stand for some time before being filtered for use. If tolerably fresh eggs cannot be 
 obtained, 100 grains of dried albumen may be taken as the equivalent of i fluid ounce 
 of white of egg, but the latter is much to be preferred. 
 
 Filter the albumen solution into an 8-ounce graduate until it is two-thirds filled, 
 keeping the neck of the funnel under the surface of the filtered albumen to avoid 
 bubbles. The few bubbles that unavoidably form may be removed with a wisp of 
 clean paper. 
 
 When a plate has been washed arid rinsed as already described, observe by the 
 mark on the back which is the front side, and flow it with the albumen solution. 
 After draining the plate for a few seconds, flow it a second time precisely as before 
 and set it on a rack to dry, always keeping the same corner downward, and never 
 touching the front surface or the uppermost edges. The plates will be apparently dry 
 in the course of an hour or two after albumenizing, but the albumen should be allowed 
 to become thoroughly desiccated arid hardened by keeping in a dry place, properly 
 
24 INSTRUCTIONS FOB OBSERVING THE 
 
 protected against dust, for several days if possible. If coated with emulsion too soon 
 aftel- albumeniy.ing, the films will be more liable to blister and rise from the glass dur- 
 ing development, ;i misfortune to be avoided by the exercise of every possible precau- 
 tion. The albumeni/ed plates are finally to be examined, and any imperfect ones set 
 aside to be cleaned again. 
 
 As the permanent marks on the plates will scarcely be visible in the dark room, 
 write the number of each one with a blue pencil, or with ink, in large and plain figures 
 on a gummed label, and attach it to the back of the corresponding plate in such posi- 
 tion that when the latter is put into a plate-box the permanent mark will be seen in 
 the upper right-hand corner, and the number on the label in the upper left-hand corner 
 of the plate. Re-examine them carefully to insure that the two numbers on each plate 
 are identical, and, if found so, store them in plate-boxes for use as required. 
 
 XVII. COATING PLATES. 
 
 As the plates are necessarilv exposed for a considerable time to whatever light 
 may be in the dark room while they are being coated, it is necessary to proceed with 
 caution. But by operating near the window while the emulsion vials, and the rack hold- 
 ing the plates Already coated, are protected from direct light by interposing screens, 
 it is possible, to work with both ease and safety. The outer door must, of course, be 
 locked. In very warm weather it is more comfortable to coat plates at night with the 
 door and window open it' there are no gas lamps or other dangerous lights outside. 
 It will be safe to use naked candles if they are so screened that, no direct light from 
 them or from anv nearly white object strongly illuminated by them can fall on the 
 plates or the emulsion. Light from gas or coal-oil lamps is more actinic than that of 
 candles, and must be used with caution if at all. Orange-glass lanterns ought to be 
 (juite safe if candles are used in them, but it must not be taken for granted that t!ev 
 are so. 
 
 Set one of the racks from the drying box where no direct light from the window 
 can fall on it, and have at hand a wide camel's-hair brush, and a bottle containing a 
 mixture of two volumes of ether and one volume of alcohol for thinning the emul- 
 sion. Begin by thoroughly shaking the bottle of emulsion it is proposed to use ; then 
 insert a tuft of filtering cotton in the throat of a perfectly clean emulsion filter and 
 moisten it with a little of the thinning mixture. Set the filter in a twelve-ounce col- 
 lodion vial, pour in emulsion from the bottle until it is nearly full, put on the cover, 
 and set an orange bell-glass over the filter and vial to diminish evaporation and pre- 
 vent the access of light. If the tuft of cotton is too large and too closelv packed, 
 filtration will be slow and tedious; if not properly inserted it will permit the passage 
 n|' small particles of sediment, itc. The emulsion should pass through in a rapid suc- 
 cession of large drops, about one per second. It may run in a thin stream at first, as 
 its pMMge will be slower \\hen the cotton is fullv saturated with emulsion. Replen- 
 ish the filter from the emulsion bottle before it becomes empty, and transfer it to an- 
 other collodion vial when the first one is two-thirds full. While plates are being coated 
 from one vial the other will be filling, and so on, using them alternately. 
 
 Take a plate from the plate-box, and, holding it near the window, see if the 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 25 
 
 albumenized surface is free from fibers and particles of dust. Such substances may be 
 carefully removed with the brush, but actual brushing will leave marks on the albumen. 
 Holding the plate by the corner that was marked with the writing-diamond, coat it 
 with emulsion, precisely as if it were collodion, and, as soon as the film is set, put the 
 plate in the rack. As emulsion is, under ordinary circumstances, rather less fluid than 
 collodion it is possible that operators who are accustomed to use thin collodion, and 
 coat their plates very deliberately, may experience some difficulty in" obtaining a 
 uniform film. Pour on a rather liberal dose of emulsion and cover the plate quickly; 
 then incline the plate but little, so as to pour off very slowly, and impart to it whatever 
 motion may be necessary to prevent the formation of lines. The precautions usual 
 with careful operators to prevent particles of dried emulsion on the lip of the vial from 
 falling upon the plate must, of course, be observed. It is sometimes recommended to 
 pour from one vial and to drain the plate into another, so that emulsion once poured 
 out may be filtered before it is used again; but this mode of operating is inconvenient, 
 and will not be necessary unless the atmosphere of the dark room is charged with 
 dust. The corner by which the plate was held while coating with emulsion is, of 
 course, left uncovered; by looking toward the window through this clear space the 
 permanently marked number of the plate may be read in the dark room, though with 
 some difficulty. When the emulsion becomes too thick to flow well (or sooner if 
 particles of foreign matter are seen in it) add as much of the mixture of ether and 
 alcohol as may be required to bring it to the proper consistency, pour the whole into 
 the emulsion bottle, and shake it until well mixed. Then transfer the filter to the 
 empty vial, and begin using the full one. The filter must not be allowed to become 
 empty. It' the vial containing it is getting too nearly full it may be set in the emul- 
 sion bottle. Do not dilute the emulsion unnecessarily by using the thinning mixture 
 too 'freely. 
 
 When the rack is full of plates remove it to the highest unoccupied position 
 in the drying-box, and avoid all further risk of accident to them by closing and bolt- 
 ing the doors. The box will contain \~2() plates, but it is better to make a smaller 
 number at one operation, filling only every second or third groove of each rack. 
 When a sufficient number of plates are prepared return the bottle and vials to the 
 emulsion chest, and wash the filter perfectly clean before the adhering emulsion 
 becomes dry and hard. 
 
 Plates freely exposed to the air of the dark room would be dry in an hour or 
 thereabout; in the drying-box a longer time will be required, and several hours at 
 least should be allowed. At night the- top and bottom of the box may be taken off- 
 When quite dry the plates are to be put in plate-boxes that are clean and free from 
 dust, in the order of their numbers, with the films toward the back or hinge side of 
 the boxes, the numbered corners uppermost, and the lowest numbers in front. Wrap 
 each box in thick paper, secure it with stout twine, and mark plainly on the package 
 the numbers of the plates it contains. 
 
 AVith each batch of plates prepared for observing the transit at least two test- 
 plates are to be made, one near the beginning and one near the end of the operation, 
 using for that purpose some of the glass not selected for the transit work. The test- 
 4 INS 
 
26 INSTRUCTIONS FOE OBSKK'VING THE 
 
 plates are to be dried with the others and afterward exposed and developed. If they 
 prove to be good the rest of the lot is likely to be equally so. 
 
 \\iii. KxrosriM-; OK IM,ATI:S IN THK 
 
 The sun's image, as seen on the reticule-plate of the photoheliograph, generally has 
 an irregular vibrating motion in different directions, arising from various causes. As the 
 exposure of different parts of the plate? lying in the direction of motion of the exposing 
 >lide is not simultaneous but successive, any movement of the image as a whole will pro- 
 duce a distortion in the resulting photograph symmetrical with respect to its vertical 
 diameter, and inversely proportional to the velocity of the slide. Moreover, the limb 
 itself is in a .Mate of constant and rapid local agitation, which in some conditions of the 
 atmosphere is so exaggerated as to produce the boiling or Haming appearance familiar 
 to all observers. It results from this that, quite apart from any distinctively photo- 
 graphic effect, the longer the exposure the larger the photograph will be, and that it 
 the motion of the slide is not uniform the limb on the side where the velocity was 
 least will be extended more than the other. So brief is the time of exposure that these 
 effects are indeed minute, but it cannot lie assumed that the\ are in all cases inappre- 
 ciable. It follows, therefore, that a given exposure is more advantageously made with 
 a wide opening of the slit and a rapid motion of the slide than, with a narrow opening 
 and a slow motion, and that the velocity should be in all cases as nearly uniform as 
 possible. 
 
 It is expected that the chief astronomer will himself expose the plates on the day 
 of the transit, or, at least, that he will supervise the manipulation of any person to 
 whom this duty is delegated, and see that such assistant lias the requisite instruction 
 and previous piactice. The movement of the slide should be as rapid as can lie given 
 with ease and uniformity, retaining control of it throughout, and avoiding any approach 
 to violence. Toward the end of its course the motion should be slackened so as not 
 to endanger the stops, but it should continue until the slide rests against them. As the 
 regulation of exposure depends entirely on the uniformity of this movement, it should 
 be practiced until a fixed habit is acquired. It is of course important that there should 
 be no great difference in velocity, whether the motion is from east to west or from 
 west to east. To increase the exposure, widen the slit by separating the sliding plates; 
 to diminish it, bring them nearer to each other: always setting them so that the center 
 of the slit shall be in line with the middle pair of screws. The milled nuts must be 
 screwed up so as to clamp the plates securely without using too much force. 
 
 In exposure considerable latitude is allowable; indeed the appearance of the sun's 
 image is so similar with very different exposures that the real importance of this 
 element is liable to be underrated. The planet requires less exposure than the sun's 
 limb, but good definition of the latter is the principal end to be attained. If with 
 chemicals in good order and proper development the image comes out reluctantly and 
 remains very thin, or if there is any material falling oil' in density near the sun's limb, 
 the exposure i> too short. To find the correct exposure, begin with one known to be 
 sufficient, and gradually lessen it until signs of under-exposure just begin to appear. 
 If the image is diMorted. or looks as if the plate-frame were out, of focus, the defect is 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 27 
 
 most probably due to flexure of the mirror. The slightest tension arising from 
 improper mounting, either of lens or mirror, will make itself apparent in this way. 
 
 XIX. THE DEVELOPMENT. 
 
 The water used for washing the plates before development, and the developer itself, 
 should have a temperature of at least 60 Fahr.; 90 to 100 is still better. If the 
 weather at any of the northern stations should be cold, it will be well to have a small 
 supply of warm water at hand for these purposes. 
 
 The following solutions will be required : 
 
 Alcohol and tannin. 
 
 Tannin - - - 20 grains. 
 
 Strong alcohol i fluid ounce. 
 
 Water i fluid ounce. 
 
 Pyrogallic solution. 
 
 Pyrogallic acid 3 grains. 
 
 Water - i fluid ounce. 
 
 This solution is decomposed by keeping, and only so much must be made at one 
 time as can be used immediately. 
 
 Dilute ammonia. 
 
 Strong ammonia 30 minims. 
 
 Water - i fluid ounce. 
 
 Bromide solution. 
 
 Bromide of potassium 20 grains. 
 
 Water i fluid ounce. 
 
 To these may be added : 
 
 Alkaline citrate solution. 
 
 Citrate of ammonia 30 grains. 
 
 Strong ammonia 30 minims. 
 
 Water i fluid ounce. 
 
 * 
 
 The dilute ammonia, bromide, and alkaline citrate are to be transferred for use 
 to dropping-bottles, so conspicuously labeled that they can be easily distinguished 
 from each other in the dark-room. It must be borne in mind that a drop from one of 
 the tubes is much smaller than one from the lip of a bottle. 
 
 Having put one fluid ounce of pyrogallic solution intooneof the small, wide-mouthed 
 bottles issued as developing glasses, begin by treating the film with alcohol and tannin. 
 One principal object of this application is to harden the substratum of albumen. The 
 solution should therefore be flowed back and forth over the plate for at least a minute, 
 
28 INSTRUCTIONS FOR OBSERVING THE 
 
 and be returned to the bottle when the film is thoroughly saturated. The manipulator 
 is next to be wetted and applied to the back of the plate unless the operator, fearless 
 of stained fingers, prefers.to dispense with it. Wiisli the plate under the tap, or with 
 warm water if necessary, until the \vator flows smoothly over the film. Then add two 
 drops of bromide solution and two drops of dilute ammonia to the pyrogallic solution in 
 the developing glass and apply it to the plate, keeping it in gentle motion over the film to 
 p'r< unote equal development. The image should appear quickly and gradually increa sc 
 in strength. Subsequent additions of dilute ammonia are to be made, a couple of 
 drops at a time, as the appearance of the plate may indicate, accompanying every 
 alternate addition with an equal quantity of bromide. 
 
 If it is preferred to use the alkaline citrate, the developer is to be prepared at 
 first as given above, and the subsequent additions will be of the alkaline citrate solu- 
 tion only. This treatment has given excellent results in some cases and is provision- 
 ally recommended. 
 
 The formula given above is offered as a starting point for such modifications as 
 circumstances may require, and not as an absolute standard of universal application. 
 The character of the image sought, the age and quality of the emulsion, etc., are 
 varying conditions which render impracticable the adoption of any rigid rules. The end 
 in view is, by means of a developer strong in pyrogallic acid and weak iu ammonia to 
 bring out an image of equal density throughout, while the rest of the [date is kept clean 
 by a sufficient quantity of bromide, or of bromide and citrate of ammonia. A greater 
 strength of pyrogallic acid than that prescribed can hardly be required, but this solution 
 may sometimes be advantageously diluted. If the image come out slowly it must be 
 allowed to take its time, and not be forced with too much ammonia. It is scarcely 
 possible to produce dense fog, but a veiled image is usually the result of an excess of 
 ammonia Operators accustomed to the development of gelatine plates must be espe- 
 cially cautious in this respect. 
 
 The finished photographs must belong to one of two classes. It is not possible 
 in the case of the sun to obtain an outline as sharp and distinct as in photographs of 
 the moon or of terrestrial objects. .But the nearest possible approach to such a defi- 
 nite line is precisely what will most facilitate measurement of the plates, and this must 
 l>e sought as being of much greater importance than the attainment of any special 
 standard of density. An image that is dull and blurred, when held ov<!r a black sur- 
 face and seen as a positive by reflected light, and that, is thin and ha/y when viewed 
 as a negative by transmitted light, will be almost worthless, while one bright and clear 
 as seen in either one of the two ways will be valuable. If, therefore, the operator 
 finds it easy to produce a sharp and clear image of the amhrotype variety he may do 
 so, taking care, of course, not, to discontinue the development until quite certain that 
 it is einial all around. Hut if there is from any canst' a tendency to discoloration of 
 the film, giving a dull appearance li\ reflected light, a greater degree of density will 
 be requisite. A slight veiling, even, is not very injurious if the image is clear :lnd 
 -tnuiLr bv transmitted light. No effort should be \\asted in striving to attain an un- 
 iiece--;ir\ rli^rei- ,,\' density, for, although it can hardly be too great, a very moder- 
 ate density will suffice. 
 
TRANSIT OP VENUS, DECEMBER 6, 1882. 29 
 
 XX. FIXING AND VARNISHING. 
 
 As soon as development is complete wash off all traces of the developer and fix 
 with a weak solution of hyposulphite of soda. Cyanide of potassium must not be 
 
 used. 
 
 Fixing solution.' 
 
 Hyposulphite of soda, J oz. to i oz. 
 
 Water, - - - 16 fluid ounces. 
 
 As the silver bromide dissolves rapidly, it is most convenient and safest to pour a 
 little of the fixing solution on the plate, flow it about until the film is clear, and finally 
 dismiss it into the sink. Then wash the plate thoroughly and set it in a rack to dry. 
 After the lapse of sufficient time, several hours at least, varnish in the usual manner. 
 
 XXL BLISTERING AND LIFTING OF THE FILM. 
 
 If the film separates from the glass, or if small blisters form near the sun's limb,, 
 the planet, or the plumb-line, the plate is worthless, and no precaution must be neg- 
 lected that can tend to prevent so great a misfortune. A substratum of albumen that 
 is too thick, such as would be obtained by using a solution materially stronger than 
 that of the formula given above, may be imperfectly coagulated by the alcohol and 
 tannin, and thus aggravate the evil it is designed to prevent. It the directions already 
 given for drying the albumenized plates thoroughly, and for saturating the film with 
 the solution of alcohol and tannin, are followed, it is probable that no trouble will be 
 experienced; but should a tendency to this defect manifest itself during the preliminary 
 practice, special caution will be necessary in conducting the development of the transit 
 plates. Too much ammonia will act on the substratum and must be avoided. The 
 development must be stopped as soon as the least allowable density is reached, and 
 the plate must be fixed by pouring on the hyposulphite solution, and not in a pan. 
 
 XXII. SPOTS AND OTHER DEFECTS. . 
 
 Small circular transparent spots may appear on the sun's disk, which, if numerous 
 and of a certain size, might resemble the image of Venus and cause time to be lost in 
 measuring the plates. The emulsion furnished is remarkably free from any tendency 
 to produce these spots. If they appear it will be from one of two causes: i. Particles 
 of foreign matter in the emulsion; 2. I'articles of dust that have settled on the plate 
 after coating, especially if charged with chemical substances, such as might arise from 
 solution of bichromate of potash spilled on the floor of the dark-room. The former 
 cause may be removed by properly filtering the emulsion; the latter must be avoided 
 by cleanliness and greater general care. 
 
 Certain other defects, such as pin-holes, crapy lines, &c., do not materially detract 
 from the value of the plate, but every operator who cares for his reputation will never- 
 theless seek to avoid them. 
 
 XXIIL PREPARAT< >RY PRACTICE. 
 
 To familiarize themselves with all the details of their work and with the materials 
 they are to use, the photographers will begin to prepare, expose, and develop plates, 
 
:!(> INSTIMVTIONS FOR OBSERVING THE 
 
 in accordance with tin- foregoing instructions, as soon as possible alter arriving at their 
 station, ami will continue such exercise until the chief of the party is satisfied that 
 they can produce solar photographs of satisfactory quality with ease and certainty. 
 They will avoid any unnecessary expenditure of materials, and will take care that a 
 sufficient supply of everything is reserved for the operations of the transit day. If 
 the bottles of emulsion are labeled with different letters it signified that pyn>x\ lines of 
 slightly different character have been used. Test the contents of each bottle, and if 
 notable differences of quality 7 are found to exist among them, reserve the best for the 
 transit platrs, or mix the various kinds judiciously, as may appear to be preferable. 
 
 It has been already remarked that the person who is to expose plates during the 
 transit must acquire a fixed habit of manipulating the exposing slide, and that unless 
 an approximately uniform motion can be thus obtained there will be no means of 
 regulating the exposure. Having secured a tolerably uniform action of the slide by 
 \vliateverpractice may be requisite, proceed by trial according to the instructions given 
 for ascertaining the correct exposure, to find the proper opening of the slit when the 
 sky is quite clear and the atmosphere in the most favorable state. This will be the 
 least opening that can lie used at any time, and the one with which the observation of 
 the transit will be commenced if the weather should be good. 
 
 If the sun is obscured bv clouds so as to be visible for a few minutes only during 
 the transit, it will be necessary to work as rapidly as possible. The manual of opera- 
 tions prescribed for the observation of the transit will permit plates to be exposed at 
 intervals of only a few seconds if every one is perfectly familiar with his duties. In 
 order that each may be prepared to perform his part of the work promptly, the whole 
 party must be drilled from time to time, going through with all the details of making 
 the record. A: c., except that, plain glass will be used instead of sensitive plates, and 
 there will, of course, be no development. 
 
 XXIV. I'KKI'AKATION FOK THE TRANSIT. 
 
 When the photographers have become sufficiently familiar with the process they 
 are to use, they will begin to prepare a stock of dry-plates to be used in observing 
 the transit. Kighteen plate-boxes, capable of holding twelve plates each, are supplied 
 to each party. Seventeen of these an; to be filled with dry-plates, leaving one box 
 empty. The plates are to In- arranged in the; boxes in the order of their numbers, as 
 already directed, and the boxes must be so marked that the plates can lie exposed in 
 
 ( secutiv -der. The remaining ylass should be cleaned and albuineni/.ed, to lie 
 
 used with wet emulsion in case of emergency, and a sntlicieiit quantity of the develop- 
 ing solutions for fifty or more plates must be provided 
 
 Everything belonging to the photoheliograph must be in correct adjustment 
 and in good working order, especially the clock-work of the heliostat, which must be 
 M well ri-ii-iilat.-d that it can be left to itself for several minutes without allowing the 
 sun's imaire to get too far from the center of the plate. See that the mirror, lens, and 
 reticule-plate are perfectly dean, and remove anv dust that may adhere to the la -I 
 named by wiping it with chamois leather. If artificial light is required for reading 
 the chronometer and making the record, as it probably will be, an orange-glass lan- 
 tern must be used. 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 
 
 31 
 
 XXV. PHOTOGRAPHING THE TRANSIT. 
 
 Photographs are to be taken only while Venus is completely within the limb of the 
 sun. When the chief astronomer decides that the planet, has progressed sufficiently far 
 upon the sun's disk, the automatic break-circuit key of the exposing slide will be put in 
 circuit with the chronograph, that instrument will be started, the members of the party 
 will repair to their respective stations, and, after locking the outer door of the photo- 
 graphic house, the exposure of plates will begin. The services of four persons will be re- 
 quired; if the party consists of but four, their duties will be as provided in what follows: 
 
 Tin' tiKHixlimt (ixtniitoiiicr will be stationed at the heliostat, where he will watch the 
 image of the sun on the target of the exposing slide, and keep them nearly concentric 
 .by an occasional movement of the tangent screws. The image must never be allowed 
 to get so far from the center as to partly uncover the black disc of the target. He 
 will also note the condition of the sky, and give prompt warning to those in the pho- 
 tographic house when the sun is obscured by clouds, and when it reappears. He will 
 occasionally see that the chronograph is working properly, and give notice when it 
 must be stopped to renew the paper. And finally, he will read the barometer and 
 thermometer as directed in another part of these instructions. 
 
 'f/ic chief astronomer will make the exposures and keep the record in the following 
 form : 
 
 RECORD OF PLATES EXPOSED DURING TI IK TRANSIT OF VENUS, DEC KM HER 6, 1882 
 At. . ; Chronometer 
 
 Plate ex- 
 posed. 
 
 Time of exposure by 
 
 Plumb-line 
 pointer. 
 
 Slide Temp, 
 moved. Fahr. 
 
 Remarks. 
 
 Chronometer. 
 
 Chronograph. 
 h. in. s. 
 
 JVff. 
 
 //. HI. S. 
 
 
 
 u 
 
 > 
 
 When the number of a plate is called he will enter it in the first column, and, 
 taking the time from the chronometer, he will make the exposure at some beat of the 
 latter, and, after giving the chronograph signal, or "rattle," with the break-circuit key, 
 he will enter the chronometer time of exposure in the second column. The time re- 
 corded by the chronograph will be read oft", and filled in, subsequently. 
 
 In the column headed "plumb-line pointer" he will write K. or W., as the case 
 may be. The pointer should be frequently reversed, but if the change is made after 
 the exposure of every plate, the plumb-line will probably never be quite at rest. A 
 much better way is to expose the plates in groups of six, the plates of a group follow- 
 ing each other in rapid succession. Then reverse the pointer and give the plumb-line 
 time to come to rest before exposing the next group of plates. 
 
 In the column headed "Slide moved" write E. or W., according to the direction 
 of that motion The movement should be alternately eastward and westward, and 
 the slide must always be left resting against the stops that limit its course. 
 
32 INSTRUCTIONS FOE OBSERVING THE 
 
 flu- temperature of the photographic house in its proper column at intervals 
 of half MM liiuir, or more frequently if the change is rapid. 
 
 In the column tor remarks any peculiarity of the plates, of the exposure given, 
 or of atmospheric conditions, and any other circumstance likely to iitl'ert the result, 
 should be noted. 
 
 When tin- chief astronomer desires a pause in the work to follow the exposure of 
 any plate he will notify the photographer who changes the plates of his intention 
 In -fore giving the exposure, so as to avoid unnecessary handling 1 of the plates. 
 
 When a plate has been developed he will examine it, and, with the advice of the 
 photographers, decide whether any change in the opening of the slit is desirable. 
 
 Our at' tlir /il/ii/of/ni/il/i'i-x will put the plates into the plate-frame, and remove them 
 after their exposure. He will commence operations by placing the empty plate-box' 
 on the right-hand end of the shelf erected for that purpose, and the box containing the 
 plates that bear the lowest numbers of the series, beginning with No. i, at the left- 
 hand end of the shelf. When directed to proceed he will open the left-hand box, take 
 out the nearest plate, which should, of course, be No. I, carry it to the plate-frame, 
 keeping the same edge of the plate uppermost, and carefully avoiding any contact 
 with the plumb-line. After securing the plate with the curved spring he will close tin- 
 plate-box, and then call the number of the plate, reading it from the label, which will 
 be in the upper left-hand corner of the plate-frame. 1 le will then watch the back of the 
 plait- closely to see that the sun's image is not too near the edge of the plate. If the 
 distance should be less than about an inch he will so report to the chief astronomer. 
 
 When the image appears for an instant on the plate he will open the right-hand 
 box, take the plate from the plate-frame, carry it to the farthest groove in the box, and 
 then close the box. Without waiting for orders he will then open the left-hand box, 
 take out the nearest plate as before, and so on; always keeping the back of the plate 
 toward himself, and the numbers uppermost, and always replacing the plate in the 
 farthest unoccupied groove of the right-hand box. 
 
 When the right-hand box is full the other will he empty; the full box must then 
 have a gummed label pasted over the hook in front, so that it cannot be easily opened. 
 and must be put in some place specially designated for the reception of exposed plates. 
 Then remove the empty box to the right-hand end of the shelf, place the next box of 
 the series to the left of it as before, and so proceed until all the plates are exposed. 
 To avoid mistakes, the boxes should be arranged beforehand so that they will lie taken 
 up in consecutive order. 
 
 Tin nflirr photographer "ill develop plates during the whole time of the transit, 
 beginning with the one first exposed. As each plate is tixed and washed he will exam- 
 ine it carefully to see if the exposure is correct, and will call the attention of the chief 
 astronomer to any alteration that maybe required. lie will then develop another 
 plate, always taking the one last exposed. The grooves in tin* boxes belonging to 
 plates taken out for development are to be left vacant, and not filled by other plates. 
 
 If the sky is clear the groups of plates exposed should be equally distributed 
 over the whole time of the transit; but if it is probable that the entire transit \\ill not 
 be seen, a considerable number of photographs should be secured at once, reserving 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 33 
 
 some plates to be used if the weather should prove better than was anticipated. If 
 the sun is visible at intervals between passing 1 clouds, everyone must be at his station 
 to take instant advantage of every opportunity that may offer. 
 
 The sun may be partially obscured by thin clouds or by a hazy atmosphere, so 
 as to require very long exposures. After the slit has been opened as widely as pos- 
 sible the exposure can be still further increased by a slower movement of the slide, but 
 good results can hardly be expected under such circumstances. 
 
 XXVI. DEVELOPMENT AND PACKING OF TRANSIT PLATES. 
 
 As soon after the transit as possible the photographers will begin to develop the 
 transit plates, and will continue that work without unnecessary intermission until it is 
 completed. After drying and varnishing the plates they will be put in the plate-boxes, 
 and kept from shaking about by small rolls of Joseph paper, long enough to reach 
 crosswise over the tops of the plates, and just large enough to hold all the plates 
 firmly, without too much force, when pressed down by the lid of the box. Two such 
 rolls will be put in each box. The boxes are then to be covered with strong paper 
 and tied with stout twine. 
 
 In separating the plates into two or more lots, to be forwarded at different times 
 to Washington, the plates of each lot should be selected so as to include all periods of 
 the transit. If there are only two lots, one should contain all the even-numbered 
 plates, and the other all the odd-numbered ones. 
 
 XXVII. WET EMULSION PLATES. 
 
 Wet plates may be prepai-ed with emulsion if any accident should cause the loss 
 of a great part or all of the dry -plates when it is too late to replace them. A glass 
 plate cleaned, albunienized, and coated with emulsion, as already directed, is to be 
 immersed in clean water contained in a pan or dipping-bath as soon as the film is set. 
 When the water flows smoothly over the film, as the plate is lifted, the exposure may 
 be made, but the plate may remain in the water for any reasonable length of time 
 without detriment. Such plates are developed precisely like dry ones, omitting the 
 preliminary treatment with alcohol and tannin. They develop more rapidly than dry- 
 plates made from the same emulsion. 
 
 XXVIII. BATH WET PLATES. 
 
 Collodion shrinks greatly in drying, but when once dry it swells but little if 
 wetted either with alcohol or water. A film that has been dried before it is exposed 
 is to be preferred, therefore, to one that is exposed while wet, for photographs that 
 are to be accurately measured. And dry-plates can be exposed so rapidly in the 
 photoheliograph that if the sun should be visible for a short time only during the 
 transit, a most important advantage would be gained by employing them. For these 
 and other reasons, it is expected that they will be used. But if from accidental loss 
 of materials, from lack of experience, or from any other cause, the photographers are 
 unable to prepare and develop emulsion dry-plates successfully, they will have recourse 
 to the usual bath process. It must be definitely decided in advance which process 
 will be used, and preparations must be made for that one only. 
 
 5 INS 
 
34 INSTRUCTIONS POBOB8BBYQTG TI1K 
 
 If a resort to the bath wet process is deemed advisable, the services of at least 
 one additional photographer must lie secured if possible. With three or four photog- 
 raphers it will probably be best to have the chief photographer collodionize all the 
 plates and immerse them in the baths, keeping his hands clean. The other photogra- 
 phers will withdraw the plates from the baths and drain them, put them in the plate- 
 frame, and call their numbers. After exposure they will remove the plates from the 
 plate-frame, develop, fix, and wash them, and, finally, put them in the drying racks. 
 Kadi photographer will go through the whole course of those manipulations with 
 every plate he takes from the bath, and the different operators will follow each othei 
 as rapidly as the limited accommodations of the photographic house will permit. 
 
 The best collodion is one that gives a rather hard and patchy negative, for soft- 
 ness, and correct rendering of light and shade, are rather to be avoided than otherwise. 
 The following formula is recommended : 
 
 Bromo- iodized collodion. 
 
 m 
 
 Alcohol - 10 fluid ounces. 
 
 Ether 10 fluid ounces. 
 
 Iodide ammonium 40 grains. 
 
 Iodide cadmium 60 grains. 
 
 Bromide ammonium 20 grains. 
 
 At least three silver baths will be required. They should contain 40 grains of 
 nitrate of silver to the ounce of water, and enough nitric acid to redden blue litmus 
 paper slowly. On removing the plates from the bath drain them as thoroughly as is 
 consistent with rapid work, and wipe their backs. Develop with: 
 
 Protosulphate of iron 1 ouncfe. 
 
 Acetic acid No. 8- 1 fluid ounces. 
 
 Water - - - - - 20 fluid ounces. 
 
 These proportions may be varied, however, if it is found advisable to change 
 them. Fix with solution of cyanide of potassium of such strength as to clear the film 
 rather quickly. 
 
 The instructions already given for the manipulation of dry-plates will be followed 
 so far as they are applicable. If bath wet plates have been exposed in the photo- 
 heHograph, the plate-frame must be thoroughly cleansed from all traces of nitrate of 
 silver before dry-plates are put into it. 
 
 x x i x . ( i !: N !: i ; A i , i i ; i :< ' A r T i < >xs. 
 
 Photographer* who are accustomed to the use of dry-plates will be careful from 
 habits already formed, but those who have practiced only the usual wet process must 
 !" \ery cautious indeed to ;t void the loss of plates by accidental exposure to light. 
 
 Plate-boxes must lie opened only to put in or to take out plates, and must Ill- 
 closed again a> soon as possible. The drying-box must never be allowed to siand 
 
 with open doon to avoid the inconvenience of opening and closing them frequentlv. 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 35 
 
 Emulsion bottles and vials must be replaced in the emulsion-chest immediately 
 after using them, even if they will soon be wanted again. The lid of the chest 
 should be secured by a strap or otherwise, so that it cannot be left open. The chest 
 must be kept locked, with the key in some specially designated place and not in the 
 key-hole. 
 
 When filtering emulsion, and while making or exposing plates, the outside door 
 of the house must invariably be locked. 
 
 And, finally, the various manipulations that have been described in these instruc- 
 tions must be conducted with constant and scrupulous regard to cleanliness. It is only 
 by conscientious attention to details like the foregoing, which are by no means trivial, 
 that successful results can be confidently anticipated. 
 
 XXX. THE CHRONOGRAPH. 
 
 The speed of the chronograph is governed by a vibrating spring whose normal 
 rate of motion is one hundred and thirty-two vibrations per second. In regulating it, 
 the time of revolution of the cylinder must be made correct within about two per 
 centum by moving the sliding weight near the root of the spring, and then the final 
 adjustment can be effected while the instrument is running by slightly loosening or 
 tightening the capstan-headed screw confining the cheeks between which the spring is 
 held. 
 
 The speed of the train is so great that to avoid detrimental friction all the pivots 
 must be frequently oiled, but especially the escape- wheel pivot, and those near it. 
 Weights of one hundred, fifty, twenty-five, and twenty-five pounds are furnished with 
 the instrument, but in ordinary summer weather one hundred pounds drives it well. 
 In winter, a little more may be needed. The weight should have space to fall three 
 feet. This suffices to run the instrument two hours, and if it is wound when a fre"sh 
 sheet of paper is put on the cylinder, it will not require winding again till the paper 
 is changed. 
 
 XXXI. TIME OBSERVATIONS AND CHRONOMETER COMPARISONS. 
 
 Should the station not be in telegraphic communication with a fixed observatory 
 from which local time is received, two azimuth stars above the pole, two below the pole, 
 and six or eight time stars should be observed with the transit instrument on every 
 night when it is practicable. One-half the observations of each class should be made 
 with clamp east, the other half with clamp west. Should the observer be able to get 
 his local time from a fixed observatory, his own determinations may be omitted when 
 not necessary to the success of the expedition. They must, however, be carefully 
 attended to, so far as may be required, either for the determination of his own longi- 
 tude, or for comparing his local time with that of other parties in the neighborhood. 
 In any case, enough observations must be made to determine the azimuth of the photo- 
 heliograph and detect any changes that may occur in it. 
 
 All chronometers at the station must be compared daily, when they are wound, 
 unless the local time and longitude are so well determined that no interest attaches 
 to their running. It will sometimes be necessary to carry one of them about, 
 
3G INSTRUCTIONS FOR OBSERVING THE 
 
 but the others should never be moved when it can be avoided. Kvery care; should be 
 taken to keep them at as uniform a temperature as possible, and the degree of heat to 
 which they are exposed should be noted and recorded three or four times a day. 
 
 XXXII. EXCHANGE OF TIME WITH OTHER PARTIES. 
 
 Should any opportunity offer for the comparison of local time or chronometers 
 with parties from other countries, it must be improved. In such case, the observer 
 must be careful to keep a complete copy of the comparison, and to assure himself that 
 he has all the data necessary for determining the difference of longitude between the 
 stations compared. 
 
 XXXIII. LATITUDE AND LONGITUDE OF STATION. 
 
 The latitude of the station must be determined with the meridian instrument, 
 used as a zenith telescope, and not less than thirty-six observations should be made 
 upon at least twelve pairs of carefully-selected stars. 
 
 Special attention must be paid to getting the true longitude of the station, but 
 the best method of doing this will depend upon circumstances. If the station is in a 
 region covered by an accurate trigonometrical survey, or if it is in telegraphic com- 
 munication with a fixed observatory, the determination of its longitude will be com- 
 paratively easy. In any case, the observer must be on his guard against depending 
 upon a single result. If accurately known trigonometrical points are available, the 
 position of the station must not be determined from a single one of them, but from at 
 least three, whenever possible. If telegraphic signals are exchanged with a fixed 
 observatory, the exchange should be continued through not less than three evenings. 
 At places where neither the trigonometrical nor the telegraphic method is available, 
 recourse must be had to occultations and moon culminations. In observing the latter, 
 care should be taken that the number of observations before and after full moon are 
 nearly equal, and that in each class about as many observations are made by the 
 assistant astronomer as by the chief of party. Instructions respecting occultations 
 are given in section XXXV. 
 
 XXXIV. THE EQUATORIAL TELESCOPE. 
 
 At stations where it is necessary to observe occultations, the mounting of the 
 equatorial must be commenced as soon as possible, taking precedence of that of the 
 photographic piers. The site selected must be such that the instrument commands a 
 good, view of the eastern and western sky, to within live degrees of the horixon if 
 practicable. 
 
 Caution. The shade glasses supplied with the double-image micrometer are so 
 constructed that they can be employed tolerably >afely upon a bright sun with the 
 full aperture of the telescope, but as a matter of precaution, they should lie wanned 
 a little before using them, and the telescope should never be left pointed at the sun a 
 moment longer than is necessary. With the shades supplied for the Airv-I luvgenian 
 eye-pieces it is quite otherwise. If the sun's rays from the full aperture of the tele- 
 scope are allowed to ].;i>s through one of them with imdiminished force, it will split at 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 37 
 
 once, to the great danger of the observer's eye. For that reason they must never be 
 used apart from the Herschel solar prism. 
 
 XXXY. OCC ULTATIONS. 
 
 At stations whose longitude is not otherwise determined, all visible occultations 
 of stars by the moon which occur during the stay of the party, must be carefully 
 observed. To facilitate this work, the instants of the emersions which happen after 
 the full moon may be computed in advance. 
 
 From the time the new moon first becomes visible until her full, she is to be care- 
 fully watched with the telescope to see what stars will be occulted. These can be 
 recognized from thirty to sixty minutes beforehand by remembering that the course of 
 the moon is nearly at right angles with the line joining her cusps, and that she moves 
 nearly her own diameter in an hour. Whenever there is a chance of seeing an occul- 
 tation, a map of the relative positions of the moon and the stars in its neighborhood 
 must be made; and if the occultation is actually observed, the exact point of the 
 moon's limb at which the star disappeared must be noted on the sketch. Nothing 
 must be recorded as an immersion or emersion except the actual sudden and distinct dis- 
 appearance or re-appearance of the star at the moon's limb. If the star is lost in the 
 moon's rays at that moment, the fact should be stated. Every observation must also 
 specify the maker's name and number of the time-piece employed, and whether the 
 occultation was well observed, and if not, what amount of uncertainty attaches to it. 
 
 Great care must be taken to guard against errors of io s in the record, and, to 
 this end, it will be well to have an assistant call aloud every tenth second, "o," "10," 
 
 "20," etc. 
 
 The chronograph may be used in observing occultations, but in that case a frac- 
 tion of a second will be required for the observer to become conscious of the phe- 
 nomenon, and to give the signal, and this interval must always be estimated by the 
 observer, and recorded in the memorandum-book. But, the chronograph should never 
 be trusted to exclusively, and, when used, either the observer himself or his assistant 
 should note the chronometer time of the occultation, or of the signal with the key. 
 
 XXXVI. GENERAL INSTRUCTIONS RESPECTING OBSERVATIONS OF CONTACTS. 
 
 The first question which the intending observer of contacts has to consider is 
 whether the appliances at his disposal and the circumstances in which he is placed 
 will permit of his making observations of astronomical value. If they do, especial 
 pains and minute attention must be devoted to all the necessary preparations. The 
 following is an outline of the general plan of operations: 
 
 Determination of time. The most essential requirement is that the observer shall 
 have the means of determining his local time within at least one or two seconds At 
 fixed observatories there need be no difficulty in this respect. For the benefit of 
 observers at other points it is intended to make arrangements with the Western Union 
 Telegraph Company to transmit time-signals from the Naval Observatory to every 
 part of the country. Individual observers who receive their time in this way should 
 communicate with the authorities at the nearest telegraph station, and, in the event of 
 
:5S INSTRUCTIONS FOR OBSERVING THE 
 
 any doubt, address the Superintendent of the Xaval Observatory, Washington, on the 
 subject. Detailed information and instructions for receiving and understanding the 
 time-signals will be sent to all who desire it in advance of the transit. 
 
 Sue and quality of telescope. The aperture df telescope to be preferred in the 
 observations is from 5 to 6 inches. In order that all observations may be as nearly as 
 possible comparable with those made in the Southern hemisphere, it is recommended 
 that observers with telescopes exceeding 6 inches in aperture reduce the aperture to 6 
 inches in observing all the contacts. Apertures as small as 4 inches may be used 
 without seriously detracting from the accuracy of the observation. Below 4 inches 
 the value rapidly diminishes, and 3 inches may be regarded as the smallest with 
 which observations of real value can be made. It is important that the objective 
 should be of good quality, forming round, neat images of stars', with a power as high 
 as 200. To test the objective, alternately push the eye-piece in and draw it out so 
 that the star shall present the appearance of a disk of light. If the objective is good, 
 this disk will be round and of uniform brilliancy ; if the disk is irregular in outline, 
 with permanent bright or dark regions in it, it shows the telescope not to be good. 
 
 Magnifying power. The eye-piece should have a magnifying power not less than 
 150 nor much more than 200. Between these limits the choice may be regarded as 
 indifferent. 
 
 Mounting. An equatorial mounting is nearly indispensable to an accurate observa- 
 tion. Observers practiced in the use of an altazimuth mounting may possibly make an 
 observation with one of that class, but they must be able to keep an object in the 
 center of the field. A clock-motion is desirable, though not indispensable. If there 
 is no clock-motion the telescope must be firmly mounted, and the observer must be 
 well practiced in moving the eye-piece steadily with his hand so as to keep an object 
 in the center of the field. 
 
 Micrometer. A regular filar micrometer will not be of any use as an instrument of 
 measurement, but spider-lines of some sort are desirable for the double purpose of 
 insuring an accurate adjustment of focus and of estimating the brilliancy of the sun's 
 disk. If the telescope is not supplied with a micrometer the observer should have a 
 positive eye-piece, in the focus of which he should insert a spider-line or, better yet, if 
 he is able to do it, a pair of spider-lines at such a distance that the angle between 
 them shall be i" or 2" of arc. In a 6-foot telescope the required distance will be about 
 .ji, of an inch. The observer should find by previous trials on the sun and stars the 
 exact point when the spider-lines are in the focus of the objective so as to insure their 
 being in proper position on the day of the transit. This point may be indicated by a 
 mark on the eye-piece. 
 
 Shcuh (/liivscs. The common sun-shades, consisting of a single piece of thick 
 jlax, are very apt to split, and thus endanger the observer's eve, if the rays of a 
 bright sun are concentrated upon them by an objective of larger aperture than two 
 inches for ;I local distance of thirty inches, or three inches for a focus of five or six 
 feet. My making the shade of three thicknesses of -lass, the piece next the eye 
 being thickest and darkest in color, the other pieces being successively thinner 
 and lighter in color, and all being lifted loosely into their cell so that they can 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 39 
 
 expand freely, it will be possible to use with safety an aperture of five inches upon 
 a telescope of six feet focus. It is, however, recommended that, wherever possible, 
 some other means of diminishing the "sun's light be employed. Silvering the object- 
 ive might be recommended, except for the possibility of cutting off too much light 
 in a hazy atmosphere. The polariscopic eye-piece is commended for its convenience 
 in use. If the observer cannot avail himself of it, a diagonal eye-piece with a re- 
 flector of plain unsilvered glass is recommended. In such an eye-piece the reflector 
 is placed a little in front of the focus at an angle of 45 with the axis of the telescope. 
 Being unsilvered, 92 per cent, of the light passes through it, and should be permitted 
 to leave the telescope through an opening so as not to heat the air or the reflector. 
 The remaining 8 per cent, of the light is reflected from the two surfaces of the glass. 
 In order that these two systems of reflected rays may not cause confusion, the glass 
 should be ground wedge-shape, and so arranged that only the reflection from the first 
 surface may reach the eye. Since 4 per cent, of the sun's light will in nearly all cases 
 be too great for the eye, the observer should also be provided with shade-glasses to still 
 further diminish it. A neutral tint is to be preferred for all such glasses. 
 
 Day of the Transit. It is essential that every observer intending to make a really 
 accurate observation should have little else to attend to during at least an hour or two 
 before the first contact he is to observe, and should be entirely free from visitors and 
 inquirers. The points to be particularly attended to are .the firmness of the telescope, 
 his ability to move it in such a way as to keep any required part of the sun's limb 
 steadily in the center of the field, and the accuracy of the focal adjustment. A mere 
 estimate of an accurate focus about the time of observation should not be trusted to 
 if it can be avoided, because the eye itself is liable to change its accommodation in 
 this respect. The surest course is to have a pair of spider-lines previously fixed in the 
 astronomical focus and to adjust the eye-piece so that these lines shall be sharply defined 
 on the sun's disk. The observer can then be certain that his focus is right so long as 
 the definition of the wires continues good. 
 
 The degree of brilliancy of the sun's disk as seen by the eye is to be particularly 
 attended to. It was recommended by the Paris International Conference that the disk 
 should be darkened to the point at which a pair of spider lines i" apart could just be 
 seen distinctly separated. But as this test may not suffice, and as the observer may 
 find insuperable difficulty in fixing the wires so close to each other, some other tests 
 should be employed. We may lay down limits as follows: 
 
 I. If the brilliancy of the disk is such as to be at all unpleasant to the eye, or if 
 there is any appearance of glare* surrounding the sun's limb, then the light is too 
 bright and must be diminished. 
 
 II. If there is any difficulty in seeing the limb well and brightly defined, then the 
 light is too faint. Perhaps a good rule will be to shade off the light to such a degree 
 that with the center of the sun in the center of the field, the whole field is as bright 
 as the observer finds it not unpleasant to look at continuously, and yet not so bright 
 as to render the mottling of the photosphere indistinct Then, since the sun's limb is 
 
 * Tins word is here used, not in the s* use of a general atmospheric illumination, but in the sense of such a reful- 
 gence as lei produce ;m ap]ii arance ot ; imlist inetm ss of outline through that excitation of the eye itself known as 
 
40 INSTKMTIONS FOROr.SKKYINC TIIK 
 
 less than half as bright as the center of its disk, it may be presumed that the latter 
 will be about of the right shade. But it must always be remembered that the slightest 
 glare indicates too great a brilliancy. 
 
 Vft another guiding rule will be that the most distinct arid easy view of the sun's 
 limb is to be aimed at. 
 
 ilrtcrnnl contuct. To make a really good observation of this contact two conditions 
 are essentially necessary in addition to all which have been described. The observer 
 must have had some previous practice in observing first contacts, and he must know 
 exactly where to look for the contact. 
 
 The first condition can be well fulfilled by the artificial transit of \ r enus apparatus, 
 of which it is intended to have one or more in Washington available for observers. 
 
 Fur the second condition it is essential that the observer shall have the means of 
 setting the spider-lines in the field of view to any required angle of position. Within 
 the United States the first contact will occur at a point of the limb found by measuring 
 147 from the north point towards the east. The spider-lines should be set at right 
 angles to that radius of the solar disk which terminates at this point of the limb. Then 
 cutting off a segment of the disk by the spider-line, first contact will be seen in the 
 middle of this segment. The tabular time of first contact at any point on the earth's 
 surface may be found within a minute by subtracting 21 minutes from the time of 
 internal contact. The interval between these contacts may be found with yet greater 
 precision from the tables in the American Ephemeris for 1882. The tabular Green- 
 wich time of internal contact may be taken at sight from the proper map accompa- 
 nying this paper. Within the United States the tabular time of first contact may be 
 regarded as 20 hours 55 minutes Washington time. In civil reckoning this is five 
 minutes before nine a. m. Owing, however, to the errors of the tables, which obser- 
 vations of the transit will help us in correcting, it is quite possible that first contact 
 will occur a large fraction of a minute earlier than the predicted time. To allow for 
 this possible error, it is recommended that the observer begin to look exactly one 
 minute before the computed time. The following is a specimen of part of the com- 
 putation which the observer should make by the aid of the chart in order to know 
 when to begin looking. Suppose the place to be Cincinnati. We find from the chart : 
 
 ll. U). 8. 
 
 Greenwich mean time of internal contact . 2 24 52 
 
 Longitude of Washington 5 8 12 
 
 Washington mean time 21 16 40 
 
 Induction to external contact 21 15 
 
 Washington time of tabular contact 20 55 25 
 
 Washington time to begin looking 20 .54 25 
 
 The observer should avoid looking before this time in order not to fatigue his eve. 
 
 The time to be noted is that at which the notch made bv the advancing planet 
 tir>t liei-Mines visible. The observer may have to wait a few seconds to be sure that 
 what he M6S is n-ally a permanent notch, but, the time to be given is that when it was 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 41 
 
 first certainly seen. If he did not catch the first moment when he could see it, that 
 fact must be stated. 
 
 Internal contact. Owing to the importance of this observation and the necessity 
 of special attention to it, it is recommended that the observer have little else to 
 attend to during the 2 1 minutes between it and external contact. It is now believed 
 that measures of the cusps with a double-image micrometer should not be undertaken 
 during this interval, owing to their fatiguing the eye and distracting the attention of 
 the observer. 
 
 It is essential that the observer should allow his eye nearly perfect repose for 
 several minutes before the contact. It is quite right and proper that he should take a 
 general view of the phenomenon at short intervals, and note the appearance presented 
 by the outline of the planet, but he should not exercise his eye and attention in endeav- 
 oring to make any difficult observation. 
 
 His serious attention will be first required some two minutes before the expected 
 time of contact. There is every reason to believe that the entire outline of the planet 
 will then be visible, that portion not on the solar disk being bounded by a fine line of 
 light, supposed to be due to the refraction of the atmosphere of the planet. Indeed, 
 this line may be visible from the first moment of the planet's appearance, and the 
 changes which it undergoes in the relative brilliancy at different points will be a sub- 
 ject of great scientific interest. Although observers of accurate contacts must guard 
 against fatiguing their eyes by minute observations on this arc of light, observers 
 who have not the appliances for making the best observations of contact might well 
 devote themselves to its careful study. 
 
 One of the great difficulties in the observation of internal contact will be to 
 avoid confusing this line of light, which may grow brighter as contact approaches, 
 with the direct light from the sun's limb, which will be seen after contact. The 
 distinction of the two is a matter of judgment which must be left with the observer. 
 In what follows we, for the most part, make abstraction of this appearance, describing 
 phenomena as if it were not present, and the observer must in like manner seek to 
 observe as if it were not present. 
 
 The moment to be observed as that of true internal contact is when the limb of 
 Venus is exactly tangent to that of the sun. It is, however, found by experience that, 
 although easy to think of this tangency, it is difficult to observe it with the requisite 
 precision, owing to the imperfection of vision, and especially to the irradiation produced 
 by the earth's atmosphere and by any imperfections in the telescope. The phenomena 
 to be really observed are defined as follows in the instructions of the International 
 Conference held at Paris in 1 88 1 : 
 
 "At ingress the moment to be noted is that at which the observer sees for the hist time an 
 evident and persistent discontinuity in the apparent limb of the sun near the point of contact with 
 Venus." 
 
 "At egress the moment of the first appearance of a well-marked and persistent discontinuity 
 in the illumination of the apparent limb of the sun at the point of contact." 
 
 However well these definitions may apply to the actual phenomena, they are not 
 sufficient, withoi;t further explanation, to enable the observer to know what is contact 
 6 INS 
 
42 INSTRUCTIONS FOR OBSERVING THE 
 
 under all circumstances. Indeed, the Conference itself added a number of instructions 
 of what was to be looked for under special circumstances. For these instructions the 
 following are, however, substituted by the American Commission. 
 
 We must first remember that just before internal contact at ingress the sun's 
 limb will be broken off by the advancing planet, and that portion which is visible 
 near the point of contact will present the appearance of two fine sharp horns, the 
 points of which will slowly approach each other. The moment of true internal con- 
 tact is evidently that at which these points exactly meet. But since they cannot be 
 seen by the eye to meet until the completed line of light becomes thick enough to be 
 seen, the observer must not expect to see the thread of light actually complete until 
 after the contact has passed. As a general rule, therefore, he must note what is to 
 be seen just before this thread of light becomes evidently complete. What he sees 
 will depend very largely upon the clearness and steadiness of the air. The most 
 favorable circumstances for observing true contact are those in which the cusps appeal- 
 steady and sharply defined against the black background of the sky. There will 
 then be little difficulty in catching the moment at which they are first about to meet, 
 which will be that of true contact. 
 
 In most cases, however, especially if the sun is low, the outline of the cusps will 
 be wavy and serrated, their ends will be more or less rounded instead of being sharp, 
 and their outline will be continually changing in consequence of the apparent undu- 
 lating motion produced by the atmosphere. The greater this vibratory motion and the 
 more the cusps are blunted the more difficult it will be to catch the moment of true con- 
 tact. The following rules are then to be borne in mind by the observer: So long as the 
 dark region between the cusps which connects the black disk of Venus with the black 
 sky outside the sun retains its full darkness, without any apparent motion or undula- 
 tion going across it, so long contact has not occurred; and this although the planet 
 may seem entirely within the true outline of the sun. It would be well for the 
 observer to have an assistant at the chronometer to whom he can from time to time 
 call out the words "not yet." The assistant should write down the second by the 
 watch or chronometer at which the observer commenced to pronounce these words. 
 
 Instead of the cusps uniting into a fine, steady line of light, the observer may at 
 a certain moment begin to see an undulating motion extending all the way across this 
 dark space. He will soon after see that this motion is due to the continually increas- 
 ing line of light, which is broken into threads and waves bv atmospheric undulation. 
 From and after the time that this undulation is permanently seen contact is certainly 
 passed. It would be well, when it is first fully recogni/ed, that the observer should 
 call out "past" to his assistant, who should note the time at which the word is spoken. 
 If he has made no mistake in his estimates the time of contact will be limited between 
 the last moment at which "not yet" was spoken and the first moment at which "past" 
 was pronounced. 
 
 In tlie event of the cusps appearing much rounded, the further Venus appears 
 inside the disk of the sun, as completed in the imagination by continuing its outline 
 ai-ro-s the dark region, the more careful must the observer be fo catch the first line of 
 true sunlight extending across. It may be assumed that if the seeing is at all good the 
 undulating light of the sun's limb will be clearly recognized in a very few seconds after 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 43 
 
 the true time of contact. On the other hand, he must be on his guard against mistaking 
 some slight haziness around the point of contact for the appearance of true sunlight. It 
 is also possible, in case of a very bright but undulating image, that the sharp cusps may 
 from time to time be momentarily brought together by atmospheric undulations before 
 contact really occurs. These are points upon which the observer must be left to his own 
 judgment. He must in all cases try to estimate what the phenomenon would be if there 
 were no undulation^, and he will be assisted in this by taking particular note of the 
 appearance at those moments of steadiness which generally occur every few seconds in 
 the worst atmosphere. The trouble to which observers are prone is that of catching 
 some phenomenon or undulation, which really occurs only from time to time, and 
 fixing the attention on it as though it were permanent. It is so easy to imagine that 
 one sees irregular phenomena that the observer must be especially careful to distin- 
 guish what is permanent from what is an accidental product of atmospheric vibration. 
 
 The preceding directions apply principally to those cases in which the air is clear 
 and the sky blue. If the observation is made through a sky so covered by clouds or 
 haze that there is no striking contrast between the brilliancy of the sun and that of the 
 surrounding sky, the observation may be extremely difficult, because the completion 
 of the thread of light will probably not be seen until a considerable period after 
 actual contact. It is therefore best in such cases that the observer should note the last 
 moment at which he felt sure the limbs did not become tangent and the first moment 
 at which it became permanently evident that the planet had passed entirely within the 
 sun. Perhaps no better definition can be given of contact under such circumstances 
 than that it is the moment when the limbs are really tangent. 
 
 Whatever moment the observer may note, it is indispensable that he give an accu- 
 rate statement of the appearance presented by the sun and planet at that moment, 
 accompanied by a drawing if necessary. If he is able also to give a drawing or 
 description for the moment at which he last spoke the words "not yet," and at which 
 he first said "past," it will be well to do so. At the same time the useless multiplica- 
 tion of times is to be guarded against, owing to the distraction thus produced. 
 
 Egress. At egress the phases occur in inverse order, so that the same directions 
 will apply when properly interpreted with the respect to time. The following points 
 are, however, to be especially noted: 
 
 As the thread of light between the limbs of the sun and planet becomes very thin 
 it will probably appear to darken, partly from atmospheric irradiation and partly from 
 the eye being less affected by a thin line than by a broad band of equal brilliancy. 
 If the atmosphere is undulating this thread may be expected to break up intb a mass 
 of undulating threads of light, continually changing their form and appearance. So 
 long as this undulating mass continues to be seen in motion all the way across the 
 dark space near the point of contact, so long contact has not occurred. If, however, 
 the appearance of congealing into a hard immovable mass is presented, the moment 
 of such seeming congelation is that of true contact. 
 
 In looking for the complete disappearance of the undulating light the observer 
 must be 011 his guard against mistaking the illumination of the outline of Venus, pro- 
 duced by the atmosphere of the planet, for the true light of the sun's limb. There 
 were supposed to be some cases during the transit of 1874 in which the observer, 
 
44 INSTRUCTIONS FOR OBSERVING THE 
 
 watching for the fading line of light to disappear, found himself really observing this 
 atmospheric, outline after contact was past. This is a point on which, in the absence 
 of complete experience, no definite instruction can be. given to the observer, and he 
 must rely upon his own judgment to guard against a mistake of this kind. 
 
 In the case of parties supplied with double-image micrometers it is recommended 
 that measures of the thickness of the band of light between the- two limbs be com- 
 menced as soon after internal contact as the observer has made all his necessary records 
 and notes. It will also be well, twenty minutes before second internal contact, to com- 
 mence similar measurements of the thickness of the point of light between the linil>s. 
 Great care must, however, be taken to stop these measures and replace the micrometer 
 by the eye-piece in good time to make a careful observation of contact. 
 
 Last contact. The observer should note the last moment at which he certainly 
 and distinctly saw the vanishing-notch made by the receding planet. To assist in 
 this it will be well to pronounce the word "notch" from time to time and have the 
 times noted by the assistant. 
 
 XXXVII. METHODS OF RECORDING CONTACTS. 
 
 To make the best possible observations of contacts the observer must be well 
 prepared to note the times of such phenomena as he may see, and this without any 
 liability to errors of 10, 20, or 80 seconds, or a whole minute. If he has to look at 
 and read a time-piece himself there is danger of such errors. They may be avoided 
 by employing a chronograph, but in observing contacts there are two difficulties con- 
 nected with the use of this instrument. The first is that in the event of other signals 
 than those of contact being made, whether by accident or design, it may be difficult 
 to recognize the meaning of the several signals. The second is that in general the 
 instant of contact can be recognized only by watching the course of the phenomena 
 both before and after that event, and thus the observer is not ready to record the con- 
 tact till some seconds after it has occurred. Still, with proper precautions against 
 these difficulties, a chronograph may be used. 
 
 Experience has shown that when an observer notes the times himself, the surest 
 way of guarding against errors in the seconds is to have an assistant at the clock or 
 chronometer to beat every second with a key, or small hammer, upon a board. By 
 this plan a pocket watch may be used in the absence of a better time-piece. At the 
 moment of each beat the assistant must call out only the units or the tens of the sec- 
 ond. Thus, beginning at ten seconds, the calls will be ten, one, two, three, etc., 
 twenty, one, two, etc.. thirty, one, two, etc. The reason for not calling the numbers 
 in full, twenty-one, twenty-two, etc., is that their distinct pronunciation would require 
 such a considerable portion of a second that the observer might be in doubt which 
 beat anyone number belonged to. The simple numerals from one to ten may be 
 pronounced simultaneously with the beats, so as to leave no doubt. An assistant 
 beating in this way may give time to several persons. 
 
 If the observer employs an assistant at the time-piece to read off and record his 
 times, he must also arrange beforehand a system for registering notes respecting the 
 p'"' neiia. Such notes will be "not yet," "haziness near the point of contact," 
 
TEANSIT OF VENUS, DECEMBER 6, 1882. 
 
 45 
 
 "flashes around the planet before contact," "atmosphere of Venus clearly illuminated," 
 etc. If several such notes have to be made, two assistants will be necessary one to 
 write them down and the other to record the times. To co-ordinate the notes with 
 the times, the letters A, B, C, etc., may be employed. The assistant at the chronome- 
 ter is then to record the chronometer time at which the letter was spoken, and opposite 
 it the letter itself, while the other assistant is to write down first the letter and then 
 the note. 
 
 XXXVIII. DIAMETEE OF VENUS. 
 
 If the chief of party can spare any time from the photographic operations 
 between second and third contact, it should be employed in measuring the diameter of 
 Venus with the double-image micrometer. Such measures may be made in groups of 
 sixteen, viz : Foiir measures of the polar diameter, two of them being made with the 
 index to the right of zero, and two with it to the left ; eight measures of the equato- 
 rial diameter, four being with index to right, and four with index to left ; and 
 lastly, four more measures of the polar diameter, two being with index to right, and 
 two with index to left. By this arrangement the zero point and all errors symmetrical 
 with the time are eliminated. FORM II contains some observations of the diameter 
 of Mercury, made during its transit in May, 1878, at Austin, Texas, which are given 
 as a specimen of such work. The numbers in the first column are, the chronometer 
 time when the measurements were begun, namely 9.14 = 9 b 14; the reading of an 
 aneroid barometer, 28.98 inches ; the temperature of the air, 92 Fahrenheit ; and the 
 chronometer time when the measures were finished, 9.30 = g h 30"': The other columns 
 are sufficiently explained by their headings. One revolution of the micrometer-screw 
 is equivalent to i 7". 208, and the diameters are the product of the sums by one-quarter 
 of this value. 
 
 FORM II. 
 
 Time, 
 Barom. 
 Ther. 
 
 Diameter 
 Measured. 
 
 Readings of Screw. 
 
 Differences. 
 
 To Left. 
 
 To Right. 
 
 Polar. 
 
 Equatorial. 
 
 9.14 
 
 Polar . . . 
 
 r. 
 14.91 
 
 .91 
 
 14.91 
 .92 
 
 93 
 .92 
 
 14.91 
 .91 
 
 r 
 "3-Si 
 51 
 
 I3-5I 
 54 
 
 53 
 52 
 
 13-51 
 
 52 
 
 r. 
 1.40 
 
 1.40 
 
 1.40 
 1-39 
 
 r. 
 
 
 28.98 
 92.0 
 
 Equatorial . 
 
 1.40 
 
 38 
 .40 
 1.40 
 
 
 
 9-3 
 
 Polar . . . 
 
 
 
 
 
 Sums 
 
 5-59 
 24"-5 
 
 5.58 
 
 Diameter of Planet .... 
 
 24". co 
 
46 INSTRUCTIONS FOR OBSERVING THE 
 
 
 
 \\XIX.-DATA REQUIRED. 
 
 For convenience of reference, the data which will be required in reducing the 
 observations an- hero enumerated. D 3, 4, 5, 6, 7, G 3, 6, 8, 9, 10, II 3, and K 4 
 can be best determined in Washington. All the others must be determined in the field, 
 and it will be the special duty of the chief of party to see that nothing needful is 
 omitted from the record. 
 
 A. Name of station. 
 
 B Latitude and longitude of station. 
 
 C. With every observation, the name of each person employed in making it, and the 
 
 part he took in the work, must be recorded. 
 D For the meridian instrument: 
 
 1. Maker's name and number. 
 
 2. Size and power of telescope. 
 
 3. Correction for flexure. 
 
 4. Correction for inequality of pivots. 
 
 5. Value of scale of striding level. 
 
 6. Value of scale of zenith distance level. 
 
 7. Value of one revolution of the micrometer screw. 
 
 8. Intervals of transit wires. 
 
 9. Intervals of micrometer wires. 
 K. For the chronometers: 
 
 1. Makers' names, numbers, and descriptions, whether mean time or sidereal, num 
 
 ber of hours on dial, break-circuit or not, etc. 
 
 2. The correction of each chronometer to local time whenever it is used. 
 F. The maker's name, and number of the chronograph. 
 
 G. For the photoheliograph: 
 
 1 . Number of objective. 
 
 2. Number of measuring-rod. 
 
 3. Length of measuring-rod at some definite temperature. 
 
 4. Temperature of measuring-rod whenever it is used. 
 
 5. Number of jaw-micrometer. 
 
 6. Correction to be applied to readings of jaw-micrometer. 
 
 7. Distance from back surface of objective to front surface of reticule-plate. 
 
 8. Distance from back surface of the objective to its second principal point. 
 
 9. Thickness of reticule-plate. 
 
 10. Refractive index of reticule-plate. 
 
 11. Interval between reticule-plate and collodion-film. 
 
 12. A/imuth of the center of the reticule-plate. 
 
 13. Level, or zenith distance, of the center of the reticule-plate. 
 
 14. When the instrument is in use. the temperature of the atmosphere in the 
 
 shade, the temperature in die photographic house, and the reading of the 
 barometer and its attached thermometer, must be recorded every half 
 hour. 
 
TRANSIT OF VENUS, DECEMBER 6, 1882. 47 
 
 G. For the photoheliograph Continued. 
 
 15. Every negative must have its number marked upon it by a diamond. With 
 each negative must be recorded the chronometer time of its exposure, a 
 sidereal chronometer being used; the direction (east or west) of the small 
 arm on top of the frame, from the center of which the plumb-line is 
 suspended ; and the direction of motion of the exposing-slide (east or west). 
 
 H. For the engineer's level, or level of precision : 
 
 1. Maker's name and number. 
 
 2. Size and power of telescope. 
 
 3. Value of level-scale. 
 
 4. Value of micrometer-screw. 
 I. For the theodolite: 
 
 1. Maker's name and number. 
 
 2. Size and power of telescope. 
 
 3. Diameters of limbs, and least reading. 
 K. For the equatorial telescope: 
 
 1. Maker's name and number 
 
 2. Size of telescope. 
 
 3. Powers of eye-pieces. 
 
 4. Value of one revolution of the screw of the double-image micrometer for each 
 
 of the two front lenses. 
 
 5. With every observation, the power employed in making it must be recorded. 
 
 The utmost care must be taken to have accurate knowledge of the local time on 
 the day of the transit, Owing to the uncertainty of weather, no fair night must 
 be allowed to pass during the week preceding December 6 without the observation of 
 star-transits for time and azimuth ; but if clouds prevent sucli observations, then, if 
 possible, the transit of both limbs of the sun must be observed daily during the same 
 period, the telescope being reversed between the limbs. On the day of the transit all the 
 chronometers at the station must be compared in the morning, and again in the evening. 
 
 On December 5th all the apparatus must be inspected to make sure that it is in 
 good working order, and care must be taken that the adjustments 4 and 5 of section 
 V are correct. Both on the day of the transit and on the day preceding, G 7 and 
 G 13 must be very accurately determined; and G 12 must be deduced from the tran- 
 sits of stars observed on nights before and after the transit of Venus, but as near to 
 that event as possible. 
 
 Examples showing how observations of G 13 should be made and recorded are 
 appended. In explanation of them it is only necessary to say that FORM III refers to 
 observations with an engineer's level, and FORM IV to observations with a level of pre- 
 cision. The theory of these observations has been already given in section V. That 
 end of the bubble which gives the largest reading is toward the high end of the line, 
 and the amount of elevation is found by multiplying the number in the line "Differ- 
 ence" into one-sixteenth of the value of one division of the level scale. For the level 
 Stackpole and Brother, No. 1510, the valve of one division was 6". 54. One-sixteenth 
 of this is o".4O9, which multiplied by 9.8 divisions gives 4".oi. For the level 
 
48 
 
 INSTRUCTIONS FOR OBSERVING THE 
 
 Stackpole and Brother, No. 1489, the value of one division is i".74, one-sixteenth of 
 this is o".iO9, which multiplied by 22.0 divisions gives 2".4O. The large collimation 
 error of the telescope of this level is noticeable. 
 
 FORM III. Observations made with the engineer's level STACKPOLE and BROTHER, No. 
 1510, to deter mitir the inclination of the line of collimation of the horizontal jilidlo'icl la- 
 graph at Hobart Town, Tasmania, December 9, 1874. 
 
 Object 
 Observed. 
 
 End of Bubble. 
 
 North. 
 
 South. 
 
 North. 
 
 South. 
 
 Reticule . 
 
 d. 
 45-0 
 
 d. 
 '3-9 
 
 d. 
 
 d. 
 
 
 44-6 
 
 13-9 
 
 
 
 
 Transit 
 
 
 
 . 
 
 12. 
 
 43-0 
 
 
 
 
 
 
 12.5 
 
 43-4 
 
 
 
 
 
 
 12.2 
 
 43-3 
 
 
 
 
 
 
 12.2 
 
 43.0 
 
 Reticule . 
 
 44.0 
 
 13.0 
 
 . 
 
 
 
 44-0 
 
 13.0 
 
 
 
 
 
 Sums . 
 
 177.6 
 
 53-8 
 
 48. y 
 
 172.7 
 
 Sums . . 
 
 23'-4 
 
 221 .6 
 
 Difference 
 
 9.8 
 
 North end high, 4".or 
 
 FORM IV. Observations made with, the level of precision, STACKPOLE and BROTHER, No. 
 1489, to determine Hie inclination of the line of collimation of the horizontal jihotohclio- 
 graph at Washington, D. C., September g, 1882. 
 
 Focusing 
 Pinion. 
 
 tnd of 
 Bubble. 
 
 Level 
 Direct. 
 
 Level 
 Reversed. 
 
 Right . 
 
 N. 
 
 23-5 
 
 44-0 
 
 
 S. 
 
 51-5 
 
 15.0 
 
 Lelt . . 
 
 N. 
 
 13-0 
 
 54-0 
 
 
 S. 
 
 42.0 
 
 25-5 
 
 
 N. 
 
 12. 
 
 55-0 
 
 
 S. 
 
 4I.O 
 
 26.0 
 
 Right . 
 
 N. 
 
 23.O 
 
 44-5 
 
 
 S. 
 
 51-5 
 
 15-5 
 
 Sums .... 
 
 257-5 
 
 2/9-5 
 
 Difference . 
 
 22.0 
 
 North end high, 2".4O 
 
TRANSIT OF VENUS, DECEMBER 6, !882. 49 
 
 XL. RECORDS OF OBSERVATIONS AND OPERATIONS. 
 
 Of the journals and memorandum-books, each observing-room is to have at least 
 one for its exclusive use, in which every operation must be recorded in detail, with 
 all the particulars necessary to its being full}- understood. A journal is also to be 
 kept, in which all the operations of the party must be entered. 
 
 In addition to the original record of the observations, a duplicate record must be 
 made with ink, at the earliest possible moment. The following rules respecting the cor- 
 rection of supposed mistakes must be attended to in the case of each set of records: 
 
 Original rouc/lt record. In this record a number once written should never be 
 erased. If the observer detects a wrong number immediately after writing it, he must 
 draw a line through it and write the correct number beside it. If it is concluded 
 from subsequent observations that a number is probably wrong, that fact must be noted, 
 and the correct number indicated ; but the record as written must not be altered. Num- 
 bers should not be inserted in this record which are the result of calculation. For 
 instance, if the observer fails to note the minutes of the chronometer corresponding to 
 any observation, he must not conclude what they were from the preceding or subse- 
 quent observations, and then put them in, but must omit them entirely, unless such 
 omission would cause subsequent uncertainty. In that case the necessary numbers 
 may be inserted, provided it is done in such a manner as to show that they were not 
 directly observed, but are concluded from other parts of the record. To indicate this, 
 a circle may be drawn around them. 
 
 Should it be found necessary from any cause whatever to make the first rough 
 notes of an observation upon loose slips of paper, these slips must be carefully pasted 
 into the proper note book, in immediate proximity to the formal record which has been 
 copied from them. 
 
 The duplicate or fair copy. The second record is to be copied from the first as 
 soon as practicable after making the observations, so that if mistakes exist they may 
 be detected and corrected. If blank forms for the observations are provided, they 
 may be used for the second copy as well as for the first; but everything for which 
 they are not available must be copied in chronological order into a single book. In 
 this duplicate copy greater liberty will be allowed respecting additions and alterations 
 than in the original, the object being to make a complete and correct record; but in the 
 event of numbers being added as the result of calculation, they should lie underscored 
 with red ink, or otherwise indicated. 
 
 All records of observations must include every particular necessary to their being 
 completely understood. For instance, wherever time is given, the particular time- 
 piece employed must be designated; where level readings are given, the direction of 
 each end of the level, east, west, north, or south, must be recorded; when the images 
 of lines on the plate-holder are observed with the transit, the direction of the image 
 from the middle wire, whether right, left, north, or south, must be stated, as well as the 
 direction of the eye-piece of the instrument, east or west; and all photographs of the 
 sun must indicate the exact time at which they were taken, and must be so marked 
 that the position of the plate Mien in the holder that is, the top, bottom, east and 
 west sides can always be distinguished. 
 
 7 INS 
 
50 INSTRUCTIONS FOK OBSK1!VIN<! THK TK'ANSIT OF YKNUS. 
 
 In using the chronograph, the minutes .and seconds must always be marked upon 
 the sheet at least twice during each series of observations. All chronograph sheets 
 must be preserved and sent home with the other original records, but in addition to 
 this they must also be read off and recorded in the proper books. 
 
 XLL TRANSMISSION AND PUBLICATION OF OBSERVATIONS. 
 
 All members of the parties are prohibited from publishing their observations, or 
 results, without authority of the commission; but this prohibition is not intended to 
 prevent any general statements respecting the nature and success of the work which 
 the observers may choose to make. In cases of co-operation with any other individual 
 or party, the chief of the party is authorized to communicate to the other copies of 
 all observations necessary to the special end for which the co-operation was entered 
 into. 
 
 The chief of each party will transmit all the records to the president of the com- 
 mission as soon as practicable after the completion of the observations. They will be 
 sent in separate packets, one containing the journal and all the original pencil memo- 
 randa of the observations, the other the fair copy already directed to be made. If 
 practicable, the two packets must be sent at different times and by different convey- 
 ances. From ports where an American consul is stationed, thev may be forwarded 
 by him through the Department of State, in which case he must be notified to send 
 the two packets by different ships. 
 
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