24 Photography at Greenwich Observatory. 



horizontal position for a few minutes,, and is then hung up to 

 dry in the air ; when dry, it is placed in a drawer till used. 



" Second Operation. — Eendering the paper sensitive to the 

 action of light. 



' • A solution of nitrate of silver is prepared by dissolving 

 fifty grains of crystallized nitrate of silver in one ounce of 

 distilled water, adding in hot weather a few drops of acetic 

 acid. 



" Then the following operation is performed in a room illu- 

 minated by yellow light : — 



" The paper is pinned as before upon a board, and (by means 

 of a glass rod as before) its surface is wetted by fifty minims 

 of the solution. It is allowed to remain a short time in a hori- 

 zontal position, and if any part of the solution remains unab- 

 sorbed, the superfluous fluid is taken off by the application of 

 blotting-paper. 



The paper, still damp, is immediately placed upon the in- 

 terior glass cylinder, and. is covered by the exterior glass cylin- 

 der, and is mounted upon the rotating apparatus to receive 

 the spot of light formed by the mirror which is carried by the 

 magnet. 



" Third Operation. — Development of the photographic 

 trace. 



" When the paper is removed from the cylinder it is placed 

 upon a board, and a saturated solution of gallic acid, to which 

 a few drops of acetic nitrate of silver are added (in hot weather 

 this solution is used at the temperature of the air, in cold 

 weather it is heated to the temperature of seventy or eighty 

 degrees), is spread over the paper by means of a glass rod, 

 and this action is continued until the trace is fully developed. 

 When the case is well developed the paper is placed in a 

 vessel of water, and repeatedly washed with several successive 

 supplies of water, a brush being passed lightly over both sides 

 of the paper to remove any crystalline deposit. 



" Fourth Operation. — Fixing the photographic trace. 



" The photograph is placed in a solution of hyposulphite of 

 soda, made by dissolving four or five ounces of the hyposul- 

 phite in a pint of water. It is plunged completely in the 

 liquid, and allowed to remain from one to two hours, until the 

 yellow tint of the iodide is removed. After this the sheet is 

 washed repeatedly with water, allowed to remain twenty-four 

 hours in water, and afterwards placed within the fold of linen 

 cloths till nearly dry. Finally, it is placed between sheets of 

 blotting-paper, and a heated iron passed over it.-" 



From the original records thus obtained, after they have 

 had placed upon them with pen and ink the necessary data of 

 the times of the breaks in the curve to obtain the time scale, 



