ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 137 



After using collodion for several evenings, it is well to allow it to stand 

 for some days, and to decant about three-fourths into a fresh vessel. 



Before pouring the collodion on to the glass plate, the usual precaution 

 of cleaning away with the fingers any dried collodion from the lip of the 

 containing vessel must be attended to ; moreover, each time, just in the act 

 of pouring, a few drops should be allowed to fall to waste on the floor ; by 

 attention to these remarks, much vexation will be avoided. 



Exposure of the Plate in the Telescope. — On taking the plate from the 

 nitrate of silver bath, it is desirable to drain it well before it is put into the 

 slide, first on the edge of the bath, then on white blotting-paper, shifting its 

 position two or three times, but always keeping the same point downwards. 

 It must be carried to the telescope as quickly as possible, and the picture 

 developed immediately after it has been removed from it. 



The sensitized plate rests on angles of pure silver, let into the square 

 plate-holder, or in the circular plate-holder within a ring of pure silver, the 

 face resting on three prominent places. I have found that contact with 

 wood is liable to produce stains which occasionally extend across the plate 

 during the development. The circular plate-holder is entirely of metal, 

 and I would recommend metal holders in preference to those of wood for 

 celestial photography, because they are not liable to warp and become set 

 from damp when left in the observatory. The plate-holder should be wiped 

 with a clean cloth after each operation, and the hands also washed each time 

 before a fresh plate is taken, on which it is intended to pour collodion. 



In order to subject the sensitized plate to the action of light when the 

 telescope is used as a Newtonian, I remove a very light cover, previously 

 placed over the mouth of the telescope, and replace it when I wish to dis- 

 continue the action ; this cover is made of black merino, stretched on a 

 whalebone hoop and is provided with a handle of bamboo. In the direct 

 method, I turn up or down, through an arc of 90°, a little hinged trap, 

 interposed between the great mirror and the sensitive plate. This motion is 

 given by means of a lever fixed on a light axis, supported by the arm which 

 holds the small camera ; the axis extending beyond the edge of the telescope 

 tube, and carrying a milled head by which it is turned. 



Regulation of the Time of Exposure. — A journeyman-clock, beating seconds 

 distinctly, should be near the telescope, in order that the operator may be 

 enabled to regulate the time of exposure, which requires great nicety with 

 such sensitive chemicals as must be employed. 



The time occupied in taking lunar pictures varies considerably ; it depends 

 on the sensitiveness of the chemicals, on the temperature, on the altitude of 

 the moon and her phase. An almost imperceptible mist in the atmosphere 

 will sometimes double the time of exposure, but, curiously enough, a bright 

 fleecy cloud passing over the moon scarcely stops any of the actinic rays. 

 I have recently produced an instantaneous picture of the full moon, and 

 usually get strong pictures of the moon in that phase in from one to five 

 seconds. The moon as a crescent, under like circumstances, would require 

 about 20 to 30 seconds, in order to obtain a picture of all the parts visible 

 towards the dark limb. 



Development of the Picture. — Of all the developing mixtures tried, I give 

 the preference to the aceto-pyrogallic acid solution, which is generally used in 

 the ordinary proportions ; namely, pyrogallic acid, three grains ; glacial acetic 

 acid, one fluid drachm; distilled water,three fluid ounces; but, in cold weather, 

 I sometimes reduce the quantity of acetic acid to one half, to render the 

 solution more active. The developing fluid retains its properties for a week or 

 more after mixing. It is desirable to pour out the requisite quantity of fluid 



