A SILVERED GLASS TELESCOPE. 



49 



mass of the telescope being at rest. A pasteboard screen is put in front of the 

 telescope, and the yellow glass taken out. After 20 seconds the instrument re- 

 maining still untouched and motionless, the screen is withdrawn, and 

 as many seconds allowed to elapse as desirable. The screen is then Fi s- 43 - 

 replaced and the plate taken back to the photographic room. 



After being again put under the tap to remove any dust or impurity, it 

 is dipped into the nitrate bath for a few seconds. Two drachms of a solu- 

 tion of protosulphate of iron 20 grains, acetic acid 1 drachm, and water 

 1 ounce, is poured on it. As soon as the image is fairly visible this is 

 washed off, and the development continued if necessary with a weak pi a te Carrier. 

 solution of pyrogallic acid and citro-nitrate of silver pyrogallic and 

 citric acids each | grain, nitrate of silver -^ grain, water 1 drachm. In order to 

 measure these small quantities standard solutions of the substances are made, so 

 that two drops of each contain the desired amount. They are kept in 

 bottles, through the corks of which pipettes descend to just below the Fig. 44. 

 level of the liquid. This avoids all necessity of filtering, and yet no 

 blemishes are produced by particles of floating matter. 



During the earlier part of the development, when the protosulphate 

 of iron is on the film, an accurate judgment can be formed as to the pro- 

 per length of time for the exposure in the telescope. If the image 

 appears in 10 seconds, it will acquire an appropriate density for enlarge- 

 ment in 45 seconds, and will have the minimum of what is called pi pe tte Bottle, 

 fogging and the smallest granulations. If it takes longer to make its 

 first appearance the exposure must be lengthened, and vice versa. 



The latter part of the development, when re-development is practised, is purposely 

 made slow, so that the gradation of tones may be varied by changing the propor- 

 tion of the ingredients. As it would be tiresome and un- 

 cleanly to hold the plates in the hand, a simple stand is used 

 to keep them level. It consists of a piece of thin wood a, 

 Fig. 45, with an ordinary wood screw, as at ft, going through 

 each corner. Four wooden pegs, as at c, furnish a support 

 for the plate d. By the aid of this contrivance and the 

 washing system, I seldom get my fingers marked, and what is much more important, 

 rarely stain a picture. 



When the degree of intensity most suitable for subsequent enlargement is reached, 

 that is, when the picture is like an overdone positive, the plate is again flooded with 

 water, treated with cyanide of potassium or hyposulphite of soda, once more 

 washed and set upon an angle on filtering paper to dry. It is next morning labelled, 

 and put away unvarnished in the case. 



To the remark that this process implies a great deal of extra trouble, it can only 

 be replied that more negatives can be taken on each night than can be kept, and that, 

 even were it not so, one good picture is worth more than any number of bad ones. 



Although the above is the method at present adopted, and by which excellent 

 results have been obtained, it may at any moment give place to some other, and is 

 indeed being continually modified. The defects it presents are two first, the time 



7 July, 1864. 



Fig. 45. 



Developing Stand. 



