A SILVERED GLASS TELESCOPE. 



49 



Fii;. 44. 



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

 t('lpsfop(% and the yellow glass taken out. After 20 s(>couds th(^ instrunuMit re- 

 nuiining still untouched and motionless, the screen is withdrawn, and 

 as many seconds allowed to (;lapse as desir;d)l('. 'I'lie screen is then 

 replaced and the plate talvcn l);ick to the pliotognipliic room. 



After b(Mng again put under the ta]) to remove any dust or im])urity, it 

 is dipped into the nitrate bath for a few s(>conds. 'l\vo driiclmis of a solu- 

 tion of protosulphate of iron 20 grains, acetic acid 1 draclini. ;ind water 

 1 ounce, is poured on it. As soon as tlie iiungc- is fairly visil)le tbis is 

 washed otf, and the developnunit contiiuu'd if necessary with a weak mate Carrier, 

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

 citric acids eacli ! grahi, nitrate of silver -,i„ grain, water 1 drachm. In order to 

 measure these small ([uantities standard solutions of the sul)stances are made, so 

 that two drops of each contain tlie desir(>d amount. 'I'liey are kept in 

 bottles, through the corks of wliich pipettes descend to just below the 

 level of the li([uid. This avoids all necessity of tiltering, and yet no 

 blemish(>s are produced by particles of floating matter. 



During the earlier j)art of tbe dcAclopment. when tbe protosulphate 

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

 per length of time for the exposiu'e in the telescop(\ If the image 

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

 ment in 45 seconds, and will liave the minimum of wbat is called pipette Bouie. 

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

 first ap])earance the exjxisure must be lengthened, and vice versa. 



The latter part of th(> de\(dopment, when re-(Uw elopmenl- is practised, is purposely 

 made slow, so that the gradation of tones may be \aried by clumging the propor- 

 tion of the ingredients. As it would lie 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 ((, 

 Fig. 45, with an ordinary wood screw, as at A, going througli 

 each corner. Four wooden i>e,gs, as at c, furnisli a su])p(nl 

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

 washing system, I seldom get my fingers marked, and what is much nH)re important, 

 rarely stain a picture. 



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

 that is, when the ]>i(tnre is like an overdone positive, the plate is again flooded with 

 water, treated witli cyanide of jjotassium or hyposu]])hite of soda, once more 

 washed and set upon an angle on filtering i)aper to drj . It is next nun-ning iabelk-d, 

 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 ke])t. and tliat, 

 even were it not so, one good ])i(tture is worth more than any nnnd)er of bad ones. 



Although the above is the metlioil at ])resent, adopted, and by which excellent 

 residts have been obtained, it may at any moment gi\(' i)lace to some other, and is 

 iudeed'bi'ing continually modified. The defects it presents are two — first, the time 



7 July, 1804. 



Fig. 45. 



Developing Stand. 



