54 



ON THE CONSTRUCTION AND USE OF 



the atmosphrric difRrultios, wliicli in r(>ality ((institute the c^reat obstacle to success, 

 would not be diniinislied by such means. The apparent advantage, that of not 

 magnifying defects in the collodion, is not of mucli moment, for when development 

 of the photographs is properly conducted, and thorough clcanliui^ss practised, 

 imperfections are not produced, and the size of the silver granules is not objection- 

 able. 



b. Hi<jh Pmoers. 



Although negatives of astronomical objects have not as yet been made wliich 

 could stand tlie higli powers of the arrangement about to l)e described, yet they 

 bear the lower powers well, and give promise of improvement in the future. 



Photography of microscopic objects as usually described, consists in passing a 

 beam of light through the transparent object into the compoimd body of the micro- 

 scope, and receiving it on its exit from the eyepiece upon a ground glass or sensi- 

 tive plate. The difficulty which best^ts the instrument generally, and interferes 

 with the productitni of fine results, arises from the luicfn'tainty of ascertaining tlie 

 focus or place for the sensitive plate. For if the collodion tihn be put where the 

 image on ground glass seems best defined, the resulting photograph will not be 

 sharp, because the actinic rays do not form tlu^r inuige there, ])ut either farther 

 from or nearer to the lenses, dep(niding on the amount of the (-hromatic correction 

 given by the optit'ian. Practically by repeated trials and variation of the ])lace of 

 the sensitive compound, an apiiroximation to the focus of the rays of maximum 

 photographic intensity is reached. 



Kig. 47. 



Microscope for Photography. 



m 



During my father's experiments on liglit, and more particularly when engaged 



the invention of portrait photography, he found that the ainmouio-sulphate of 



pper, a deep blue liquid, will separate the more refrangible rays of light, the rays 



