54 



ON THE CONSTRUCTION AND USE OP 



the atmospheric difficulties, Avhich in reality constitute the great obstacle to success, 

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

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

 of the photographs is properly conducted, and thorough cleanliness practised, 

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

 able. 



b. High Pmvers. 



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

 could stand the high powers of the arrangement about to be 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 compound 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 besets the instrument generally, and interferes 

 with the production of fine results, arises from the uncertainty of ascertaining the 

 focus or place for the sensitive plate. For if the collodion film 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 their image there, but either farther 

 from or nearer to the lenses, depending on the amount of the chromatic correction 

 given by the optician. Practically by repeated trials and variation of the place of 

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

 photographic intensity is reached. 



Fig. 47. 



rosoope for Photograpliy. 



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

 in the invention of portrait photography, he found that the ammonio-sulphate of 

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



