ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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he marked out, and the improvements in the art since then have simply 
kept pace with the gradual improvement of the objective, and especially 
in the direction of increased apertures. One notable advance in the 
technique, however, is Abbe’s happy conception of the projection 
ocular.* 
Fig. 112. 
Photograph of Artificial Spectrum. 
The collodio-iodide gave place some years ago to the gelatino- 
bromide plate, which, like its predecessor, is especially sensitive to the 
blue and more refrangible rays, and almost wholly insensitive to those 
which give the strongest visual impression. The relative sensitiveness 
of such a plate to pigment colours is clearly shown in the annexed 
photograph of an artificial spectrum. (See fig. 112.) 
In order to obtain the sharpest image on such a plate the lens must 
* Peculiar advantages have been claimed in behalf of objectives constructed 
according to the so-called “ apochromatic ” system. These lenses, however, possess 
certain disadvantages which restrain me from giving them unqualified oommenda- 
tion. 
