ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
789 
In order to test the practical value of this theoretical reasoning, I 
requested Mr. H. R. Spencer, now of the Spencer & Smith Optical Co., 
of Buffalo, N.Y., to calculate the formula and construct a lens which 
would fulfil the stated requirements. The result was a lens of 1/6 in. 
equivalent focus, and possessing a numerical aperture of 1*35. With 
this objective I have resolved and photographed the Amphipleura pellu- 
cida in lines, and have photographed both the Navicula rhomboides and 
the Van Heurckia crassinervis in lines and beads.* 
Fig. 113. 
Photograph of the Test Podura ( Lepidocyrtis curvicollis ). x 3000. 
Spencer & Smith Obj. 1/15, N.A. l f 35. 
In ordinary histological work, with amplification ranging from 200 
to 400 diameters, the results with this lens have been very much more 
satisfactory than I have been able to obtain by any other combination 
of lenses or plates. 
With a 1/4 in. objective of the same construction, made by Spencer & 
* As regards the last-named object, it has been previously photographed in beads, 
so far as I am aware, only by Van Heurck with the aid of a lens of higher power, 
1/8 in., and muchlarger aperture (N.A. 160), 
