42 H. J. Clark on the Microscope. 
In regard to the usually estimated worth of wide angles of 
aperture, I would say, that, from numerous experiments upon 
living tissues, objectives having this property are valuable, not 
so much because they can admit extremely oblique one-sided 
rays, but because they allow rays to enter from all sides at a 
very wide angle to the axis. One-sided oblique rays throw the 
shadow in a great measure, beyond any particular cell upon its 
neighbor, and this produces distortion; whereas when the rays 
converge at a wide angle, each cell becomes strongly marked at 
its periphery by a dark, broad shade. A moderately oblique, 
one-sided light, hardly twenty degrees from the axis of the ob- 
jective, always appeared to be the most frequently serviceable. 
I was surprised one day to find that the hitherto faintly visible — 
circulation in the cells of Spirogyra was rendered, by such a ~ 
light, very distinct, and the granules borne along in the current 
appeared like little specks with a very sharp, thick, black outline. 
At first thought, there would appear to be an insuperable | 
objection to the wide angle of such objectives, and that is the — 
shortness of the working distance, which will not allow one to — 
