E. M. NELSON ON BrNOCULARS. 



59 



Depth is an all-important element. There can be neither 

 displacement nor stereoscopism in either ordinary or micro- 

 scopic vision without depth. In this department of microscopy 

 there is any amount of room for theorizing. For instance, we 

 know that depth is reduced in the direct ratio of the increase of 

 aperture and in a greater ratio of that of power. Therefore 

 we might reasonably conclude that because stereoscopism 

 is a function of depth it might be greatly reduced by an increase 

 of power. Practically, however, such is not the case, for if you 

 change from a two-inch to a one-inch eye-piece, you will have 

 about a quarter of the depth, but the stereoscopism will remain 

 constant. Again, because parallactic displacement increases 

 with aperture, and because it is found that too great an aper- 

 ture in a low power gives an hyperstereoscopic effect on opaque 

 objects with reflected light, we might, therefore, conclude with 

 Dr. Carpenter that there is a limit of aperture for perfect 

 stereoscopism ; in practice, however, we do not find any such 

 limit. I have profitably used an oil immersion | of 1*4 N.A., 

 but I do find that increase of power and aperture materially 

 degrade the quality of the image by accentuating the disturbing 

 effect of the prism. This points the moral — the fewer prisms 

 and the fewer surfaces the better. It is obvious, too, that the 

 division of the back of the objective by the prism also divides 

 the spectra, so that with a central axial cone 

 and a stop (Fig. 4) over the back of the objec- 

 tive we obtain an arrangement as in Fig. 11, 

 consisting of half a dioptric beam and four 

 spectra (three of the first order and one of the 

 second) unsymmetrically arranged with re- 

 gard to that beam ; but if we use duplex 

 illumination * by means of a stop (Fig. 12) at 

 the back of the condenser v/e secure a much 

 better arrangement, viz., a whole dioptric beam 

 and four symmetrically placed first order spectra 

 as in (Fig. 13). The stop at the back of the 

 lens is an advantage, because it keeps the 



Jiy.l2. 



* Duplex illumiuatiou by means of mirrors was first introduced by Eiddel, 

 Stei)henson used a condenser composed of cylindrical lenses and a double 

 stop as in Pig. 12. I do not discover any advantage to be gained by the 

 use of cylindrical lenses, and prefer the ordinary achromatic condenser. 



