3io 



ACCESSORY APPARATUS 



of "5 N.A. ; for such illumination, in fact, it is perhaps the best 

 illuminator extant, and shows objects on a dark ground with 

 sparkling brilliancy, and may be used with polarised light. 



A chromatic condenser, somewhat similar in construction to this, 

 and of low price, is made by Messrs. Powell and Lealand ; but it is 

 of much higher power, so that the distance between the foci for 

 the central and peripheral rays is not so great, and on this account 

 it yields a somewhat larger aplanatic cone. This instrument with 

 its diaphragms is shown in fig. 256. It is more convenient in form, 



FIG, 255. Abbe's chromatic condenser as applied to the Zeiss microscopes. 



and can be handled and adjusted with greater facility, than that oi 

 Abbe. The size of their respective back lenses is significant in this 

 regard, that of Powell's being fa inch, and that of Abbe's being 

 1 fa inch. This instrument of Powell's, if fitted in the usual way, 

 would be now a very efficient instrument of its kind and quality. 

 The particular quality of oblique illumination was in fact still 

 further advanced by a modified form by the same makers known as 

 Powell's truncated condenser, which gives great obliquity with 

 abundance of light, but it is as a matter of course very, chromatic. 

 The diaphragms (fig. 256, A) have a central aperture for the 



