3 io 



AC( ESSDRY APPARATUS 



of ") X.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 chmiiiiitic condenser, somewhat similar in construction to this, 

 and of lo\v price, is made by Messrs. Powell and Lealaiid ; 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 \ ields a someuhat larger aplanatic cone. This instrument with 

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



FIG. '2~>r>. AMie's chromatic r.nidciiscr as applied to the Zeiss microscopes. 



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



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



regard . that of Powell's being ,",, inch, and that of Abbe's being 



1 n; ii"-h. This instrument, of l'o\\ ell's, if fitted in the usual way, 



vould l>e now a very ellicient inst rumenl of its kind and quality. 



'I'l"- particular quality of oUiniie illumination was in fact still 



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



truncated condenser, \\hich gives great obliquity with 



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



liaphra.-iiis (tig. iT.f,. A) have a central aperture for the 



