THE CONDENSER. 
71 
the apochromatic condenser of Powell and Lealand, 
available N.A. nearly 1. For ordinary medical and 
similar work this condenser is unnecessarily costly, but 
for critical work it is desirable (fig. 18). 
Those who propose to use immersion condensers are 
presumably ambitious of the very finest results, and for 
this we recommend the best condenser at present known 
to us—the achromatic oil-immersion of Powell and 
Lealand, N.A. 1-4. This is not usually fitted with an 
iris stop, but it is altogether as near perfection as any¬ 
thing that can be obtained. It is costly (see Appendix) 
but is not dear (fig. ig). 
A very important point not usually noticed in books 
is that good objectives may be used as condensers with 
the best results. All that is needed is a fitting of such 
nature that the objective can be screwed into the sub¬ 
stage, and an iris can easily be added to the arrange¬ 
ment. In this way we have produced excellent results, 
using as condensers both achromatic and apochromatic 
objectives; the chief point to note is that the condenser- 
objective should be about the same N.A.—less rather 
than more—as the objective used. Dr. Bousfield has 
used two immersion objectives, one as condenser, the 
other as objective. 
Chromatic condensers are less expensive than achro¬ 
matic or apochromatic, and the cheaper ones act fairly 
well in general work ; at the same time, considering 
that good achromatic condensers are not so very costly, 
we do not advise the purchase of chromatic. Zeiss 
makes two chromatic condensers, one for air, the other 
