164 THE AMERICAN MONTHLY [Jun 
are arranged to work satisfactorily with the front lens 
removed, and by this means high and low power effects 
are secured in one combination. 
Size of field lens.—The reason for the popularity of 
the Abbe illuminator, with its glaring imperfections, is 
on account of its large field lens and the ease with which 
it can be worked. A high power condenser must of ne- 
cessity have comparatively small lenses, and requires as 
great care in manipulating as the objective itself. The 
Abbe achromatic condenser was an attempt to maintain 
the easy working of the Abbe illuminator in a corrected 
form, but it is really too heavy and clumsy and restricts 
the movements of a mechanical stage. The best conden- 
sers have, as a rule, the largest field lenses that can be 
advantageously fitted, but this point is deserving of spe- 
cial consideration when making a decision. 
Recommendations. —From the foregoing it will be pos- 
sible, with given objectives and a maker’s catalogue, to 
choose the most suitable condensers. Ifa man proposes 
to restrict himself to low and medium powers, not ex- 
ceeding say @ in., he can readily make a choice, and we 
would like to specifically mention the new condenser in- 
troduced by Mr. C. Baker, of 244, High Holborn: in this 
a specially large field lensis provided ; the power (4-10in.) 
is exactly the right one for histologists and workers with 
medium power objectives, while the aplanatic aperture 
closely approaches .90. We have found it most effective 
in some work we have been doing recently, and great cred- 
it is due to the maker for its introduction. 
The worker who does not go beyond an aperture of 1:25 
can do all that his lens will permit with a dry condenser 
having the nominal aperture of 1.0 and yielding an apla- 
natic cone of ‘90 as several of them do. If higher aper- 
tures are used, an oil immersion condenser is necessary. 
This advice has an appalling sound,but it is too little rec- 
ognized that such systems can usually be worked dry,and 
