120 THE AMERICAN MONTHLY [Jun 
easily obtaining it without putting the sub-stage con- 
denser out of focus ; that method will not be thought de- 
sirable by any one. who has tried the experiments re- 
commended above. Neither is the expedient of placing 
an auxiliary condenser in front of the lamp flame to be 
altogether commended, although a properly arranged 
and focussed aplanatic “‘bull’s-eye” thus used is, in skil- 
ful hands, capable of affording good (but not strictly 
critical) results when compared to the images obtainable 
with an out-of-focus sub-stage condenser. The proper 
and easy way to obtain perfectly critical representations 
of objects viewed under a 14-in. or l-in. objective, with 
the whole field fully illuminated, is to employ a very 
low power achromatic condenser affording a sufficiently 
large aplanatic cone for the purpose. This maximum 
aperture need not exceed ‘3 for the 1-in., and may be, 
of course, considerably less for the 14-in. ; but as it will 
be frequently found advantageous to utilize such an ap- 
pliance with a 4-in., especially in photomicography, it 
should be capable of yielding a solid cone of N.A. °5. 
Now, it is a remarkable fact that no cheap very-low power 
achromatic condenser of the kind above indicated is at 
present on the market, so that clearly no great demand 
for it exists, although there must be hundreds of earnest 
investigators to whom it would prove a boon could they 
once realize the advantages derivable from it. As a 
makeshift it will be found,» however, that one of the or- 
dinary triplet combination loups magnifyiug 6 times, 
when mounted in a suitable fitting and provided with an 
iris diaphragm, will answer well for 2-in. objectives, af- 
fording a large image of the light source, and practically 
filling the field when the broad part of the lamp is fo- 
cussed. The Zeiss Abbe achromatic condenser with top 
lens removed gives even better results; and I would es- 
pecially call attention to the modified semi-apochromatic 
form recently introduced by Baker, which, without the 
