1901) MICROSCOPICAL JOURNAL. 163 
differentiated coloration is plainly or distinctly visible. 
SuBSTAGE CoNDENSERS.—It is gratifying toobserve the 
number of first-class substage condensers that are offer- 
ed by manufacturers, and it is a distinct indication of 
growing knowledge and spissalep inal of good things on 
the part of workers. 
It was at one time an easy matter to make achoice when 
only two or three systems were available, but it is evi- 
dently presenting some complexity now, and in response 
to correspondents’ enquiries we propose to give a few 
hints.on the subject. 
The main features of a condenser are: (1) The achro- 
matism, (2) aplanatism, (3) magnifying power, and (4) 
the size of the fixed lens. 
Achromatism and aplanatism can be considered togeth- 
es, but the latter is more important. Recognizing this, 
there is a tendency on the part of makers to claim greater 
aplanatism than is actually yielded; this can however, 
easily be verified by the methods described in the text- 
books. Achromatism is adesirable quality but we doubt 
the advantage of an apochromatic over an achromatic con- 
denser ; we would as readily work with the latter as the 
former provided the aplanatism were as well corrected, 
and this is frequently the case. Expense may therefore 
be avoided without loss of efficiency in this respect. The 
solid illuminating cone that an objective will bear has 
been frequently discussed. It is generally stated that 
three-fourths the full aperture is the best, but it will be 
found that the majority of lenses will not bear more than 
two-thirds without deteriorating in performance; there 
are some exceptional ones that will take more thana three- 
quarter cone, but this is not the rule, and a light filter is 
usually requisite. 
The Power.—The magnifying power of the condenser 
should not exceed half that of the objective, less rather 
than more than half is always preferable. Many systems 
