1902 -MICROSCOPICAL JOURNAL 121 
front lens, has a power of eight-tenths-in., and an aplan- 
atic N.A. of ‘5, thus rendering it available for use with 
a wide-angled 3-in. apochromat, while with the top lens 
in situ, it may be satisfactorily employed with the 
higher powers, being then a four-tenths-in. of aplanatic 
N.A. -9; but in this latter respect the high power dry 
and oil-immersion condensers of all our leading opticians 
now leave little to be desired. 
Regarding the third point—i. e., the apparent care- 
lessness and lack of interest in all optical matters evinc- 
ed by many naturalists—it is difficult to believe that such 
men, most of whom will put themselves to infinite pains 
in the collection, careful preparation, and preservation 
of their specimens, could fail to be equally fastidious 
and painstaking with respect to the manner in which 
they exhibited to themselves and others the fruit of their 
hard work, toil none the less taxing because it happens 
to be entirely alabor-of love, without thought of gain, 
were they once convinced that any practical advantage 
might be derivable in their own particular branches of 
research by a little more careful attention to optical 
principles. In some instances these workers may be de- 
terred from any attempt to grapple with the subject by 
the mistaken idea that considerable study of an intricate 
nature in a, to them, uncongenial field is requisite, or 
that the cost of a proper instrumental outfit to secure 
optical results of a high order is quite beyond their 
means. 3 
For the attainment of a strictly critical image the only 
essentials are :—(a@) That the source of light (preferably 
the edge of a lamp flame) be sharply and centrally focuss- 
ed on to the object by means of a sub-stage condenser of 
suitable aplanatic aperture, the microscope being either 
pointed directly to an illuminant, or,if the angle at which 
the instrument must be inclined to effect this should from 
any cause prove inconvenient, the plain mirror may be 
