118 THE AMERICAN MONTHLY (Jun 
five to ten minutes before searching for faint detail. Fre- 
quent rests of this kind should also be taken in the 
course of all observational work. Thirdly, it should be 
remembered that the eye itself is an optical instrument | 
which varies much in its capacity at different times. I 
find that from about an hour after lunch to 6 or 7 p. m. 
is decidedly the best period, but the evening hours can 
also be usefully employed. The morning is to me indi- 
vidually the most unfavorable part of the day for obser- 
vational purposes, my eyes then becoming quickly and 
easily fatigued. It may well be, however, thatthe ex- 
perience of others will differ in this respect, but each 
observer will probably find that his best work can only 
be accomplished during some certain hours of the day or 
evening. | 
But probably the easyest means by which the beginner 
will be enabled to most speedily convince himself of the 
advantages of critical microscopy for ordinary working 
purposes, lie in the proper employment of a good 1-in. 
or 3-in. objective of about N.A. ‘30. Experiments with 
such a lens will be the more likely to prove instructive, 
as it has been over and over again asserted that, however 
necessary critical images may be when the higher mag- 
nifications are in question, the rough-and-ready method 
is quite satisfactory for low-power general work. To ab- 
solutely disprove the truth of this so frequently reiter- 
ated statement, you have only to place a common entom- 
ological object, such as the proboscis, wing, or leg ofa . 
fly,or other similar preparation, under, say, a cheap 
Leitz No. 3 objective (§-in.) of N.A. 28, and sharply focus 
on the selected specimen the image of the edge of the 
lamp flame, using a low-power achromatic sub-stage con- 
denser. Having done this, carefully examine the appear- 
ance of the smaller features of the object : (a) with the 
condenser diaphragm quite open, thus giving a full cone, 
i. e. when the eyepiece is removed the back of the objec- 
