198 A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 



from five to ten minutes before ssarching for faint detail. 

 Frequent 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. In my own case I find that 

 from about an hour after lunch to 6 or 7 p.m. is decidedly the 

 be->t period, but the evening hours can also be usefully employed. 

 The morning is to me individually the most unfavourable part 

 of the day for observational purposes, my eyes then becoming 

 quickly and easily fatigued. It may well be, however, that the 

 experience of others will differ in this respact, 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 easiest 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 |-in, objective of about N.A. "30. 

 Experiments with such a lens will ba 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 

 magnifications are in question, the rough-and-ready method is 

 quite satisfactory for low-power general work. To absolutely 

 disprove the truth of this so frequently reiterated statement, 

 you have only to place a common entomological object, such as 

 the proboscis, wing, or leg of a fiy, 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 appearance 

 of the smaller features of the object : (a) with the con- 

 denser diaphragm quite open, thus giving a full cone, i.e., when 

 the eyepiece is removed the back of the objective appears evenly 

 illuminated over its entire area ; (5) with the diaphragm a little 

 closed so as to illuminate |ths of the central portion of the back 

 lens of the objective, leaving a narrow dark outer annulus ; (c) 

 with the diaphragm successively more and more closed up, so as to 

 light respectively about |, |, 5, and gth of the central part 

 of the back lens. Then, on the same object, carefully observe 

 the effect, of lowering or raising the substage condenser beyond 

 its focal point, an all too common expedient for obtaining an 



