and Laboratory Methods. 2595 



comes to determining some faint, though nevertheless distinctive, structures on 

 certain materials which the microscopist has to examine, the best are found to 

 be none too good. 



The achromatic series of objectives are, on the whole, very good, though at 

 present the series par excellence is without question the apochromatic ; the greatest 

 drawback to them from the standpoint of the purchaser being the fact that, for cor- 

 responding powers, they are from two to four times as expensive as the achro- 

 matic. If the achromatic objectives are finally chosen, the Huyghenian oculars 

 will be satisfactory ; but for the apochromatic series the compensating series 

 should be used. This is necessary, since, as the name indicates, they compen- 

 sate for certain overcorrections of color made in the present series of apochro- 

 matic lenses. 



To test the defining power of an instrument it is usual to examine such 

 objects as starch from potato or bean, diatoms and scales from a butterfly's 

 wing or other objects possessing delicate markings or structures. Each of these 

 objects possesses well defined markings, which with proper conditions of light 

 should be well shown by an instrument intended for exact work. 



The starches possess rings, usually concentric in the bean or eccentric in 

 the case of the potato. For examination small samples are mounted in water or 

 dilute glycerine. On account of the transparent nature of the grains the sub- 

 stage light must be cut down by the substage diaphragm until by trial the most 

 exact detail of markings is obtained. With a good instrument the outlines of 

 the grains should appear sharp and distinct with no (or at least very little) color. 

 Bright colors of course will not be expected since they usually take the form of 

 a faint suggestion of blue or yellow around the edge of the object, depending on 

 which side of the focal plane the grain happens to lie. Hence, by changing the 

 conditions from one side to the other by focusing, the colors may be seen to 

 alternate. 



According to the writer's experience this phenomenon is also ordinarily 

 accompanied by the grains appearing to be covered by a slight fog or haze. 

 The latter may, however, be caused by soiled lenses, or because that upon one or 

 more of the lenses there has been deposited some form of film.' The feature 

 of non-achromatism in low or medium power objectives can be strikingly shown 

 by placing a fine wire gauze or other opaque object having openings at short 

 intervals over the surface, in the field and focusing upon it. In this condition 

 the colors will be strongly shown if the lens is not accurately corrected. 



In a similar manner to that described for the starch, examine also the butter- 

 fly scales and diatoms. The former is characterized by having longitudinal 

 markings upon the surfaces and also (in some species at least) fine connecting 

 lines. The diatoms show various kinds of markings, depending upon the species. 

 One form is shown in Fig. 2. 



In using the microscope when focusing upon a new specimen it is a matter 

 of wise precaution against the accident of thrusting the objective forcibly down 

 upon the slide to lower the objective nearly to the cover-glass, meanwhile watch- 



1 Edward Bausch, "On Determination of Supposed Defects in Microscope Objectives." 

 Journal Applied Microscopy and Lab. Methods, Vol. I, No. i. 



