ORGANIC TEST- OBJECTS. 133 



testing uf high-power objectives, have only very recently come 

 into general use, but they now form the most usual resource of 

 the optician. Although we do not consider the results obtained 

 with these test-objects strictly and decisively reliable, yet it 

 cannot be denied that they furnish an easy mode of approximately 

 judging of the performance of a Microscope. A knowledge of 

 the ordinary test-objects is therefore always of practical value, 

 and every microscopist may be recommended to take note of the 

 sharpness and clearness of the images produced of them in a good 

 instrument with a definite amplification. We therefore subjoin 

 a brief list of the butterfly-scales and diatom- valves which may 

 be advantageously used as test-objects, with a short description 

 of the more important. 



With regard to the scales of butterflies, they usually show 

 distinct longitudinal stria (which run parallel to each other, 

 or somewhat diverging from the base of the scale to its upper 

 edge), with less distinct transverse strict; , which cross the others at 

 right angles and come into view with somewhat lower focal 

 adjustment. The series of markings ought to be seen distinctly 

 with the amplifications indicated. 



1. Lepisma saccharinum. Longitudinal striae of the larger 

 scales, distinct with an amplification of 40, of the smaller with 

 100 150. The larger scales are wedge-shaped, the smaller ones 

 rounder, with paler and closer markings. 



2. Hipparchia Janira. (a) Scales from the wings of the female. 

 Longitudinal striae, according to Schacht, visible with a power 

 of 80, transverse strife by oblique illumination with Hartnack's 

 No. 5, Beneche's No. 7, and Zeiss's C objectives, that is, with a 

 power of 120 250. With higher powers the transverse striae 

 should be clearly seen, even with direct (axial) light, as sharp lines 

 (not knotted or broken). The long transparent scales are always 

 more difficult than the darker coloured ones, (b) Scales of the 

 upper side of the wings of the male. Longitudinal stride, according 

 to Mohl, visible only with the highest objectives of Amici and 

 with oblique illumination. There is no trace of transverse striae, 

 which, without doubt, are also present. 



3. Lycmna Argus. Scales of the upper surface of the fore-wing. . 

 (a) Those which by reflected light appear blue, and by transmitted 

 light yellow. Longitudinal striae visible with lower powers (50 

 80 linear), transverse striae only with about 300. Deserving especial 



