72 SELECTION AND USE 



to see these lines, it is necessary that the 

 stand be a good one, and that the light 

 be very carefully managed, but, even 

 with the most perfect arrangements or- 

 dinarily used, we cannot, with such an 

 objective, discover similar markings upon 

 the Pleurosigma Angulatum, although 

 they exist there in great perfection. But 

 if for the French sixth we substitute a 

 first-class objective of less than half its 

 magnifying power, but of wider angular 

 aperture, we shall be able to see the lines 

 quite distinctly. We have now before us 

 an objective of four- tenths of an inch 

 focus, which does not correct for thick- 

 ness of cover, but which, with any ordi- 

 nary thickness of covering glass, is capa- 

 ble of resolving the lines on the Angu- 

 latum perfectly, and we have seen objec- 

 tives of even lower magnifying power 

 which would accomplish the same thing. 

 That the effect depends here chiefly 

 upon angular aperture, was shown very 

 clearly by Dr. Goring, from whose work 

 we take the following figures, engraved 

 from seven drawings showing the ap- 

 pearances presented by the scale of a 

 butterfly's wing, viewed with the same 

 magnifying powers, but different angular 

 apertures. A well corrected lens of wide 

 aperture showed the scale as in G; re- 

 ducing the aperture, while all else re- 

 mained the same, the appearance was as 

 shown in F, and by successive reduc- 

 tions the stages shown in E, D, C, B } 

 and A, were reached. The slightest ex- 

 amination shows that features which were 

 quite distinct under a high angle, became 

 invisible when the angle was reduced, Fig. 17. 



