976 INSECTS AND ARACHNIDA 



achromatism to the microscope ; for, as the clearness and strength 

 with which they could be shown were found to depend on the 

 decree to which the angular aperture of an objective could be opened 

 without sacrifice of perfect correction for spherical and chromatic 

 aberration, such scales proved very serviceable as 'tests.' The 

 Author can well remember the time when those of the Morpho Mem- 

 laus (fig. 723), the ordinary and ' battledore ' scales of the Polyom- 

 matus Argus (figs. 724, 725), and the scales of the Lepisma saccharina 

 (fig. 726), which are now only used for testing objects of loiv or 

 medium power, were the recognised tests for objects of high power ; 

 while the exhibition of alternating light and dark bands on a 

 Podura scale was regarded as a first -rate performance. It is easy 

 for anyone possessed of a good apochromatic objective of 6 mm. 

 (i- inch) to obtain all the characteristic features of the scale ; but 



the determination of the method of 

 construction of the scale and the proper 

 interpretation of the ' markings ' is a 

 matter that the wise microscopist will 

 prefer to relegate to the days when the 

 apertures of our best present lenses will 

 be looked upon comparatively as we now 

 look upon the earliest achromatic ob- 

 jectives. No one can give a fairly 

 comprehensive and satisfactory sugges- 

 tion of the true nature of the Podura 

 scale, and yet on no one object has 

 microscopy lavished so much labour for 

 so many years. 



The easier test scales are furnished 

 by the Lepidoptera (butterflies and 

 moths), and among the most beautiful 

 of these, both for colour and for regu- 

 larity of marking, are those of the 

 Morpho Menelaus (fig. 723). These are 

 of a rich blue tint, and exhibit strong 

 longitudinal striae, which seem due to 

 ribbed elevations of one of the superficial layers. There is also an 

 appearance of transverse striation, which cannot be seen at all with 

 an inferior objective, but becomes very decided with a good objective 

 of medium focus ; and this is found, when submitted to the test of a 

 high power and good illumination, to depend upon the presence of 

 transverse thickenings or corrugations (fig. 723), probably on the in- 

 ternal surface of one of the membranes. The large scales of the Poly- 

 ommatus Argus ('azure blue ' butterfly) resemble those of the Menelaus 

 in form and structure, but are more delicately marked (fig. 724). 

 Their ribs are more nearly parallel than those of the Menelaus scale, 

 and do not show the same transverse striation. When one of these 

 scales lies partly over another, the effect of the optical intersection 

 of the two sets of ribs at an oblique angle is to produce a set of 

 interrupted striations (6), very much resembling those of the Podura 

 scale. The same butterfly furnishes smaller scales, which are com- 



FIG. 723. Scale of Morpho 

 Menelaus. 



