SCALES OF INSECTS. 



657 



peculiar markings whicli many of these scales exhibit, very early 

 attracted the attention of those engaged in the improvement of 

 the Microscope by the application of the principle of achromatic 

 correction (p. 41); since these markings are entirely invisible, 

 however great may be the magnifying power employed, under 

 microscopes of 'the older construction, owing to the necessary 

 limitation of their angular aperture ; whilst, as they are brought 

 into view with a clearness and strength that are proportionate to 

 the extension of the angular aperture and the perfection with 

 which the aberrations are corrected, they serve as "test objects" 

 of the goodness of an achromatic combination. At first, the 

 scale of the Podura (Fig. 281) was the most difficult test known 

 for the highest powers ; and a microscope which could only 

 exhibit an alternation of dark and light bands or striae upon its 

 surface, was considered a good one. But even the complete 

 "resolution" of these strife into their component markings, is 

 now considered as but a very ordinary " test" for the medium 

 powers of the Microscope ; and tests of much greater difficulty, 

 and therefore more suitable for the higher, are affiarded (as we 

 have seen, § 102, III) by the valves of the Diatomacese. Still, 

 the test scales of Insects have their use, in enabling us to ap- 

 preciate the performance of achromatics of medium power 

 (§ 102, ni); and it' will therefore be advantageous here to 



Fig. 279. 



Fig. 2S0. 



Scale of Morpho Menelaus. 



Batlledoor Scale of Polyommnius argus 

 (azure blue). 



notice a few of those which are most commonly employed for 

 this purpose. 



.380. Among the most beautiful of all these scales, both for 

 color and for regularity of marking, are those of the butterfly 

 termed Morpho Menelaus (Fig. 2T9). These are of a rich blue 



