65G TEGUMENTARY APPENDAGES OF INSECTS '. — SCALES. 



very much the appearance of being tiled like the roof of a house. 

 Such an arrangement is said to be 'Imbricated.' The forms of 

 these Scales are often very curious, and frequently differ a good 

 deal on the several parts of the Wings and of the Body of the same 

 individual ; being usually more expanded on the former, and 

 narrower and more hair-like on the latter. The peculiar markings 

 which many of these Scales exhibit, very early attracted the atten- 

 tion of those engaged in the improvement of the Microscope by the 

 application of the principle of Achromatic Correction ; since these 

 markings are entirely invisible, however great may be the magnify- 

 ing 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 ai*e 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 combina- 

 tion. At first, the Scale of the Poclura (Fig. 335) 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 Stria? into their component markings, 

 is now considered as but a very ordinary test for the medium, 



powers of the Microscope ; and 

 Fig. 334. Tests of much greater difficulty, 



and therefore more suitable 

 for the higher, are afforded (as 

 we have seen, § 132, in.) by 

 the Valves of the Diatomaceae. 

 Still, the resolution of their 

 markings depends more upon 

 angle of aperture than upon 

 other qualities of an Objective ; 

 and for testing definition, pene- 

 tration, and that general excel- 

 lence which is needed to con- 

 stitute a good ' working power, ' 

 there is probably no better 

 object than a Podura scale. 

 Further, as even the easier 

 Test-scales of Insects have 

 their use, in enabling us to 

 appreciate the performance of 

 Achromatic Objectives of me- 

 dium power (§ 132, n.), it 

 will be advantageous here to 

 notice a few of those which are 

 most commonly employed for 

 Scale of Morpho Menelaus. this purpose. 



