TEST-OBJECTS FOB, LOW AND MEDIUM POWER. 195 



tracheae of Insects (Fig. 291), the spires of which ought to be 

 distinctly separated from each other, without any appearance of 

 intervening chromatic fringes. 



II. We may consider as object-glasses of medium power, those 

 which range from half to one-fifth of an inch focus; whose 

 magnifying power is from about 100 to 250 diameters under the 

 shallower eye-piece, and from about 150 to 375 diameters with 

 the deeper. These cannot be advantageously employed in the 

 examination of opaque objects, save of such as are of unusual 

 minuteness ; but their great value lies in the information they 

 enable us to obtain, regarding the details of organized structures 

 and of living actions, by the examination of properly prepared 

 transparent objects by transmitted light. It is to these lenses, 

 that the remarks already made respecting angular aperture ( 101) 

 especially apply ; since it is in them that the greatest difference 

 exists, between the ordinary requirements of the scientific in- 

 vestigator, and the special needs of those who devote themselves 

 to the particular classes of objects for which the greatest resolv- 

 ing power is required. A moderate amount of such power is 

 essential to the value of every objective within the above-named 

 range of foci ; thus, even a good half-inch should enable the 

 markings of the larger scales of the Polyommatus argus (azure- 

 blue butterfly) to be distinguished, these being of the same kind 

 with those of the Menelaus, but more delicate, and should clearly 

 separate the dots of the small or "battledoor" scales (Fig. 280) of 

 the same insect, which, if unresolved, 'are seen as coarse longitu- 

 dinal lines; a good 4-10ths in. should resolve the larger scales of 

 the Podura (Fig. 281) without difficulty; and a good l-4th or 

 l-5th in. should bring out the markings on the smaller scales of 

 the Podura, and should resolve the markings on the Pleurosigma 

 hippocampus into longitudinal and transverse lines. Even the 

 4-10ths (a power for which Messrs. Smith and Beck have attained 

 a deserved celebrity) may be made with an angle of aperture 

 sufficiently wide to resolve the objects named as fair tests for the 

 powers above it ; and so the l-4th inch may, by the enlargement 

 of its angular aperture to 120 (which has been accomplished by 

 Mr. Ross) be made to exhibit the more difficult Diatomacese. 

 But it will be found that, in such object-glasses, the difficulty of 

 making the most advantageous use of them, and the loss of 

 penetrating power which necessarily attends the excessive exten- 

 sion of their angular aperture, are most serious drawbacks to 

 their practical utility in the hands of the Anatomical or Physio- 

 logical investigator ; for whose purposes, such a resolving power 

 as will show the easier tests first enumerated, combined with 

 perfect definition, with a fair amount of penetrating power, and 

 with flatness of field, constitute the best combination. For de- 

 fining power, very good tests are found in the complex hairs of 

 many animals, such as the Indian Bat (Fig. 310, c), and the 

 Dermestes (Fig. 282, B). And for that combination of the seve- 



