194 MANAGEMENT OF THE MICROSCOPE. 



it is of Qo advantage to see, whilst, through the sacrifice of pene- 

 tration, those parts of the object which are brought exactly into 

 focus being seen with over-minuteness, the remainder are en- 

 veloped in a thick fog, through which even their general contour 

 can scarcely be seen to loom ; and if the corrections be imper- 

 fectly made, no line or edge will be seen with perfect sharpness. 

 For defining power, the Author has found the pollen-grains of 

 the Hollyhock, or any other flower of the Mallow kind (Fig. 189), 

 viewed as an opaque object, a very good test; the minute spines 

 with which it is beset, being but dimly seen with any save a good 

 object-glass of these long foci, and being really well exhibited 

 only by adding such power to the eye-piece, as will exaggerate 

 any want of definition on the part of an inferior lens. For flat- 

 ness of field, no test is better than a section of Wood (Fig. 165) 

 or a large Echinus-spine (Fig. 237), under an eye-piece that will 

 give a field of the diameter of from 9 to 12 inches. Such objects 

 ought to be very well shown by the divided lens of 2 in. or 3 in. 

 focus ; but, as its corrections are rendered imperfect by the re- 

 moval of the front pair, its defining power is necessarily impaired, 

 and cannot be made even tolerable, save by such a curtailment 

 of the aperture as detracts from the brightness of the image. 

 The general performance of object-glasses of 1 in. and 8-lOths 

 in. focus, may be partly judged of by the manner in which they 

 show such injections as those of the Gill of the Eel (Fig. 330) or 

 of the Bird's Lung (Fig. 332), which require a higher magnifying 

 power for their resolution than those previously named; still 

 better, perhaps, by the mode in which they exhibit a portion of 

 the wing of some Lepidopterous insect, having well-marked 

 scales ; the same qualities should here be looked for, as in the 

 case of the lowest powers ; and a want of either of them is to be 

 distinguished in a similar manner. The increase of angular 

 aperture which these lenses may advantageously receive, should 

 render them capable of resolving all the easier "test" scales of 

 Lepidoptera, such as those of the Morpho menelaus (Fig. 279), in 

 which, with the deeper eye-piece, they should show the trans- 

 verse as well as the longitudinal markings. The tongue of the 

 common Fly (Fig. 287) is one of the best transparent objects for 

 enabling a practised eye to estimate the general performance of 

 object-glasses of these powers; since it is only under a really 

 good lens, that all the details of its structure can be clearly made 

 out ; and an objective which shows this well, may be trusted to 

 for any other object of its kind. For flatness of field, sections 

 of small Echinus- spines are very good tests. The exactness of 

 the corrections in lenses of these foci, may be judged of by the 

 examination of objects which are almost sure to exhibit color, 

 if the correction be otherwise than perfect ; this is the case, for 

 example, with the glandulse of Coniferous wood (Fig. 161), the 

 centres of which ought to be clearly defined under such objec- 

 tives, and ought to be quite free from color ; and also with the 



