MANAGEMENT OF THE MICROSCOPE. 167 



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

 Defining power, the Author has found the Pollen-grains of the Holly- 

 hock or any other flower of the Mallow kind (Fig. 277, A) viewed as an 

 opaque object, a very good test; the minute spines with which they are 

 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 Flatness of field no test is better than a section of 

 Wood (Fig. 253) or a large Echinus spine (Fig. 369), under an Eye-piece 

 that will give a field of the diameter of from 9 to 12 inches. The general 

 performance of Object-glasses of 1-inch and 2-3ds inch focus, may be 

 partly judged-of by the manner in which they show such injections as 

 those of the Gill of the Eel (Fig. 484), or of the Bird's Lung (Fig. 486), 

 which require a higher magnifying power for their resolution than those 

 previously named; still better, perhaps, by the mode in which they ex- 

 hibit 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 distin- 

 guished in a similar manner. The increase of Angular aperture which 

 these Objectives may advantageously receive up to 30, should render 

 them capable of resolving all the easier ' test' scales of Lepidoptera, such 

 as those of the Morplio menelaus (Fig. 414), in which, with the B eye- 

 piece, they should show the transverse as well as the longitudinal mark- 

 ings. The Proboscis of the Blow-fly (Fig. 428) * is one of the best trans- 

 parent 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 well shown. 

 In particular, all the outlines and edges should be seen clearly and 

 sharply, without any haze or fringe; the tracheal spires and rings should 

 be well-defined, without any color between them; and there should be no 

 indication of general mist. An Objective which shows this well, may be 

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

 small Echinus-spines (Plate II., fig. 1) are very good tests. The exact- 

 ness of the corrections in lenses of these foci may be judged-of by the ex- 

 amination of objects which are almost sure to exhibit Color if the correc- 

 tion be otherwise than perfect. This is the case, for example, with the 

 so-called glandules of Coniferous wood (Fig. 248), the centres of which 

 ought to be clearly defined under such objectives, and to be quite free 

 from color; and also with the trachece of Insects (Fig. 432), the spires of 

 which ought to be distinctly separated from each other, without any ap- 

 pearance of intervening chromatic fringes. 



ii. We may consider as Objectives of medium power the Half -inch, 

 4-10ths inch, l-4th inch, and l-5th inch; the magnifying power of which 

 ranges from about 90 to 250 diameters under the A eye-piece, and from 

 about 150 to 400 diameters with the B eye-piece. The first three, when 

 used by reflected light, can be advantageously employed in the examina- 

 tion of such small opaque objects as Diatoms, Polycystina, portions of 

 small Feathers, capsules of the lesser Mosses, Hairs, etc. ; they should be 

 so mounted on cones as to allow of side illumination; and the l-4th 

 should have sufficient working distance to permit its easy use for these 



1 This object should be mounted in Glycerine- jelly; for when mounted in Bal- 

 sam, the parts are usually flattened-out and squeezed together, so that their real 

 forms and relative positions cannot be seen. 



