TEST-OBJECTS. 177 



Hollyhock or any other flower of the Mallow kind (Fig. 230, 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 (Plate xn.), or a large 

 Echinus-spine (Fig. 289), 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 2-3rds 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. 386), or of the Bird's Lung 

 (Fig. 388), which require a higher magnifying power for their reso- 

 lution than those previously named ; still better, perhaps, by the 

 mode in which they exhibit a portion of the wing of some Lepi- 

 dopterous 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 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 Morpho menelaus (Fig. 334), in which, with the B eye- 

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

 markings. The Tongue of the common Fly (Fig. 350) 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 well shown ; so that 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 (Plate ii., fig. 1) 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 Colour if the correction be otherwise than perfect. This 

 is the case, for example, with the Grlanduhe of Coniferous wood 

 (Fig. 210), the centres of which ought to be clearly defined under 

 such Objectives, and ought to be quite free from colour ; and also 

 with the Tracheae of Insects (Fig. 347), the spires of which ought 

 to be distinctly separated from each other without any appearance 

 of intervening Chromatic fringes. 



n. We may consider as Object-glasses of medium power the 

 Half-inch, 4-10ths-inch, l-4th-inch, and l-5th-inch; the magnify- 

 ing 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. These can only be advantageously employed in the 

 examination of Opaque objects, when they are of unusual minute- 

 ness ; 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 trans- 



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