344 



MICROSCOPE. 



drawing out of the eye-piece until the end is 

 accomplished answers the object most simply 

 and effectually. In the use of the micro- 

 metric eye-piece, also, which will be presently 

 described, the capability of adjusting the mag- 

 nifying power to a certain definite amount will 

 be found of very great utility. It is to be 

 borne in mind, however, that for giving the 

 highest effect to the achromatic objectives, a 

 certain fixed distance of the eye-piece is neces- 

 sary ; this is usually adjusted by the maker; 

 but it may be easily determined by trial, since 

 at any other the want of correction of the chro- 

 matic aberration will make itself apparent 

 (however slightly) by the presence of coloured 

 fringes around the images. 



The degree of perfection in the construction 

 of the optical part of a microscope, whether 

 simple or compound, is judged of by the dis- 

 tinctness and comfort (by which we mean the 

 freedom from strain or effort on the part of the 

 observer) with which it exhibits certain objects, 

 the details of which can only be made visible 

 by combinations of lenses of high magnifying 

 power and a near approach to correctness. 

 Such are called test objects. They are of 

 various degrees of difficulty. For testing the 

 penetrating powers of a microscope, the lined 

 scales on the wings and bodies of certain in- 

 sects are commonly employed. The scales 

 from the vvm"s of many Lepidoptera are so 

 coarsely marked, that a good ordinary com- 

 pound microscope, or deep single lens, will 

 make the lines apparent. But there are many 

 others, which, under such magnifying powers, 

 only show a flat unmarked surface, requiring 

 lenses of large angular aperture to make their 

 lines visible. One of the most beautiful of 

 these, and at the same time most easily re- 

 solved, is the scale of the Menelaus butterfly; 

 its longitudinal stria may be seen in the best 

 ordinary microscope, but they require a cor- 

 rected object-glass to be well made out; and 

 its transverse markings are only to be seen 

 distinctly with a superior instrument. A scale 

 with more delicate lines than these is the 

 larger of those of the Lyctena Argus; the smal- 

 ler one will be presently noticed. The most 

 difficult of the test-scales, however, is that of 

 the Podura plumbea, or common spring-tail ; 

 and a microscope which will distinctly exhibit 

 its markings may be regarded as, for this class 

 of objects, of the highest order For defining 

 power, however, another class of objects is 

 needed. Among these one of the best is the 

 small scale of the Lycana Argus (or battledore), 

 which, with an inferior instrument, appears 

 covered with coarse longitudinal lines; but 

 these, when more perfectly defined, are found 

 to be resolvable into separate circular or oval 

 dots, arranged in a linear manner. The curi- 

 ous hair of the Dermestes, and that of the 

 Bat, require a microscope of good defining 

 power to represent their forms with clearness 

 and accuracy. We have ourselves been accus- 

 tomed to employ the branching hairs of the 

 common bee as tests of the correctness of a 

 microscope of moderate power; for they have 



a remarkable tendency to produce fringes of 

 colour, under most of the ordinary modes of 

 illumination, unless there is a perfect freedom 

 from chromatic aberration. The spiral fibres 

 lining the tracheae of many insects, also, will 

 be found good tests of the defining power, 

 and freedom from aberration, of a microscope. 

 For still lower powers, we consider the glan- 

 dular dots in the woody fibre of the resinous 

 trees (especially those of the order Coniferae) 

 as very advantageous tests. They ought to be 

 rendered distinctly visible with an object-glass 

 of half an inch focus; and the depression in 

 the centre should be clearly made out, with a 

 perfect freedom from colour. We have seen 

 objectives of English construction (in which a 

 lan;e aperture for showing opaque objects was 

 the chief point aimed at) so defective in this 

 respect, as to be far inferior in their exhibition 

 of these arid similar objects to French acro- 

 matics of much smaller aperture. We may 

 here repeat the general rule, that those micro- 

 scopes are, ceteris paribus, the best, which will 

 show the most with the lowest magnifying 

 power. It will sometimes happen that, al- 

 though the details of an object may be made 

 out with tolerable clearness, there is a sort of 

 thin fog or mist over the whole field. This 

 fault may proceed from the too great enlarge- 

 ment of the aperture of the objective, or from 

 a faulty mode of illumination ; or it may result 

 from the imperfect extinction of the rays re- 

 flected within the body of the microscope from 

 the surfaces of the lenses of the eye-piece, 

 and from the interior of the tube itself,*- 

 a fault which may be obviated by carefully 

 coating the inner surface with a black covering, 

 adapted to absorb all the false light. Black 

 velvet may be advantageously used for this 

 purpose. If the aperture be too great, the 

 fault may generally be corrected by the use of 

 stops beneath the stage, by which it may be 

 diminished as required. This plan, which will 

 be presently described more in detail, will be 

 found very much to increase the applicabi- 

 lity of low-power achromatic lenses of that 

 large aperture which is desirable for opaque 

 objects. 



II. OF THE MECHANICAL ARRANGEMENTS 

 OF MICROSCOPES. 



Having now described, with as much detail 

 as the nature of this article permits, the prin- 

 ciples on which the operation of the micro- 

 scope depends, we shall next proceed to con- 

 sider the means of arranging the optical por- 

 tion of the instrument, so as to confer upon it 

 the best and most varied application, keeping 

 especially in view, however, the wants of the 

 anatomist and physiologist. We shall begin 

 by stating what, in any form of microscope, 

 we regard as the essential conditions to be 

 attained in its construction. 



1. Steadiness and firmness in all its parts, 



* [Or it arises, as suggested to the Editor by 

 Mi-. Powell, the eminent optician, from imperfection 

 ia the correction ol the objectives. El).] 



