260 MICKOSCOPE constructed, spherical aberration is almost wholly got rid of, and chromatic dispersion is so slight that the angle of aperture may be considerably enlarged without much sacrifice of distinctness. Such " doublets " and " triplets," having been brought into use in England while the com pound microscope still retained its original imperfections, proved very serviceable to such as were at that time prosecuting minute biological investigations : for example, the admirable researches of Dr Sharpey on ciliary action in animals (1830-35) and Mr Henry Slack s beautiful dissections of the elementary tissues of plants, as well as his excellent observations on vegetable cyclosis (1831), were made by their means. No one, however, would now use Wollaston " doublets " or " triplets " of high power in place of a compound achromatic microscope; and for the simple microscopes of low power that are useful either for dissecting or for picking out minute specimens (such as diatoms) other constructions are preferable, as giving a larger field and more light. As a hand-magnifier the " Coddington " lens which is a sphere of glass with a deep groove ground out of its equatorial portion has many advantages. 1 By making this groove sufficiently deep, both spherical and chromatic aberrations can be rendered almost insensible ; and, as the rays falling on any part of the spherical surface can only pass to the eye either through or near the centre, the action of every part of that surface is the same, so that the image of the object will be equally distinct (when properly focussed) whether its parts lie nearer to the axis of the sphere or more remote from it, or the axis be itself turned to one side or the other. Again, it was mathematically shown by Sir JohnHerschel in 1821 that by the combination of a meniscus with a double con vex lens the four surfaces of these lenses having certain proportionate curvatures spherical aberration could be entirely extinguished for rays parallel to the axis, the combination being thus an " aplanatic " doublet, while another combination, which he termed a " periscopic " doublet, gives a remarkable range of oblique vision with low powers, and almost entirely extinguishes chromatic aberration, although at the expense of residual spherical aberration. These combinations have been mounted both as hand-magnifiers and as single microscopes, for both which purposes they are much superior to single lenses of the same magnifying power. But such combinations have been greatly improved by the introduction of concaves of flint glass, so as to render them achromatic as well as aplanatic ; and nothing, according to the writer s experi ence, can now be used with greater advantage for all the purposes answered either by the simple microscope or the hand-magnifier than Browning s " platyscopic " lenses or the "achromatic doublets" of Steinheil of Munich. Each of these combinations gives a large flat field, with plenty of light, admirable definition, and freedom from false colour. At the period when " doublets" of very short focus were used in order to obtain high magnifying power, it was requisite to mount these on such a stand as would enable the focal adjustment to be made, and would admit the use of a special illuminating apparatus with great exactness. But now that comparatively low powers only are employed the ordinary rack-and-pinion movement is quite sufficient for their focal adjustment, and nothing more is required 1 It is difficult to understand how the name of Coddington came to be attached to the grooved sphere, seeing that he neither was nor claimed to be the inventor of it. Dr Wollaston s first "doublet" consisted of a pair of plano-convex lenses with their plane surfaces opposed to each other, and a diaphragm with central aperture placed between them. Sir D. Brewster showed that this construction is most advantageous when the two lenses are hemispheres, and the central aperture between their two plane surfaces is filled up by a transparent cement having the same refractive index as glass. And from this the transition is obvious to the grooved sphere, which had been made for Sir D. Brewster long before the high commendation it received from Mr Coddingtoii brought it into general repute. for the illumination of the object than a concave mirror beneath the stage when it is transparent, and a condensing lens above when it is opaque. The various patterns of simple microscope now made by different makers vary in their construction, chiefly in regard to portability, the size of their stages, and the mode in which "rests" or supports to the hands are provided. These, in Continental in struments, are very commonly attached to the stage ; but, unless the stage itself and the pillar to which it is fixed are extremely massive, the resting of the hands on the supports is apt to depress the stage in a degree that affects the focal adjustment ; and where portability is not an object it seems better that the hand-supports should be independent of the stage. For a laboratory microscope, the pattern represented in fig. 2 has been found very convenient, the framework being of mahogany or other hard wood, the stage FIG. 2. Laboratory Dissecting Microscope. being large enough to admit a dissection or carry a water-trough of considerable size, and the bent arm that carries the "powers" being made capable of reversion, so as to permit the use of lenses of very long as well as of very short focus. As it is desirable that the stage should not be acted on chemically by sea-water, acids, or other reagents, it may be made either of a square of plate-glass or of a plate of ebonite with an aperture in the middle ; and either of these may be made to slide in grooves in the side supports, so that one may be substituted for the other. The arm may be easily made (if desired) to carry the body of a compound microscope, so as to apply it to the examination of objects dissected or otherwise prepared under the simple microscope, without transferring them to another instrument. A portable form of simple microscope is shown in fig. 30. Compound Microscope. The placing of two convex lenses in such relative positions that one should magnify an enlarged image of a small near object formed by the other naturally soon followed the invention of the telescope, and seems to have first occurred to Hans Zansz or his son Zacharias Zansz, spectacle-makers at Middelburg in Hol land, about 1590. One of their compound microscopes, which they presented to Prince Maurice, was in the year 1617 in the possession of Cornelius Drebell of Alkmaar, who then resided in London as mathematician to king James I. In order to make clear the successive stages by which the rude and imperfect microscope of that period has, after remaining for two centuries unimproved in any essential particular, been developed within the last half- century into one of the most important instruments of scientific research that the combination of theoretical acumen and manipulative skill has ever produced, it is necessary to explain the principle of its construction, and to show wherein lay the imperfection of its earlier form. In its simplest construction, as already stated, the compound microscope consists of only two lenses, the "object-glass" CD, fig. 3, which receives the light-rays direct from the object AB placed near it, and forms an enlarged but reversed image A B at a greater distance on the other side, and the "eye-glass" LM, which receives the rays that diverge from the several points of this image as if they proceeded from the points of an actual object occupying the position and enlarged to the dimensions A B , and
brings these to the eye at E, so altering their course as to