(A) INTRODUCTION (cont.)

Wollaston Doublet.—This consists of a combination of two plano-convex lenses, whose focal lengths (as directed by Dr Wollaston) should be as 3 to 1, with their plane sides turned towards the object,—the sinaller lens being placed lowest, and the upper lens at a distance of one and a half times its focal length above it. This construction, however, has been subsequently improved—(1) by the introduction of a perforated diaphragm between the lenses; (2) by a more effective adjustment of the distance between the two lenses, which seems to be most satisfactory when it equals the difference of their respective focal lengths, allowance being made for their thickness; and (3) by the division of the power of the lower lens (when a shorter f ocus than 1/10 inch is required) into two, so as to form a "triplet." When combinations of this kind are well constructed, spherical aberravion 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 compound 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. [Footnote 260-1] 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 John Herschel in 1821 that by the combination of a meniscus with a double convex 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 anpther 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 experience, 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 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 coinstruction, 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 instruments, are very commnonly 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 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.





Footnote

260-1 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 lemes with their plane surfaces opposed to each other, and a diaphragni 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 Coddington brought it into general repute.


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