This combination was first proposed by Dr. Wollaston, and is known as his doublet. It consists of two plano-convex lenses, whose focal lengths are in the proportion Of one to three, or nearly so, having their convex sides directed towards the eye, and the lens of shortest focal length nearest the object. In Dr. Wollaston's original combination no stop was interposed, and the distance between the lenses was left to be determined by experiment in each case. A great improvement was subse quently made, however, by the introduction of a stop between the lenses, and by the divi sion of the power of the smaller lens between two; this is due to Mr. Holland.* By these means a combination may be produced, in which the errors are made to correct each other so nearly that all the advantages of a wide aperture with a very short focus may be gained. The general nature of the performance of a doublet or triplet may be understood from the adjoining figure, (fig.160,) in which L 0 L' is of the axis 0 P ; so that each becomes af fected by opposite errors, which to a certain extent balance and correct one another. To take the pencil L, for instance, which enters the eye at It I3, It B; it is bent to the right at the first lens, and to the left at the second ; and as each refraction alters the direction of the blue rays more than of the red, and more over, as the blue rays fall nearer the margin of the second lens, where the increased power of the refraction, consequent upon the distance from the centre, compensates in some degree for the greater focal length of the second lens, the blue and red rays will emerge very nearly parallel, and are therefore colourless to the eye. At the same time the spherical aberration has been diminished by the circumstance that the side of the pencil, which passes one lens nearest the axis, passes the other nearest the margin. This explanation applies only, how ever, to the pencils near the extremities of the object. The central pencil, it is obvious, will pass through the same relative portions of the two lenses, and only an imperfect correction will therefore take place, and of those issuing from the intermediate points the amount of correction will vary with their proximity to the centre or to the circumference. Hence a dou blet is not a perfect magnifier; but it is very much superior to a single lens, and may be so constructed as to show many of the usual test objects,—especially those in which a moderate amount of penetration is sufficient, provided the definition be good,—in a very beautiful manner. Its angle of aperture, however, by which is meant the angle of the apex of the conical pencils of rays admitted by it, cannot be advantageously increased much beyond or 45°. But when the smaller lens is replaced by a combination of two others, so as to form a triplet, their joint aberration is so much less the object, P a portion of the pupil, and D D the The pencils of light from the two extremities, L L', of the object cross each other in the stop, and consequently pass through the two lenses on the opposite sides that it is more counterbalanced by the third lens placed above the stop. In this manner the transmission of a still larger angularpencil, —even to 65°,—is rendered compatible with distinctness; and great penetrating power is thus combined with perfect definition, as well as with brilliancy of illumination. For the purposes of anatomical investigation, as we shall hereafter state, we consider good doublets and triplets, where circumstances admit of their employment, superior to any other kind of magnifying instrument. The principal disadvantages which the use of them involves are the close proximity to the object required by their very short focus when a high magnifying power is employed; and the strain ing of the eye, which is occasioned by their very minute aperture. Thus a triplet in our possession, which will show the most difficult test-objects, has a focal distance of only about th of an inch, and an aperture through which the smallest pin would scarcely pass. But the first of these disadvantages is more apparent than real. The object should be always co vered with talc, (which may be easily split into lamina: of the of an inch in thickness,) for the purpose of protecting both it and the lens from injury by accidental contact ; and the magnifier should not be screwed into the arm which carries it, but loosely fitted, so that, if the observer should happen to bring the arm too near the stage, he may not force down his lens upon the object. As Mr. Holland justly ob serves, " Should the proximity of the object to the lowest lens of the triplet be urged as a material objection to its usefulness, it may be answered that the whole microscope is a mass of delicacies ; consequently, it cannot be al lowed that a line be arbitrarily drawn, beyond which every thing is to be considered as too delicate." The second of the above objections must be obviated by never continuing the use of deep powers in a simple microscope for any length of time at one sitting, and by taking care to adjust the instrument in such a manner that the head may be as little inclined forwards as possible.
The only other form of simple microscope which we shall notice is one commonly known under the name of the Coddington lens. The first idea of it was given by Dr. Wollaston, who proposed to cement together two piano convex, or hemispherical lenses, by their plane sides, with a stop interposed, the central aper ture of which should be equal to ith of the focal length. The great advantage of such a
lens is that the oblique pencils pass, like the central ones, at right angles with the surface ; and that they are consequently but little subject to aberration. The idea was further improved upon by Mr. Coddington, who pointed out that the same end would be much better an swered by taking a sphere of glass, and grind ing away the equatorial parts, the groove being then filled with opaque matter, so as to limit the central aperture. Such a lens gives a large sphere of view, admits a considerable amount of light, and is equally good in all directions; but its powers of definition are by no means equal to those of an achromatic lens, or even of a doublet. This form is very useful, there fore, as a hand lens, in which a high power is not required, but has no particular advantages for magnifiers of short focus, nor for the object glasses of a compound microscope.* It may be desirable to mention that a magni fier, now known under the name of the Stan hope lens, and much praised by those interested in its sale, is nothing more than a double con vex glass, much thicker than ordinary, so that an object in contact with one of its surfaces shall be in focus to the eye placed behind the other. This is an easy method of applying rather a high magnifying power to scales of butterflies' wings and other similar flat and minute objects ; but the instrument is totally destitute of value as a means of scientific re search, and must be regarded as an ingenious philosophical toy.
Compound Micro scope. — The com pound microscope es sentially consists, as already stated, of two lenses, which are so disposed that one of them receives the rays of light from the object, and forms an image by its refraction of them ; and this image is seen by the eye through the second lens, which acts upon it as a simple mi croscope. The princi ple of such a micro scope will be at once understood from the adjoining diagram (fig. 162). According to the laws already stated, if the object be at a less distance from the lens than its diameter of cur vature (supposing it to be a double - convex lens, or twice that dis tance of a plano-con vex) the lunge will be larger than the object; and this in proportion as the latter is brought nearer to the principal focus, at which it can give rise to no image, as its rays after refrac tion become parallel. Hence, by the use of the same object-glass, a considerable variety of power might be ob tained; for, if the image be formed near the lens, it will be small; but if the object be caused to approach it, the image will be thrown to a con siderable distance, and will be proportionably magnified. The eye piece would of course require, however, a cor responding re-adjust ment; and, in fact, the construction of the whole instrument would need modifica tion. Further, the optical disadvantages of such a plan would be considerable, for the nearer to the principal focus of the lens the object is brought, the more obliquely will the rays fall upon its surface, and the greater, therefore, will be the errors of aberration. This method of augmenting the power of a microscope has been adopted in spite of these disadvantages,• but it is not found to answer. Nevertheless, it is capable of being made of great utility, as we shall presently show, to a limited extent. A much more generally convenient method of varying the power of the microscope is to employ, as object-glasses, lenses of different foci ; and thus, as the same distance between the image and the lens is constantly maintained, whilst that of the object varies, the number of times that the latter is amplified is changed in a like proportion. In whatever mode addi tional amplification be obtained, two things must always result from the change; the por tion of the surface of the object, of which an image can be formed, must be diminished, and the quantity of light spread over that image must be proportionably lessened. In the use of high magnifying powers, the compound microscope has the great advantage over the simple, that the object need not be brought to nearly the same proximity with the lens, and that much more of it can be seen with comfort at a time. The long focus and large aperture with which the eye-piece is usually made pre vent even the prolonged use of the instrument from acting prejudicially on the visual powers, except in cases of peculiar tendency to nervous disorders of the eye. And as the power of the eye-piece as well as that of the object-glass can be raised, there are scarcely any limits to the magnifying power that may be obtained. Practically, however, there are limits, arising from the fact that, as the amplification is greater, the aberrations will be increased in even an augmented proportion ; so that these com pletely antagonise the benefit otherwise deri vable from the employment of high powers. The aberrations can only be diminished by contracting the aperture of the object-glass ; and this renders the image so dark that no real advantage is gained. Moreover, the imperfec tions necessary to the best compound micro scope, in which ordinary lenses are employed, are further augmented by the slightest error in the centering of the lenses, so that their axes do not coincide.