OF THE DOUBLE OR COMPOUND MICROSCOPE.
In the compound microscope, the image is viewed instead of the object, which image is magnified by a single lens, as the object is in a single microscope. It consists of an object lens N L, Fig. 5. [Plate I.] and an eye glass F G. The object B O is placed a little further from the lens than its principal focal distance, so that the pencils of rays proceeding from the different points of the object through the lens, may converge to their respective foci, and form an inverted image of the object at Q P; which image is viewed by the eye through the eye glass F G, which is so placed, that the image may be in its focus on one side, and the eye at the same distance on the other. The rays of each pencil will be parallel, after passing out of the glass, till they reach the eye at E, where they will begin to converge by the refractive powers of the humours; and after having crossed each other in the pupil, and passed through the crystalline and vitreous humours, they will be collected in points on the retina, and form a large inverted image thereon.
It will be easy, from what has been already explained, to understand the reason of the magnifying power of a compound microscope. The object is magnified upon two accounts; first, because if we viewed the image with the naked eye, it would appear as much larger than the object, as the image is really larger than it, or as the distance f R is greater than the distance f b; and secondly, because this picture is again magnified by the eye glass, upon the principle explained in the foregoing article on vision, by single microscopes.
But it is to be noted, that the image formed in the focus of a lens, as is the case in the compound microscope, differs from the real object in a very essential particular; that is to say, the light being emitted from the object in every direction, renders it visible to an eye placed in any position; but the points of the image formed by a lens, emitting no more than a small conical body of rays, which arrives from the glass, can be visible only when the eye is situate within its confine. Thus, the pencil, which emanates from o in the object, and is converged by the lens to D, proceeds afterwards diverging towards H, and, therefore, never arrives at the lens F G, nor enters the eye at E. But the pencils which proceed from the points o and b, will be received on the lens F G, and by it carried parallel to the eye; consequently, the correspondent points of the image Q P will be visible; and those which are situate farther out towards H and I, will not be seen. This quantity of the image Q P, or visible area, is called the field of view.
Hence it appears, that if the image be large, a very small part of it will be visible; because the pencils of rays will for the most part fall without the eye glass F G. And it is likewise plain, that a remedy which would cause the pencils, which proceed from the extremes B and O of the object, to arrive at the eye, will render a greater part of it visible: or, in other words, enlarge the field of view. This is effected by the interposition of a broad lens D E, Fig. 5, of a proper curvature, at a small distance from the focal image. For, by those means, the pencil D N, which would otherwise have proceeded towards H, is refracted to the eye, as delineated in the figure, and the mind conceives from thence the existence of a radiant point at Q, from which the rays last proceeded. In like manner, and by a parity of reason, the other extreme of the image is seen at P, and the intermediate points are also rendered visible. On these considerations it is, that compound microscopes are usually made to consist of an object lens N L, by which the image is formed, enlarged, and inverted; an amplifying lens D E, by which the field of view is enlarged, and an eye glass or lens, by which the eye is allowed to approach very near, and consequently to view the image under a very great angle of apparent magnitude. It is now customary to combine two or more lenses together at the eye glass, in the manner of Eustachio Divinis and M. Joblot; by which means the aberration of light from the figure is in some measure corrected, and the apparent field increased.
OF THE SOLAR MICROSCOPE.
In this instrument, the image of the object is refracted upon a screen in a darkened room. It may be considered under two distinct heads: 1st, the mirror and lens, which are intended to reflect and transmit the light of the sun upon the object; and 2dly, that part which constitutes the microscope, or which produces the magnified image of the object, Fig. 10. [Plate I.] Let N O represent the side of a darkened chamber, G H a small convex lens, fixed opposite to a perforation in the side N O, A B a plane mirror or looking glass, placed without the room to reflect the solar rays on the lens C D, by which they are converged and concentrated on the object fixed at E F.
2. The object being thus illuminated, the ray which proceeds from E will be converged by the lens G H to a focus K, on the screen L M; and the ray which comes from F will be converged to I, and the intermediate points will be delineated between I and K; thus forming a picture, which will be as much larger than the object, in proportion as the distance of the screen exceeds that of the image from the object; a small object, such as a mite, &c. may be thus magnified to eight or ten feet in diameter.
From what has been said, it appears plainly, the advantages we gain by microscopes are derived, first, from their magnifying power, by which the eye is enabled to view more distinctly the parts of minute objects: secondly, that by their assistance, more light is thrown into the pupil of the eye, than is done without them. The advantages procured by the magnifying power, would be exceedingly circumscribed, if they were not accompanied by the latter: for if the same quantity of light be diffused over a much larger surface, its force is proportionably diminished; and therefore the object, though magnified, will be dark and obscure. Thus, suppose the diameter of the object to be enlarged ten times, and consequently the surface one-hundred times, yet, if the focal distance of the glass were eight inches, provided this were possible, and its diameter only about the size of the pupil of the eye, the object would appear one-hundred times more obscure when viewed through the glass, than when it was seen by the naked eye; and this even on the supposition that the glass transmitted all the light which fell upon it, which no glass can do. But if the glass were only four inches focal distance, and its diameter remained as before, the inconvenience would be vastly diminished, because the glass could be placed twice as near the object as before, and would consequently receive four times as many rays as in the former case, and we should, therefore, see it much brighter than before. By going on thus, diminishing the focal distance of the glass, and keeping its diameter as large as possible, we shall perceive the object proportionably magnified, and yet remain bright and distinct. Though this is the case in theory, yet there is a limit in optical instruments, which is soon arrived at, but which cannot be passed. This arises from the following circumstances.[27]
[27] Encyclopædia Britannica, last edition, vol. xiii, p. 357.