Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/277

MICROSCOPE 261 act as a simple microscope in magnifying that image to the observer. It was early found useful, however, to interpose another lens FF, fig. 4 (the " field-glass "), between the object-glass and the image formed by it, for the purpose of giving such a slight convergence to the pencil of rays as shall reduce the dimensions of the image, and thus allow a larger part of it to come within the range of the eye- FlG. 3. Diagram of Simplest Form of Compound Micro scope. FIG. 4. Diagram of Complete Compound Microscope. glass, so that more of the object can be seen at once. And it was soon perceived that the eye-glass and the field-glass might be advantageously combined into an "eye-piece," in which a perforated diaphragm might be inserted at the focal plane of the image (i.e., in the focus of the eye-glass), so as, by cutting off the peripheral portion of the field of view, to limit it to what can be seen with tolerable distinctness. It is obvious that the magnifying power of such an instrument would depend (1) on the proportion between the size of the image formed at BB and that of the actual object, and (2) upon the magnifying power of the eye-glass. And further the proportion which the size of the image bears to that of the object depends upon two factors, (1) the focal length of the object-glass, and (2) the distance between the object-glass and the plane BB occupied by the image it forms. If we diminish the focal length of the object-glass, the object must be brought nearer to it, so that, while the distance of the image on the other side remains unchanged, that distance comes to bear a larger proportion to the distance of the object, and the size of the image is augmented in a cor responding ratio. On the other hand, the object-glass remaining unchanged, the distance at which it forms the image of the object can be increased by a lengthening of the tube of the microscope ; and, as this involves a shortening of the distance between the object-glass and the object, the proportion which the former bears to the latter is augmented, and the image is correspondingly enlarged. Thus an increase in the magnifying power of the compound micro scope may be gained in three modes, which may be used either separately or in double or triple combination, viz., (1) shortening the focus of the object-glass, (2) lengthening the tube of the microscope, and (3) increasing the magnify ing power of the eye-glass by shortening its focus. This, it may be remarked, also lengthens the distance of the image from the object-glass, by bringing the focal plane BB nearer the eye-glass. The second of these methods was not unfrequently used in the older microscopes, which were sometimes made to draw out like telescopes, so as to increase the amplifying power of their object-glasses. But, whilst very inconvenient to the observer, such a lengthen ing of the one distance involved such a Lhortening of the other as greatly impaired the distinctness of the image by increasing the aberrations of the object-glass, so that this method came to be generally abandoned for one of the other two. When lenses of from 1 to 4 inches focus were used as object-glasses, and their apertures were restricted by a stop to the central part of each, tolerably distinct images were given of the larger structural arrangements of such objects as sections of wood or the more transparent wings of insects, which images would bear a further moderate enlargement by the eye-glass without any serious deterioration either by want of definition or the introduction of colour-fringes. But when lenses of less than 1 inch focus were employed in order to obtain a higher magnifying power, the greater obliquity of the rays so greatly increased their aberrations that defective definition and the introduction of false colours went far to nullify any advantage obtainable from the higher amplification; while the limitation of the aperture required to keep these aberrations within even moderate limits occasioned such a loss of light as most seriously to detract from the value of the picture. On the other hand, the use of deeper eye-pieces to enlarge the images formed by the object-glasses not only brought out more strongly all the defects of those images, but introduced a new set of errors of their own, so that very little was gained by that mode of amplification. Hence many of the best of the older microscopists (notably LEEUWENHOEK, q.v.) made some of their most valuable discoveries by the use of the simple microscope ; and the amount of excellent work thus done surprises every one who studies the history of microscopic inquiry. This was still more the case, as already stated, when the use of single lenses of very short focus was superseded by the introduction of the Wollaston doublet. And the substitution of these doublets for the single lenses of object-glasses, while the single lens of the eye-glass was replaced by a Herschel s aplanatic doublet, and the field-glass was a convex lens whose two curves had the proportion of 1:6 (the form of least spherical aberra tion), constituted the greatest improvement of which the instrument seemed capable in pre-achromatic times. 1 It has been only within the last sixty years (1820-30) that the microscope has undergone the important improve ment which had been worked out by Dollond in the refracting telescope more than sixty years previously, namely, the correction of the chromatic aberration of its objectives by the combination of concave lenses of flint- 1 This combination was made in the h rst microscope of which the writer became possessed, about the year 1830 ; and he well recollects the great superiority to any compound microscope of the old construc tion which was proved by its power of separating the lines on the Menelaus scale, and of bringing into view the details of the structure of animalcules, with a clearness that only an achromatized object-glass

could surpass.