For upwards of a century the compound microscope, netwithetanding tho a 'vantages above inentiemed, was a comparatively feeble and inefficia nt instrument, owns to the distance which the light had to traverse, and the consequent Increase of the chromatic and spherical aLerrat:ons To explain this a e have drawn in /g. 12 a second image near A're, the fact being that the object-glass would not form one image, as has been supposed, but an infinite number of variously coloured and various-sized images, occupying the space between the two dotted arrows. Those nearest the object-glass would be red, and those nearest the eye-glaas would be blue. The effect of this is to produce so much confusion, that the instrument was reduced to a mere toy, although these errors were diminished to the utmost possible extent by limiting the aperture of the object-glass, and thus restricting the angle of the pencil of light from each point of the object. But this involved the defects, already explained, of making the picture obscure, so that on the whole the beet compound inseruments were inferior to the simple inicroscopee of a mingle lens, with which indeed all the important observation,' of the last century were made.
Even after the improvement of the simple microscope by the use of doublets and triplets, the long course of the rays and the large angular pencil required in the compound instrument deterred the most sanguine from anticipating the period when they should be conducted through such a path free both from spherical and chromatic ermrs. Philo sophers of no less eminence than N. Riot and Dr. 'Wollaston had predicted that the compound would never rival the simple microscope, and that the idea of achromatising its object-glass was hopeless. Nor can these opinions be wondered at when we consider how many years the achromatic telescope had existed without an attempt to apply its principles to the compound microscope. When we consider the small ness of the pencil required by the telescope, and the enormous increase of difficulty attending every enlargement of the pencil—when we con sider further that these difficulties had to be contended with and removed by operations on portions of glass so small that they are themselves ahnoet microscopic objects, we shall not be surprised that even a cautious philosopher and most able manipulator like Dr. Wollaston should prescribe limits to improvement.
Fortunately for science, and especially for the departments of animal and vegetable physiqlogy, these predictions have been shown to be unfounded. The compound microscope has been elevated from the condition we have described to that of being the most important instrument ever bestowed by art upon the investigator of nature. It now holds a very high rank among philosophical implements, while the transcendent beauties of form, colour, and organisation which it reveals to ue in the minute works of nature, render it. subservient to the most delightful and instructive pursuits. To these claims on our attention
it appears likely to add a third of still higher importance. The microscopic examination of the blood and other human organic matter will in all probab.lity afford more satisfactory and conclusive evidence regarding the nature and seat of disease than any hitherto appealed to, I and will of consequence lead to similar certainty in the choice and application of remedies.
Soon after the year 1820 a series of experiments was begun in France by M, Selligues, which were followed up by Frauenhofcr at Miinich, by Amici at Modena, by M. Chevalier at Paris, and by the Late Mr. Tulley in London. In 1824 the last-named excellent artist, without knowing what had been done on the Continent, made the attempt to construct an achromatic objective for a compound microscope, and produced one of nine tenths of an inch focal length, composed of three lenses, and transmitting a pencil of eighteen degrees. This was the first that had been made in England ; and it is due to Mr. Tulley to Kay, that as regards accurate correction throughout the field, that glass has not been excelled by any subsequent combination of three lenses. Such an angular pencil, and such a focal length, would bear an eye piece adapted to produce a gross magnifying power of one hundred mid twenty. Mr. Tulley afterwards made a combination to be placed in front of the first-mentioned, which increased the angle of the trans mitted pencil to thirty-eight degrees, and bore a power of three hundred.
While these practical Investigations were in progress, the subject. of achromatism engaged the attention of eorne of the most profound mathematicians in England. Sir John Herschel, Professor Airy, Professor Barlow, Mr. Coddington, and others, contributed largely to the theoretical examination of the subject ; and though the reculta of their labours were not immediately applicable to the inicroecope, they essentially promoted its improvement.
For some time prior to 1829 the subject had occupied the mind of a gentleman, who, not entirely practical, like the first, nor purely mathematical, like the last-mentioned class of inquirers, was led to the discovery of certain properties in achromatic combinations which had been before unobserved. These were afterwards experimentally verified ; and in the year 1829 a paper on the subject, by the discoverer, Mr. Joseph Jackson Lister, was read and published by the Royal Society. The principles and results thus obtained enabled Mr. Lister to forum a combination of lenses which transmitted a pencil of fifty degrees, with a large field correct in every part : as this paper was the foundation of the recent improvements in achromatic microscopes, and as its results are indispensable to all Who would make or understand the instrument, we shell give the more hnpoitant parte of it in detail, and in Mr. Lister 's own words.