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Reports of the British Association for the Advancement of Science for 1844 and 1847. London: 8vo. 7. Report on the Origin and Functions of Cells. By William B. Carpenter, M.D., F.R.S., &c. &c. From the British and Foreign Medical Review, No. xxix. London: 1843. 8vo.

8. The British Desmideæ. By John Ralfs. The Drawings by Edward Jenner. London: 1848. 8vo.

FEW are cognizant, we suspect, of the recent improvements on the Microscope; though they have been such as to have raised it, within the last quarter of a century, from being little more than a scientific toy, to the dignity of a most perfect instrument of research. And yet fewer, probably, are aware of the varied conquests which it has achieved during that short period :—though its recent discoveries will bear comparison, in point of rapidity, novelty and importance, with those of any other epoch of similar length in the history of Natural Science. As an optical instrument, the microscope is now at least as perfect as the telescope, and its revelations altogether as surprising. The universe which it brings within our ken, in one direction, is as unbounded in its limits, as that whose remotest depths the most gigantic telescope still vainly attempts to fathom in another; and greater wonders of organization are disclosed in a speck of inconceivable minuteness, than the most mysterious nebula of the astronomer can exhibit to us. But more than this: -the generalizations in the phenomena and science of Life, to which its researches lead us, will be found as simple and comprehensive, as any which physics or chemistry have yet afforded. We propose to give our readers the opportunity of judging of the grounds of our assertions.

We need not dwell at any length upon the history of the microscope. Those who are curious in the matter will find a very complete account of it in the Introductory Chapter of Mr. Quekett's Treatise. It is sufficient to mention that, whilst the simple microscope, or magnifying glass, was known at a very remote period, the compound microscope, the powers of which, like those of the telescope, depend upon the combination of two or more lenses,—was not invented until about the end of the sixteenth century;

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the earlier microscopes having been little else than modified telescopes, and both instruments having been first constructed, it would appear, by the same optician. From that period, almost to the present time, the microscope remained nearly stationary; no essential improvement was made in the optical principles of its construction, and but little in the mechanical appliances by which those principles were turned to account in practice. Every one possessed of a smattering of optical knowledge is aware that, in the passage of light through lenses of ordinary construction, there are two serious causes of error or aberration. Of these, one depends upon the spherical curvature of the lens; in consequence of which, the rays that have passed through its outer portion, meet in a focus much sooner than those which have passed through its central part, thus communicating a certain indistinctness to the image. The other is occasioned by the dispersive power of glass; the consequence of which is, an unequal refraction of the different rays of the solar spectrum, and the production of a series of coloured fringes around every line or spot in the picture, totally destructive of a clear view of any portion of it. The first of these aberrations is termed spherical, the other chromatic. The only method at first thought practicable for preventing them from rendering both telescope and microscope virtually useless, was to contract the aperture of the lenses, so as to allow only their central portions to be in use. But this bungling expedient was necessarily attended with a great loss of light, especially when lenses of high power were employed: and in fact, it did nothing to correct the errors; it merely restrained their action within certain limits. Under these circumstances little could be expected from either the telescope or the microscope, as an instrument of scientific research, without a complete change in the principles of their construction. Despairing of such a change in the refracting telescope, Newton devoted himself to the improvement of the reflecting instrument. But in the middle of the eighteenth century, the want of uniformity between the refractive and dispersive powers of different transparent media was fortunately discovered: so that Dollond was enabled to construct compound lenses, in which the errors of the several component parts balanced or corrected each other, and an

image was produced free from both chromatic and spherical aberration. From that period to the present, refracting telescopes have been constructed upon the achromatic principle; and the only limit to their power lies in the impossibility of procuring glass of a quality sufficiently pure and homogeneous for lenses of more than fifteen or sixteen inches in diameter. But this limit, as long as it remains such, must oblige astronomers still to rely on the reflecting telescope, as their greatest and last resource.

Looking back from our present point of view upon the history of the past, it cannot but excite surprise that no systematic attempt should have been made by Dollond or his successors, to introduce the achromatic principle into the construction of the microscope, considering that its alliance with the telescope is so intimate. The idea, however, would seem to have been regarded as hopeless, in consequence, not of any failure in the principle, but of the practical difficulties that stood in the way of its application. For whilst the progressive augmentation of the capability of the telescope depends on the enlargement of the diameter of the object-glass, the increase of its focal length, and the consequent diminution of its curvature ;-that of the microscope can only be effected by the diminution of the focal length of the object-glass, the increase of its curvature, and the consequent reduction of its dimensions. It was supposed to be impracticable to make the necessary corrections and adjustments, in a combination of lenses, of a diameter sufficiently small to give even the lowest serviceable magnifying power to a microscope; far more, to make such corrections in those minute lenses of higher magnifying power, for the advantageous use of which they were more especially needed. It appears, indeed, that various attempts were made for this purpose towards the end of the last century, and in the early part of the present. But their ill success prevented them from being carried further. Accordingly, we find Professor Amici, about thirty-five years ago, laying aside his experiments respecting the refracting principle, and endeavouring to improve the reflecting microscope, as had been first suggested by Newton. In the year 1823, however, a basis was laid for the improvement of the refracting microscope, by the construction of effective achromatic object-glasses. A combination of lenses on the plan

of the object-glasses of telescopes, was brought to perfection by the late eminent optician Mr. Tulley, working on the instigation of Dr. Goring; whilst a different combination on another plan, since found more suitable to the microscope, was accomplished in Paris by M. Selligues, at the suggestion of M. Chevalier. Similar attempts were made soon afterwards in various other quarters. Professor Amici resumed his labours on the refracting instrument; and the opticians of London, Paris, Munich, and Modena, vied with each other in producing lenses of higher power. Their exertions were crowned with such success, that Dr. Goring declared, in 1829, 'that microscopes were now placed completely on a level with telescopes, and, like them, must remain stationary in their construction.' In that very year, however, Mr. J. J. Lister communicated to the Royal Society his discovery of certain properties in achromatic combinations, which had been previously unobserved. The improvements grounded upon this discovery are so important, that they constitute a new era in the history of the microscope; and manipulative skill has since realized nearly everything which theory indicates as possible. Taking warning from Dr. Goring, we nowise affirm, that microscopes have now reached their limit of perfection; but we may safely say, that they have so nearly approached it, that a new principle of construction must be discovered before any essential improvement can be made in them. The highest magnifying power has been attained, that the microscopist can conveniently employ: the limit being determined by the shortness of the focus, that is, by the closeness of the proximity into which the object-glass must be brought with the object. No lenses of a shorter focus than a twelfth or a sixteenth of an inch can be made available in practice; and achromatic combinations are now constructed with such perfection, as to give a welldefined image of objects magnified from one thousand to two thousand diameters, or from one to four million times, reckoning by superficial measure. And the completeness of the corrections made by them is truly marvellous, when we consider that every such combination consists of six distinct lenses, all whose curvatures must be accurately adjusted to the correction of each other's errors, the largest of the lenses scarcely exceeding an ordinary pin's head in

diameter, whilst the smallest may be of the dimensions of the head of the smallest 'minikin' produced by the pin-maker.

The attention of scientific men was early attracted to the compound microscope. In its original and rude form, it opened to them a field of research altogether new, and promised to add largely to their information concerning the structure of every kind of organized body. The Transactions of the Royal Society contain the most striking evidence of the interest taken in microscopic investigations two centuries ago. The early volumes, as Mr. Quekett remarks, literally teem' with improvements in the construction of these instruments, and with discoveries made by means of them. The Micrographia of Robert Hooke, published in 1667, may be fairly styled one of the wonders of his day; and in 1673, the name of Leeuwenhoek appears for the first time in the Philosophical Transactions. That with the imperfect instruments at his command, he should have seen so much and so well, as to make it dangerous even now to announce a discovery, without having consulted the works of Leeuwenhoek, in order to see whether some anticipation of it may not be found there, must ever remain a marvel to the microscopist. This is partly to be explained by the fact that he trusted less to the compound microscope, than to single lenses of very high power,-the use of which is attended with much difficulty, but which are comparatively free from the errors inseparable from the other instrument in its uncorrected form. The names of Grew and Malpighi, also, appear as frequent contributors to the early Philosophical Transactions; the researches of the former being chiefly directed to the minute structure of plants, those of the latter to that of animals. Both were attended with great success. Malpighi seems to have been the first to observe the wonderful spectacle of the movement of blood in the capillary vessels of the frog's foot ;-an ocular verification of the doctrine of the circulation expounded by the sagacious Harvey. Glimpses of the invisible world of animalcular existence were occasionally revealed to the earlier microscopists. Their curiosity must have been strongly excited; yet they do not appear to have entered on this class of minute investigations with any portion of

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