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NUMBER XXIV.

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1.—Dr. Chalmers' Life and Writings

II.-Latham's Natural History of the Varieties of Man

III.-Neander

IV.-Eastern Monachism

V.-Hearts in Mortmain, and Cornelia

VI.--Puseyite Novels

THE

PROSPECTIVE REVIEW.

No. XXI.

ART. I.-THE SCIENTIFIC VALUE OF THE

MICROSCOPE.

1. A Practical Treatise on the Use of the Microscope, in

cluding the different Methods of preparing and examining Animal, Vegetable, and Mineral Structures. By John Quekett, Assistant Conservator of the Museum, and Demonstrator of Minute Anatomy at the Royal College of Surgeons of England. London: 1848. 8vo.

pp. 454.

2. Die Infusionsthierchen als volkommene Organismen. Von

Dr. C. G. Ehrenberg. Leipzig : 1838. folio. 3. Die fossilen Infusorien. Von Dr. C. G. Ehrenberg.

Berlin : 1837. folio. 4. Mikroscopische Untersuchungen über die Uebereinstim

mung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Von Dr. Th. Schwann. Berlin : 1839.

8vo. 5. Odontography ; or a Treatise on the Comparative Ana

tomy of the Teeth, their Physiological Relations, Mode of Development, and Microscopic Structure, in the Vertebrate Animals. By Richard Owen, F.R.S., Hunterian Professor to the Royal College of Surgeons,

London, &c. &c. London : 1840-5. 8vo. 6. Report on the Microscopic Structure of Shells. By Wil

liam B. Carpenter, M.D., F.R.S., &c. &c. From the CHRISTIAN TEACHER.No. 47.

B

Reports of the British Association for the Advance

ment of Science for 1844 and 1847. London : 8vo. 7. Report on the Origin and Functions of Cells. By Wil

liam 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.

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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; 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

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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

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