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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
the persevering zeal which they devoted to the analysis of higher forms of animal and vegetable structure. Its wonders, however, were gradually unfolded; so that in the various treatises on the microscope, published during the eighteenth century, they occupy a conspicuous place. In the meantime, the microscope had serious difficulties to contend with, some from the nature of the case, some accidental. The defects inseparable from its original construction formed a bar to all discovery beyond certain limits. But it fell under a temporary cloud from another cause. When physiologists began to theorise on the elementary structure of the animal body, and to twist their imperfect observations into accordance with their theories, there soon followed a general suspicion of a want of trustworthiness in the microscope, and in everything announced upon its authority. The instrument and its advocates were brought into more or less discredit, the effect of which has continued to attach to them, in the minds of many, until the present day. We wish, therefore, to pause for a moment, and examine into the grounds of this impression.
A tendency, common to all observers, is to describe what they believe and infer, rather than what they actually see. Microscopic observers were especially open to this reproach, as long as a want of definiteness in the image presented to their eyes, left so much to be completed by the imagination. But from the moment that the visual image, presented by a well-constructed microscope, gave as good an idea of the object as we could have obtained from the object itself, if enlarged to the same size, and viewed by the unassisted eye, microscopic observations were made with the same degree of certainty in this respect, as observations of any other class. Another fallacy, common indeed to all observers, but of which the microscopic observations of former times had perhaps more than their share, arose from not sufficiently attending to a difference in the circumstances under which the observations had been made. Thus one observer described the human blood-corpuscles as flattened discs, like pieces of money, another as slightly concave on each surface, another as slightly convex, and another as highly convex, or even globular. Yet all microscopists now agree that their real form, when examined in freshly-drawn blood, is that of
discs with slightly-concave surfaces. The diversity in the previous accounts was simply due to the admixture of water or other liquids added for the sake of dilution, which, being absorbed by the blood-discs, at first rendered their surface flat, then convex, and at last changed their form into the globular. But microscopical inquiries are no longer more exposed to fallacies of this description than any other branch of physiology. And the microscopists of the present day can be only held responsible for the errors of their predecessors, by persons who are ignorant of the perfection which the instrument has now attained, as well as of the general agreement regarding facts among all who are competent to use it, however they may differ in their inferences.
But our immediate object is to introduce our readers to some of the most interesting discoveries for which we are indebted to the recent improvements in the microscope: and we propose to pass lightly over such as the public may be presumed to be acquainted with, from their having been incorporated into treatises adapted for general circulation. We shall say little, therefore, of Professor Ehrenberg's laborious investigations into animalcular life, as well fossil as recent; although his additions of new forms of organized beings to the catalogue of the naturalist greatly exceed all that have been contributed by the most industrious collector in any other department. It may give some idea of their wonderful variety, to be told that Professor Ehrenberg detected in some specimens of a tertiary deposit, brought a short time since from the island of Barbadoes by Sir R. Schomburgk, no fewer than 1,200 new specific forms. It was necessary to create 250 new genera for their reception: the mere naming of which must have constituted in itself no insignificant labour. Professor Ehrenberg first discovered, that various extensive deposits, both calcareous and silicious, are almost entirely made up of the remains of minute organisms; incalculable numbers of which must have successively lived and died, flourished and decayed, leaving the solid portion of their fabrics to accumulate during countless generations, and thus to form vast beds of chalk or strata of silicious sand. One of the most interesting discoveries of this class was recently communicated to the scientific world by Mr. W. C. Williamson.
By means of a microscopic analysis of the white mud, with which the sea-bottom of a great part of the Levant is covered, he has shown that the deposit consists almost entirely of organic forms: and among them may be distinguished the shells of about twenty-five distinct species of animalcules, some silicious, others calcareous; besides certain other bodies hereafter to be noticed, which are determined by the microscope to be fragmentary particles of shell.
We owe to Professor Ehrenberg the separation of the remarkable group of Rotifera or Wheel Animalcules, from a lower division,-to which he has given the name of Polygastrica. On the complex structure and high organization of the Rotifera, all microscopists are agreed; not so, with respect to the Polygastrica. Many distinguished naturalists question the high authority of Professor Ehrenberg in this instance: and maintain that the facts which he has observed do not bear out his inferences, and that his account of the digestive apparatus of the Polygastrica is not entitled to rank as a physiological truth. Into this question we shall not now enter. But it is right to state our conviction, that Professor Ehrenberg has included among his Animalcules many forms which are truly vegetable, and whose supposed polygastric structure is altogether a product of his imagination. The beautiful tribe of Desmidea, which forms the subject of Mr. Ralfs's admirable monograph, is now almost universally admitted, in this country at least, to be of a vegetable nature: and the recent discovery by Mr. G. H. K. Thwaites of the conjugation of the Diatomeæ makes it next to certain that this group, to which a very large proportion of the silicious fossil animalcules (so called) belongs, must be transferred in the same manner to the domain of the botanist. The phenomenon in question is one of a very singular character: and, as it is one of which the right understanding will probably lead to a remodelling of the whole fabric of Cryptogamic Botany, we shall endeavour shortly to explain it.
The attention recently paid to the simplest forms of vegetable existence has shown, what was previously suspected, that in the simple cell of the lowest Algae we may find the type of all vegetative existence. This cell grows