to be most advantageously improved, was Mr. Jackson Lister, a gentleman who had long turned his attention to scientific subjects, although he was neither professionally nor commercially engaged in the manufacture of instruments. In a paper which he transmitted to the Royal Society in 1829, and which was published in their Transactions of that year, he laid down the real facts which occur in the transmission of rays of light through lenses made of various substances. The details into which he entered are too various, too numerous, and too abstruse to be interesting in such a publication as this and such as could only be made intelligible by a number of diagrams and a full treatise on the properties of light as reflected from various substances, and refracted in various media. It may therefore be sufficient to observe that such an advance had been made by Mr. Lister's discoveries and explanations, that Mr. Andrew Ross and Mr. Hugh Powell succeeded in making instruments of so perfect a combination, that the object-glass gave a complete achromatic view to the observer. To such a degree of delicacy indeed did they attain, that the interposition of a piece of common glass or of the thinnest talc was sufficient to affect the correctness of the view.

One defect attended this arrangement. The instrument, if it may be so said, was too perfect; for the slightest accidental interruption materially interfered with the delicacy of the results which it presented. Mr. Ross therefore set himself to work to discover the rationale of the interfering causes, and in the 51st volume of the Transactions of the Society of Arts published the whole of the principles of, and detailed the means for, obtaining such counter aberrations; and at length succeeded in establishing the basis on which future corrections or improvements can be made.

The object-glass and the lenses, and their adaptation and position, having been brought to this state of perfection, the next object was to improve the eye-glass; and for that purpose

Mr. Varley applied the eye-piece of Huygens, the philosopher who in the seventeenth century had first adopted it to decrease the chromatic aberration of the telescope. The object of Huygens, who was one of the first persons who brought the telescope to such a position as led to its ultimate great improvement, was simply to increase the refractive power of the instrument. For this purpose he placed two plano-convex lenses-that is, a lens flat on one side and convex on the otherat a certain distance from each other, so as to accumulate their magnifying power; and the doing away with the rays, dimness, and uncertain colour of the objects viewed through these lenses, was altogether an accidental discovery. It is indeed even now allowed to be the best for all telescopic purposes; but a further improvement was added by Ramsden and called, from the end which it answered, the micrometer eye-piece. When it is recollected that it is frequently necessary to measure objects with dimensions at least a hundred times less than the markings on our finest rules for measurement, the advantage of applying a scale to the magnified object will be sufficiently apparent.

The grander revelations of the microscope all belong to the present century. Swammerdam and Leuwenhoeck, even with imperfect instruments, carried on the most patient and successful investigations into animal and vegetable nature; but our discoveries by the microscope, as now improved, transcend all the wondrous accounts they published. By this instrument we learn that every department of nature teems with animalculæ or with minute vegetable life; and that all are of the utmost importance in the economy of our earth, though not beheld by the naked eye. It has proved to us that classes of animals exist in which the second generation differs from the first in form and habit, the third one taking on the figure of the first, whilst the fourth again is similar to the second, and incredible as it may appear, such is the fact. The human eye, heart, brain, muscles, skin, and other parts of the body are

now discovered to be regions of animalcule life. All the larger animals carry numbers upon numbers within or without their bodies. A handful of dried herbs steeped in water will be found, in a few hours, to have produced millions of these living creatures, so small that five hundred millions of them may be contained in one drop of water. Flint, gravel, and chalk beds are found to be formed of their dead bodies; and the composition of the globe seems likely eventually to be

traced to their tribes.

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The comparative anatomist," observes the writer of a popular treatise on the microscope, " makes use of this instrument to determine from the structure of the teeth, the form, habit, and class, of animals which lived and have become extinct on our earth for many thousand years. Thus Professor Owen, from the examination of the structure of the tooth of the megatherium, by demonstrating the identity of the dental structure with that of the sloth, has yielded us an unerring indication of the true nature of its food. By the aid of high-power magnifying glasses, we are informed that our island was once possessed of a climate nearly approaching to a tropical one; for if we examine a piece of drift-wood, found in the eocene clay (so called from its being the dawn of a new creation) of the estuary of the Thames, we shall find that these fragments belong to a class of plants nearly allied to the pepper tribe, and that they flourished in company with the turtles, vultures, crocodiles, and boa-constrictors of the Sheppey district.

"Every department of nature teems with minute objects of animal and vegetable life, which are of the utmost importance in the economy of this earth, and which are totally unrecognisable by the naked vision. Many of these animals and vegetables are unrivalled for the beauty and complexity of their forms. Some are productive of great changes, which have been effected, or are still going on, in the earth's surface; and some are productive of the greatest mischief and

destruction. To illustrate the former, we need only mention the filling up now going on in the harbours of Wismar and Pillau, in the Baltic; and the production of coral-reefs, and their gradual conversion into new islands. The potato disease is an instance of devastation now going on, and is said to be caused by the development of a minute microscopic fungus in the cells of the tubers of this vegetable."

The celebrated Dr. Chalmers thus speaks of the wonders discovered by the microscope:-"While the telescope enables us to see a system in every star, the microscope unfolds to us a world in every atom. The one instructs us that this mighty globe, with the whole burthen of its people and its countries, is but a grain of sand in the vast field of immensity—the other, that every atom may harbour the tribes and families of a busy population. The one shows us the insignificance of the world we inhabit the other redeems it from all its insignificance, for it tells us, that in the leaves of every forest, in the flowers of every garden, in the waters of every rivulet, there are worlds teeming with life, and numberless as are the stars of the firmament. The one suggests to us, that above and beyond all that is visible to man, there may be regions of creation which sweep immeasurably along, and carry the impress of the Almighty's hand to the remotest scenes of the universe—the other, that within and beneath all that minuteness which the aided eye of man is able to explore, there may be a world of invisible beings; and that, could we draw aside the mysterious veil which shrouds it from our senses, we might behold a theatre of as many wonders as astronomy can unfold—a universe within the compass of a point, so small, as to elude all the powers of the microscope, but where the Almighty Ruler of all things finds room for the exercise of His attributes, where he can raise another mechanism of worlds, and fill and animate them all with evidences of His glory."