present attempt to supersede. For this purpose, I select the microscope invented by Professor AMICI of Modena, as justly deemed the most refined and perfect now in use. The optical principle of this instrument is represented in fig. 2, Pl. XIX, and is thus described in Dr. BREwSTER'S elegant treatise upon optics: "He (professor AMICI) made use of a concave ellipsoidal reflector, whose focal distance was 2 inches. The image is formed in the other focus of the ellipse, and this image is magnified by a single or double eye-piece, eight inches from the reflector. As it is impracticable to illuminate the object, m n, when situate as in the figure, professor AMICI placed it without the tube or below the line RN, and introduced it into the speculum A B by reflection from a small plane speculum placed between m n and A B, and having its diameter about half that of A B." I have marked the requisite position of the object m n to facilitate the reader's conception.

The reader will note that there are two reflections in the objective part of this instrument; one of which is useful only as introducing the rays proceeding from the object, and contributes in no respect to the magnifying virtue of the microscope. He cannot fail to observe also, that the great size of the plane forms a very serious impediment to the rays of light proceeding from the concave metal to the eye-glass. By an improvement of Dr. GORING's, the size of the plane has indeed been reduced to 3rd the diameter of the great mirror. But even in this improved state it continues so material an obstacle, besides having other and greater disadvantages to be touched upon in the sequel, that one cannot avoid wishing it removed if possible, by introducing and illuminating the object itself in the axis of the tube at the focus of the concave speculum. This improvement I have endeavoured to effect by a very simple optical contrivance, which will be easily understood by reference to figure 3, representing the form of the instrument adapted for diaphanous objects. A B is the concave ellipsoid or mirror, and C is a transparent object situated directly in its focus: D and E are the illuminating apparatus; D being a bull's-eye lens, and E a very small plane diagonal mirror, so situated as to reflect parallel with the axis of the tube rays of light coming from the lens. By this arrangement, an image of the luminous aperture F, is formed in the focus of the lens and just behind the transparent body C. It is now apparent that but one reflection is sufficient to form an image of a microscopic body in the conjugate focus of the speculum; and that the diagonal plane being of almost evanescent dimensions, presents no obstacle worth mentioning to the rays in their passage to the eye-glass.

The mere loss of light however, attendant upon the Amician construction, is a disadvantage of very secondary importance; for it is obvious

that almost any amount of loss may be compensated by an artificial condensation of light upon the object under examination. But the disadvantages of a double reflection are of a higher and more important order. It is one of the most inflexible laws of practical optics, that all superfluous refractions or reflections are to be avoided, and for this simple reason, that to form a perfect surface, either plane or spherical, or of any other figure, is beyond the power of human art, however exquisite, and hence the greater the number of reflections or refractions, the more darkened and muddy will the vision become. When we consider therefore, how extremely minute are many objects of microscopic investigation (such for instance as the marks, probably not the 10,000th of an inch in diameter, upon the dust of lepidlopterous insects), we may conceive how much vision may be impaired by a very slight error in the figure of the plane, augmented in the long passage of the rays from the mirror to the eye-lens. It is well known that NEWTON's objections to the Cassigramian and Gregorian telescopes arose from considerations of this kind. The errors (says he) of the said convex will be much aug. mented by the too great distance through which the rays reflected from it must pass before their arrival at the eye-glass. For which reason I find it convenient to make the tube no wider than is necessary, that the eye-glass be placed as near to the oval planes as possible, &c." If we conceive his own form of the instrument subjected to reversed vision, it will bear no remote resemblance to the Amician microscope; and tried in this way, I am pretty confident we should find a material difference in the performance of the most exquisite reflector. I took a onefoot Newtonian, having a very indifferent plane, but which showed objects in the day time sufficiently well; and making the rays from the object enter by the small metal, I looked with a lens directly in front, throwing the image a little out of the axis so as to escape the plane. Allowing for the necessary deficiency of light, the image was so confused and distorted, that it was almost impossible to distinguish any object whatever.

As there is some analogy between the telescopic "front view" of HERSCHELL (magnis componere parva !) and the microscope I propose, it may not be amiss to remind the reader of the advantage that great optician found in laying aside the oval plane. Besides the "capital advantage of nearly double the light of former constructions," the defining power seems also to be increased. "The 20ft. reflector having been changed from the Newtonian form to my present one, I had a very striking instance of the great advantage of the increased penetrating power in the discovery of the Georgian satellites. The improvement, by laying aside the small mirror, was as 61 to 75, and whereas

the former was not sufficient to reach these faint objects, the latter showed them perfectly well."

But to return to our humble sphere; the advantages of the new construction may be very briefly summed up. First, increased brilliancy and penetrating power, arising from the removal of the plane. Secondly, the probability of obtaining finer instruments; for the artist having the difficulties of but a single reflecting surface to contend with, manifestly enjoys at least a double chance of producing a perfect work.

Of the method of observing with this instrument, I think it hardly necessary to speak. The object must be inserted into the tube, affixed either upon a very small plate of talc, or upon the point of a fine needle; and adjusted to the focus with a delicate screw. This is a very simple affair in the hands of a skilful mechanic. For observing large objects with low powers, a speculum of long focus, say 1 or 2 inches, is obviously the best; but for more minute investigations a deeper one is desirable. With the requisite arrangements, such an instrument would, I conceive, be hardly if at all more difficult to use than a refractor.

I have wrought several metals upon this plan, and though I enjoy no opportunity of having them fitted up otherwise than in the rudest and most imperfect manner, I find their performance upon some minute and rather difficult objects very satisfactory. With a metal of about six inches focus and three aperture, without any attempt at adjustment, or any convenience for arranging the focus, several test objects have been easily developed, such as the asperities on the surface of the human hair, the striæ upon the dust of lepidlopterous tribes, &c. Fig. 5. represents the hair of the mouse, like a piece of well twisted whipcord, with longitudinal marks between the spirals, considered by microscopists as a good test. (Dr. GORING in the Quart. Journal, June 1827,) A represents the hair near the root, B near the extremity.

When I first applied myself to these constructions, I was apprehensive that the introduction of light would produce a glare in the tube detrimental to vision. But I find this not at all the case. The main tube however, should be made considerably larger than the diameter of the mirror, and a well blackened diaphragm may be placed on each side the illuminating apparatus to quench any straggling light that may intrude. The size of the diagonal plane in the above instrument is about .05, in shorter diameter, and is I find a great deal too large. One of only .025 would be sufficient, I think. I had almost omitted to mention, that as mány objects require a somewhat oblique radiance to be even seen, this may be obtained by giving the diagonal a slight revolution on its shorter axis; or perhaps as simply by closing up one half of the bull's-eye lens.