electrotype. The copies, perhaps, may not be quite so perfect as the original, yet they may be sufficiently accurate to be fine and important instruments.

The first great and obvious difficulty of the electrotype was with regard to the metal generally employed. Copper reflects light badly, and therefore was not suitable for a speculum. Mr. Talbot devised a method to overcome this difficulty. In the meantime, Professor Wheatstone had been also engaged in similar investigations. The idea of the several white metals, platinum, palladium, nickel, silver, &c. had occurred to both; and Prof. Wheatstone had preferred platinum, which he employed for the purpose by precipitation, and had obtained a reflector of sufficient polish and power. Mr. Talbot's especial thought was, whether the surface of copper might not be whitened, and thus a better reflecting-surface obtained. He employed hyposulphuret of ammonia, which does turn copper white, and moreover conquers two evils-the deficient reflecting power, and the readily tarnishable surface. For, in fact, in the latter respect the copper is changed into a sulphuret, a stronger chemical compound than an oxide of copper, wherefore it would not be acted on by the oxygen of the atmosphere.

About a year ago, when Mr. Talbot was at Munich, Professor Steinheil showed him what he considered a great novelty, namely, a method of making specula by means of the electrotype. Thus had the same notion occurred, and been practically carried out, by three eminent persons. The realisation by Prof. Steinheil was the precipitation of gold on a given speculum of copper. Mr. Talbot had thought gold would not be capable of reflecting sufficient light. It does, however, reflect more light than polished steel; and Mr. Talbot had witnessed, in a Gregorian telescope, commonly gilt, well-defined objects, although slightly tinged with yellow. The idea of Professor Steinheil, not only of the copying specula, but also other parts of the telescope, by the electrotype, had been much approved by German astronomers. In order to produce a fine gold surface, with copper back, by the electrotype, Prof. Steinheil had adopted a very ingenious and novel process. First, from the pure cyanide of gold was thrown down a gold film or coating to the required thickness; then the cyanide of copper, in certain proportions, was mixed with the salt of gold, and both precipitated as an alloy; and then, increasing the salt of the less noble metal by degrees, a back of pure copper was obtained, with perfect adhesion throughout. The principal novelty of this is the two metals going down together; for hitherto the belief has been, that each would have passed over to the negative pole singly, in the order of their electric states.

But the large specula being thus easily and cheaply procured, the question remains, how the telescope, with requisite gigantic framework, may be used. Professor Steinheil's plan was, to have the tube of a speculum, of 60 or 80 feet focal length, fixed in an invariable position, and pointing to the pole of the heavens; to have a plane mirror larger than the speculum placed over the axis revolving, to

transmit the heavenly object to the parabolic reflector, and thence to the spectator. Other mechanical arrangements of a similar kind were proposed.

Mr. Holden said, that Lord Stanhope had invented and erected a telescope on the plan, (as to the fixity of the framework,) which Mr. Talbot recommended. The instrument was stationary, and he could sit still and view more than one-half of the heavens, taking in even 20° behind his head, by means of an arrangement of mirrors.

Sir D. Brewster mentioned a similar plan, proposed twenty years ago by an American gentleman to the Royal Society of Edinburgh. There the mirror was not vertical, like Lord Stanhope's, but horizontal, consisting of a dish of metal in which a quantity of mercury was placed, and a rotatory motion was given to the disc, so that the mirror assumed a concave form, giving a parabolic surface, of the actual form required to reflect bodies; by which means a mirror was constructed in one instant of time. This appeared so impracticable to the Society, that they did not venture to give any account of it in their Memoirs; but now we saw something analogous to it. He thought the advantage would be greatly increased if, in place of the gold, a silver surface could be deposited, as it reflected nearly onethird more than gold. The reason why gold reflected more light than steel was, because its refractive power was greater. The difficulty of making a surface of silver would, he thought, be found considerable in practice, but some method might be discovered of removing the difficulty. A gentleman in Ireland had communicated to him a scheme for coating porcelain with platina similar to those vessels in ordinary use which had a metallic aspect. That attempt, he understood, was not successful; but the method seemed to be one which might be rendered useful by further experiments.

Sir W. Hamilton said, that one of the peculiarities of Lord Rosse's plan was, that he thought nothing of unpolishing a mirror in the morning, and having it ready for use again in the evening.-Literary Gazette, No. 1330.

NEW SYSTEM OF EYE-PIECES.

PROFESSOR GONNELLA has presented to the Scientific Congress, held at Florence in 1841, a theory of new systems of Eye-pieces of arbitrary lengths for the Newtonian telescope; and for spherical as well as parabolic object-glasses, according as is best suited for artists. In reducing for each telescope the length of the tubes of the eyepieces greater than the semi-diameter of the object-glass, we may substitute for the small plane metallic mirror a rectangular prism of glass, very small, whatever may be the opening of the object-glass, since the prism may be placed very near the focus without the eye of the observer being of necessity within the great tube; for the length of the eye-piece determines the place of the eye on the outside of this same tube. We thus obtain the power of constructing the best telescopes, with the prism, as pointed out by Newton, of large dimensions; for that prism being very small, we may,-1st, easily find

pieces of perfect glass for them; 2nd, we may have the total reflexion of the rays, since the quantity of light absorbed by the thickness of the prism is insensible; 3rd, we may obtain the advantage that the loss of the central rays of the object-glass, intercepted by the prism, shall be very small in relation to the diameter of the object-glass, and always smaller relative to the greatest diameters; for the size of the prism is always constant for all the object-glasses, viz. about an inch for each side of the right angle of the rectangular triangle which forms its base. In short, it only remains for us to observe, that the telescopes with the prism have double the clearness of those which have the small plane metallic mirror.-Communicated by Mr. Babbage to the Philosophical Magazine, No. 128.

POWERFUL MICROSCOPE.

MR. CAREY has constructed for the Polytechnic Institution a new Oxyhydrogen Microscope, consisting of six powers, ranging from 130 times to 74,000,000 times. The second power magnifies the wings of the locust to 27 feet in length. The fourth power magnifies the sting of the bee to 27 feet. By the sixth power, the eye of the fly, containing 750 lenses, is so magnified that each lens appears to be 14 inches in diameter; the human hair is magnified to 18 inches in diameter. The diameter of the condensing lens is 91⁄2 inches.

NEWLY-CONSTRUCTED ACHROMATIC MICROSCOPE.

THE points most deserving of attention in this improved Microscope, recently constructed by Messrs. Powell and Lealand, are the following:-The double pillar, as first made by Mr. George Jackson, which possesses the advantage of being lighter, and distributes the weight more equally upon the foot. We have also introduced a circular motion to turn the body off the stage to examine the object, and to change the object-glass to prevent it from falling upon the object. The stage is made larger and stronger than in former microscopes of this size; and the pinion and screw are of the same diameter as in the larger instrument, above alluded to. This description of stage was first constructed by Mr. Turrell. There are, however, two or three other improvements and modifications, such as the method for adjusting the object-glass to compensate for the thickness of the glass covering the object, &c.

The annexed Engraving shows the Microscope in the position for general use. As the most convenient method of using the instrument is in this position, it will be necessary, after taking it out of the case, to turn it by means of the pillars on its moveable foot, in order that the principal weight may be over one of the feet. When the person using it is standing, and the body of the Microscope perpendicular, it is then firmest as taken from the case.

A. The coarse adjustment for the body, which rests on two rollers and is moved by a rack and pinion.

B. The fine adjustment, which is a screw with a cone, against which

L

there is a spring pressing against the cradle, which carries the compound body.

C. Milled

Head, which moves the stage to the right and left.

D. Heads that move the stage at right angles to the other. (C) -There is one head on each side of the stage to this motion, in order that both hands may be employed at the same time; one on C, and the other on the opposite side, when a rotatory movement is required to search for animalcules, or other objects. When the screws, C and D, are used together, a diagonal motion, and when separate, motions at right angles with each

two

other, are obtained.

E. Arm for

holding the stops, which are used when viewing opaque objects.-To place them for use, push down the mirror G to the extremity of the stem; put the stop into the spring-hole, turn it into the centre of the stage, and raise it as high as the slide on which the object is placed will admit of.

G. Mirror for illuminating objects. When the concave side is used, it should be raised nearly to the stem, in order that the rays of light proceeding from it may reach the object before crossing, for by this means the most intense light is obtained. It is invariably used for opaque objects, together with the Lieberkuhn. At night it will be necessary to use the larger condensing lens, which should be placed

at about its focal distance from the lamp, with its convex side to the mirror, and adjusted till the rays of light fill it.

H. Spring-piece for holding the slides on the stage.-The most convenient mode of placing them is, to push up the spring-piece sufficiently high to allow the slide to go on the stage, and then compress it until it holds it. Any number of slides of the same width may then, after it has been set, be put in without that trouble. Should the slides be very wide, in order to have the whole range of the stage it will be necessary to observe if the centre of the slide corresponds with the line on the centre of the stage; if not, it must be altered until it does.

The following table exhibits the magnifying power of the objectglasses with each eye-piece.

For the complete details, see the Microscopic Journal, No. 12; whence the preceding has been abridged.

PORTABLE POCKET MICROSCOPE.

Mr. Cary, of the Strand, has constructed this simple and portable instrument of sufficient power for the general purposes of the Physician, the Naturalist, and for illustration in the Class-Room. This little microscope is very slightly modified from that invented and used by Mr. Children some years since it is so constructed as to view objects either transparent or opaque; the latter being accomplished by means of Lieberkuhn's to the half and quarter inch single lenses only. Either of these powers is well adapted for viewing such objects as the Foraminifera, portions of fine anatomical injections, parts of insects, and the like.

The instrument consists simply of a bed on which the object-holder is made to move to or from the power, by means of an endless screw, turned at one extremity by a milled head. The thread of the screw is sufficiently fine to adjust and focus the objects viewed by a doublet, which is the highest power that can be conveniently used. At the opposite extremity to the milled head, there is fixed beneath, a handle, which, by means of a hinge, folds back to the under side of the bed, as does also the part for holding the powers, rising in the opposite direction to the handle. Both of these portions may then, for portability, be placed in the same plane as the bed. The portion intended to carry the springs for holding the slide is made so that the part ordinarily employed for carrying the object may be instantly removed and replaced, or any other contrivance fitted, such as a short pin of iron wire, having at the end a ball-and-socket joint, which will enable the observer to move the object in any convenient position.