supported will be understood from the annexed diagram. The bottom of the mirror, A, is ground to an approximately

true surface, and the same thing is done with the bottom of the inner cell, B, on which it rests. Adjustments of parallelism, etc., are made by means of the screws, D D and E E; and the mirror, with its cell, can be removed from the telescope and replaced with the greatest facility, without loss of adjustment. A tight-fitting brass cap closes the inner cell, and keeps the silvered surface safe when out of use.

The reflecting prism or flat is supported, as shown in the appended diagrams, by means of thin strips of chronometer

spring. Fig. 1 gives a sectional, and Fig. 2 a perspective view. These two contrivances-the cell for the speculum and the support for the prism or flat-of Mr. Browning appear, from the trials given to them in Mr. Slack's telescope, to answer their purposes exceedingly well. The President of the Astronomical Society expressed a doubt whether the cellular plan of mounting the mirror would answer for large instruments; but, if it performs well up to seven, or eight, or ten and a quarter inches (the last being the largest size to which Mr. Browning has yet adapted it), its importance will stand very high. The new system of mounting the prism or flat has great advantages. The three slender springs do much less optical mischief than the single stout arm previously employed, and contribute, with the excellent working of Mr. With's mirrors,

to remove the defective definition which reflectors usually give of large stars. In Mr. Slack's instrument, the definition closely resembles that of a fine refractor, and the discs are remarkably small.

The Equatoreal stand was constructed according to suggestions given by Mr. Slack, the details being entirely arranged by Mr. Browning. The special ends in view were, great stability, with convenience and economy. Hitherto, moderate-priced stands have usually been of comparatively slight construction; and, though many of them possess considerable merit, none of them seemed adapted to carry a somewha heavy and bulky telescope. It will be seen, from the drawing, that the base of the new stand is very compact and solid. It is, in fact, a stout cast iron tube. The circles are twelve inches in diameter, reading to l' of an arc, and 2 seconds of time. The declination circle has considerable weight, and thus effectively assists in counterpoising the telescope. The tube of the telescope divides into two parts, each furnished with a flange fastened by screws to stout rings supported by a heavy arm. By this means the principal weights are exactly opposite each other in every position of the instrument, and they are kept near the centre of the polar axis, and at about equal distances from the centre of gravity of the pillar-stand. The hour angle motion has bearings equal to the diameter of the hour circle, twelve inches, which adds to stability, though at the cost of a little extra friction. Upon trial, this telescope is found to be remarkably steady and free from vibration under a power of between 600 and 700; and the result of this steadiness is very conspicuous in the definiteness of the division of double stars, when the lowest powers are employed that can produce such a result. Three short, stout screws run through the base, and enable the instrument to be adjusted to the indications of two spirit-levels, one of which is shown in the figure.

The eye-piece and prism or flat revolve so that the awkward positions to which an observer is subjected when an ordinary reflector is mounted equatoreally are completely obviated.

That these silvered glass telescopes will come into favour cannot be doubted, as they cost only a fraction of the price of reflectors capable of doing the same work; and perform to the satisfaction of observers like Mr. Webb, who has tried a good many, Mr. Cooper Key, Mr. Bird, and others. We must, however, caution our readers against unreasonable expectations, which we know have in some cases led to great disappointment. In the first place, if an observer who is acquainted with refractors, but has never used a reflector, gets one, and the slightest derangement occurs, he will most likely

have to serve a little apprenticeship in order to learn exactly what to do, and nothing is easier than for an unpractised hand to make a small fault ten times worse by mistaken efforts to get rid of it. Another thing is, that observers who have been using three or four-inch refractors are sometimes apt to expect that reflectors of two or three times those apertures will show all objects much better. Now they will not do this; neither will the same sized refractors. Large apertures, if good, give smaller discs, and thus permit the separation of closer stars. They render faint objects visible that were invisible with smaller instruments; and they bring out in the moon and planets details not to be made out with smaller means. But they do all these good things at the expense of taking in a larger space of disturbed air; and, except on favourable nights, the disadvantage arising from this circumstance is often so great, that it is advisable to reduce the aperture by stops when looking at bright objects. If a small aperture shows a double star well—that is, light enough, clear enough, and well divided -a larger one will seldom show it better, and often worse. The 6 size of Mr. With's mirrors, in good weather, will notch y Andromedæ, which is an excellent performance; and they find no difficulty at all with stars like Coronæ, when the atmosphere is favourable. Larger sizes, under appropriate circumstances, will, of course, do more.

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We would give another caution to telescope-buyers. Do not spend much money on large mirrors or object-glasses, and then ill-use them by mounting them on bad stands. A recent experiment showed that y Leonis was far better shown with a power of about 100, and the stand made for Mr. Slack, than with a mirror of the same size, and between two to three hundred magnification, on a shaky stand. In like manner the four stars of e Lyræ are more clearly seen with between 80 and 90, and a well mounted instrument, than with 250, and a badly mounted one. Economy is an excellent thing, and we are glad Mr. Browning has taken so much pains to provide for it; but if an observer only intends spending a given sum, we recommend him to apportion it so that he may get a thoroughly good stand, though so doing may oblige him to be content with a smaller mirror.

MR. WENHAM'S NEW BINOCULAR.

AT the Soirée of the Microscopic Society, Mr. Wenham exhibited a Binocular Microscope intended for use with the higher powers, having a new form of prism by means of which the whole of the aperture of the object-glass is transmitted into each eye. In our last number we described a binocular arrangement, invented by Messrs. Powell and Lealand, by means of which this result was first obtained. The main fault of this arrangement is the difference in the intensity of the two images, as the quantity of light reflected from the first surface of a well polished plate of glass, set at an angle of 45° is only th part or 53.66 out of 1000 of the incident rays. In Mr. Wenham's plan the light is obtained by combining two internal reflections, and the result is such that the image is nearly of equal intensity in both bodies of the microscope, the eye not being able to detect any difference. The first experiment tried by Mr. W. with the view of increasing the light was by means of two small right-angled prisms with their diagonals in contact, this formed a cube with parallel sides, and was placed behind the object glass and slightly tilted beyond the range of total reflection, so as to allow the rays from the object to pass directly through; the transverse reflection from the two contact surfaces was thrown into the required eye-piece by means of another right-angled prism. But eventually the preference was given to the annexed arrangement, as it is very compact and fits into the slide of the usual binocular microscope without any alteration being required in the instrument itself.

Its main feature consists of a prism similar to the now wellknown form but with the angles slightly more inclined, so as to allow a portion of the rays to pass through the first reflecting surface instead of being totally reflected therefrom; barely in contact with this surface is another prism whose upper plane is parallel with the base of the main prism, the rays from the object-glass will thus pass through without refraction or displacement, and the rays reflected from the two surfaces will be again reflected into the opposite body just in the same way as in the ordinary binocular microscope. The surfaces of the two prisms being almost in absolute contact both reflected images are thrown to the same point so that there is no appearance of a double image. The following illustration shows the prisms four times the real size, with the angles employed by Mr. Wenham, and of which he has furnished us with particulars; all the angles are measured from the base.

A, A, rays transmitted direct; B, B, portions of the same rays reflected into the oblique body as usual. As the rays are incident upon the back of the prism within the angle of total