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Those of our readers who are familiar with microscopic operations are aware how much depends, in the observation of structure, upon the method of illumination. In the investigation of the delicate markings of diatomaceæ, and the striæ of enamel and muscular tissue, an immense deal of the worker's success is dependent upon the volume of light which is thrown upon the object. If the flood of rays be very intense, the markings cannot be perceived, while if, on the contrary, the illumination be faint, other points of structure are lost sight of. There is, therefore, a necessity for the regulation of the quantity of light reflected to the object. In the instrument at present in use, this end is attained by the employment of a piece of apparatus termed "a wheel of diaphragms." This consists simply of a revolving black disk, placed horizontally between the stage and the mirror, and pierced by a number of hole (generally 6 or 7) of different sizes. The microscopist, when he desires to alter the illumination, turns the disk round till he finds a perforation which suits his purpose best. All however will agree that there are two very serious objections attending the use of the "wheel." In the first instance, though it may help to produce a more suitable form of illumination than that which results from the utilization of all the light reflected from the mirror, its number of apertures is limited, and hence also its application and utility. It not unfrequently occurs that between the two smallest apertures there are several possible ones which would render observation more satisfactory. Secondly—and this is a circumstance of some import,—in changing the aperture, the observer loses sight of the object, and has to trust to memory in estimating the advantage he has secured by the alteration.

These two obstacles have been overcome by Mr. Collins, of Great Titchfield-street, who has devised the diaphragm represented beneath. The instrument consists of a plate of about the size of the stage, and placed below it: this presents an aperture of the same diameter as that of the stage, so that in the illumination of objects under low powers, all the light may be employed. Beneath the first plate are two smaller ones, which move horizontally to the right and left of the stage, and have their proximal borders incised in a wedge-shaped manner. These can be either separated or approximated by means of a lever, which is connected with a screw and milled head, which is situate within easy reach of the observer's finger. When widely separated, these two plates leave a large lozenge-shaped aperture; but by the application of the screw this may be diminished in the most gradual manner till an aperture of scarcely a pin's head's diameter is produced.

In testing the new diaphragm, a full flood of light should first be thrown upon the object (some of the diatomaceæ say), and then by the rotation of the

screw the aperture should be slowly diminished till the fine markings come into view. Whether the shape of the aperture may disadvantageously affect the pencils of light as they travel to an object under examination with high powers, is a question which remains to be determined; but with such objectives as the ½-inch, and those below it, the illumination-effects of Collin's instrument are beautiful.

Proposed Association of Amateur Microscopists.—At a meeting held at 192, Piccadilly, June 14th, a provisional committee was appointed to draw up a scheme of organization, &c., to be submitted to a general meeting. We are authorized to announce that the meeting will take place at St. Martin's National Schools, Adelaide-place, Charing Cross, on Friday evening, July 7th, at eight o'clock precisely, to which all persons interested in the establishment of such a society are invited. Tickets may be obtained (free) at 192, Piccadilly; of Mr. Highley, Green-street, Leicester square; Mr. Gibson, 9, Lupus-street, Pimlico; or Mr. Bywater, 5, Hanover-square.