MICROSCOPY.

a. Instruments, Accessories, &c.

Ahrens's Erecting Microscope.-In this instrument (fig. 28), by Mr. C. D. Ahrens, the erecting prism is inserted below the body-tube, and the latter is inclined at an angle of about 45°.

The prism is similar to Nachet's erecting prism.

When the Microscope has a fine adjustment, the prism is mounted on a piece of tube, as shown in the woodcut; but when the fine

adjustment is omitted, as in the smaller forms, the prism is fixed directly on the arm.

For convenience of packing, the inclined body-tube slides off, and a cap is fitted over the top of the prism-box.

The advantages claimed by Mr. Ahrens for the instrument are

the erection of the image by a prism at the lower end of the bodytube immediately over the objective instead of over the eye-piece, "so that any objective and any eye-piece can be used without any trouble," and the convenient inclination of the tube.

Bulloch's Improved "Biological" Microscope.*-Mr. Bulloch has made further improvements in his "Biological Microscope," principally in the substage.

The substage and mirror-bars move independently, with the object as a centre, as heretofore; but immediately beneath the stage, just above where the rackwork ends, the substage-bar is cut transversely and the two parts joined together by a pinion and screw passing vertically through lateral projections cast for the purpose. About this pin the lower part, carrying the substage with its rack and centering screws, swings laterally, entirely out from beneath the stage. The space between stage and mirror is thus unobstructed by the substage, and the substage itself is practically clear of the Microscope, where it can be seen, and apparatus removed from or added to it with even more facility than if it were held in the hand.

Mr. Hitchcock regards it "as the greatest improvement in substage fitting that has been made for years, and one that is sure to be appreciated as its value becomes known."

The substage-ring is also made in two parts, and the lower part swings to one side independently. This part may carry a tinted glass to modify the light, or the diaphragms of a condenser, which could be conveniently changed. It would be better to place the condenser and its diaphragms in the upper substage-ring, while the polarizer with its plates of mica and selenite are fitted in the lower ring. Such an arrangement would give the microscopist every facility for work that could be desired. Without removing a single accessory, he would be prepared to use the light directly from the mirror by turning the substage aside. Then the condenser could be brought into use by a single motion, and the different effects of oblique light and dark-ground illumination obtained by the simplest possible operation of changing diaphragms. By bringing in the polarizer, which is always ready for use, all the effects of polarized light can be obtained.

Cox's Microscope with Concentric Movements.t-The Hon. J. D. Cox describes the new features of this stand (fig. 29) to consist in "the construction of the arm of the instrument in the form of the segment of a circle in which is a circular groove or slot; the pillars of the base have on their inner faces tongues which fit the slot in the arm. The inclination of the instrument is made by the sliding of the whole body along the fixed tongues in the pillars of the base; the centre of motion of the whole body is also the optical centre of the instrument, around which the stage, the substage bar, and the mirror bar all revolve. The body is clamped up in any position by the setscrew, with large milled head, in the base. The result is a shifting

* Amer. Mon. Micr. Journ., v. (1884) pp. 9-10.

† Proc. Amer. Soc. Micr., 6th Ann. Meeting, 1883, pp. 147-8 (1 fig.).

of the centre of gravity in changing the inclination of the instrument, so that great stability in all positions is secured, and the optical centre is thus made the centre of all the circular motions of the parts of the instrument.

The first application of the sliding motion of the body was made by Geo. Wale in his 'Working Model,' but he did not make the

centre of motion the optical centre of the instrument. Mr. Wenham in his elaborate concentric Microscope uses a separate rocker arm with the whole body of the ordinary instrument pivoted above it. My design, which has been constructed by the Bausch and Lomb Optical Company, greatly simplifies the Wenham instrument, and extends the principle contained in Wale's, and makes a very compact,

stable, and satisfactory stand. The radius of the circular motion is 4 in., the stage is 4 in. in diameter, the concave mirror has 4 in. focal length, and the diameter of the mirror may be from 2 to 3 in."

Geneva Company's Microscope.-This instrument (fig. 30), made by the "Société Genevoise pour la Construction d'Instruments de

Physique," has two specialities; one being the spring pincers for rapidly inserting the objective, described infra p. 284, and the other the mounting of the mirror.

The mirror is attached, as shown in the woodcut, to three arms articulated in such a way that "the centre of the mirror is made to describe a curved line very nearly an arc of a circle, the centre of which is on the object under examination. The most suitable illumination is thus obtained very rapidly, the observer not having to

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regulate at the same time the focal distance of the mirror and its lateral distance from the axis of the Microscope."

The condenser fits into a double cylindrical tube beneath the stage, the inner tube being moved up or down in the outer by rack and pinion. The diaphragms are also inserted in the inner tube.

The whole arrangement can be readily turned away from the axis on an excentric pivot.

The stand, in its general form, size (16 in. high), and workmanship, is one of the best that we have received from the Continent. [Since fig. 30 was cut, the Geneva Company have supplied us with fig. 31, which shows more of the mode of attachment of the arms of the mirror.]

"Giant Electric Microscope."-This Microscope (ante, p. 109) has continued to be the subject of somewhat ludicrous comments on the part of the newspaper press.

The one point of remark is the extent of the magnification