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cell of each end of the filament. In those species which have no such sheath, variations in the turgidity are also brought about by variations in the exosmotic and endosmotic phenomena of the cells.

Alveoli of Diatoms.*-A. Grunow considers that the perforation of the alveoli has been completely proved in diatoms from the Jutland cement-stone and from the London clay; but that this can only modify the previously adopted interpretation, since he considers that the diatoms from these localities have already begun to undergo dissolution-this being unquestionably the case in those from the London clay-in consequence of which the delicate closing membranes of the alveoli disappear first of all. He believes that whenever diatoms are accompanied by lime, and especially when iron pyrites is present, as is the case in these localities, alkaline reactions have been set up, which may have been very weak, but which always act more strongly on silica than even very strong acids. In valves treated with very strong acids the alveoli may sometimes be seen actually perforated; while others close by will still be closed. Coscinodiscus oculus-iridis and C. Asteromphalus the former of which (in cement-stone) has open, and the latter unquestionably closed membranes are so closely related to one another that no sharp line of demarcation can be drawn between them. The author has given a close examination to this group of diatoms from FranzJosef Land; and believes that the alveoli are closed above and below by delicate membranes proceeding from the thickening-ring. In Triceratium Favus the upper one is sometimes furnished with small spines. He considers it very improbable that in nearly related forms there should be so great a diversity of structure as that between perforation and complete continuity of the valvos.

MICROSCOPY.

a. Instruments, Accessories, &c.

Hensoldt's and Schmidt's Simplified Reading Microscopes.tDr. C. Bohn refers to the necessity for insuring that the divisions of the micrometer of these instruments ‡ shall be a simple fraction of the magnified image of the circle divisions. If the distance of the latter from the objective is altered by moving the objective or the Microscope, the power is no doubt changed, but the image no longer coincides with the micrometer scale. Shifting the scale alone is of no use, for the same reason. The only course is to alter the distance of the objective from the circle divisions and the distance of the micrometer scale simultaneously in a proper ratio, the conditions for which he discusses. A Ramsden eye-piece must be used.

* Bot. Centralbl., xvii. (1884) p. 67.

† Zeitschr. f. Instrumentenkunde, iii. (1884) pp. 87-8.
See this Journal, ii. (1882) p. 548.

Geneva Company's Travelling Microscope.-This is a very ingeniously constructed instrument, shown set up in fig. 51, and as folded for travelling in fig. 52. To fold it, the narrow

FIG. 51.

curved support between the base and the uprights is turned back within the latter, a pin which fixes it in position (not shown) being first withdrawn from the base. The uprights are then brought down to meet the base, the body-tube, stage, and mirror

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being at the same time swung so as to be horizontal. The base consists of an open frame only, but is heavy enough to give complete steadiness.

The milled head seen on the right of the body-tube clamps the socket of the latter between the uprights, so as to prevent it altering its inclination. The fine adjustment is effected by tilting the stage at one end by the screw beneath it.

When folded, the instrument measures 7 x 3 in. x 12 in.

Reichert's Microscope with modified Abbe Condenser. C. Reichert, of Vienna, with his medium stand (No. III.) supplies a modified form of Abbe condenser, shown in fig. 53, with which may be contrasted the original form by Zeiss (fig. 54).

The optical combination, consisting of three lenses with an aperture of 1.30 N.A., is screwed in a ring a attached to an arm d. This arm revolves on a pivot beneath the stage, so that it can be turned away from the stage, as shown in the figure. The fitting c of the lower lens has inner grooves to receive the diaphragm slide b, which can be drawn out entirely, for changing the five diaphragm-stops which drop into an aperture at e, or partially (to the right or left), so that the aperture may lie eccentrically to the optic axis for oblique illumination. A spring-pin falling in three holes marks the central or extreme lateral positions of the slide. The lenses with the diaphragm slide can be rotated in the ring, so that all azimuths of obliquity can be obtained. The pin f fits into a hole beneath the stage when the condenser is centered.

A slide beneath the stage for the ordinary cylinder diaphragms can be used when required on the condenser being turned aside.

This apparatus seems to supply effectively the want which has long been felt for an adaptation of the Abbe condenser to the smaller

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stands, though some of the advantages of the original form must necessarily be lost in such a case.

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Reichert's Polarization Microscope.-This (fig. 55) is an inexpensive form of stand by C. Reichert, the chief peculiarity of which is that the wheel of diaphragms with five apertures rotates at the end of a horizontal arm, which, as with the condenser in the preceding form, swings on a pivot away from the stage, as shown in the figure. The diaphragm-plate is raised above the arm on a vertical axis, so that the tube attached to the largest aperture to hold the polarizer

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may not prevent the complete rotation of the plate. A notched projection on the arm falls against a second spindle beneath the stage when the apertures of the diaphragm-plate are central. The tube which holds the polarizer has a rotating fitting, and carries the polarizer with it.

The analyser fits over the eye-piece without any attachment, which would seem to be undesirable, even though the Microscope can only be used in an upright position.

*When the polarizer is in place the plate cannot rotate completely, but no rotation is then required.

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