On the whole, while no marked novelty seemed to present itself, the whole department of maps and charts at Paris was satisfactory, as indicating a constant improvement both in the clearness with which physical features are shown and in artistic excellence.

MICROSCOPY.

By Professor HAMILTON L. SMITH,

HOBART COLLEGE, GENEVA, N. Y.

IMPROVEMENTS IN MICROSCOPICAL OBJECTIVES AND MICROSCOPICAL APPARATUS.

The New Oil-immersion Object-glass of Carl Zeiss. Dr. Dallinger publishes a letter in Nature highly commendatory of this objective. The spherical and chromatic aberrations are beautifully corrected, and all the most crucial tests readily mastered. He states that he has not been able to do more with it than with the new formula one-eighth of Powell and Lealand, but that the same results are accomplished much more readily, as there is no correction to be brought into operation by the German glass, which has simply to be brought into focus. A drawback upon the use of this objective is the fact that the oil is a solvent of most varnishes and gums used in the mounting and finishing slides, except shellac-varnish; and he further remarks that immersion objectives are of very limited service in observations continuously conducted upon minute living organisms in fluid. We may gladly call in their aid in determination of a delicate change of form, or in the more perfect detection and definition of an obscure point of structure; but for steady and constant work we are bound to avoid them; for the fluid under the delicate cover is in danger every moment of being "flooded" by coming into contact with the water (or other fluid) on the top of the cover and between it and the lens; because the movements of the organism have to be counteracted by the movements of the mechanical stage, in order to keep any form that may be studied in view constantly. Since, then, there are these difficulties in the use of immersion-lenses in biological investigations, Mr. Dallinger expresses the hope that the English, the Continental, and the American opticians will not abandon their efforts for the still

greater improvement of dry lenses. Another notice, especially of the oil-immersion one-twelfth of Zeiss, by Adolf Schultze, in a recent number of the English Mechanic, states that the field is perfectly flat, and that the brilliancy and definition leave nothing to be desired. This author states that he has failed to see, both with the one-eighth and one-twelfth oil-immersion lenses, more than with the Powell and Lealand excellent new formula, or some other first-class waterimmersion lenses; yet, considering that no adjustments were to be made to the Zeiss objectives, he could, upon the whole, see everything better and casier with these. We have been informed that the results obtained by Dr. Woodward, U.S.A., in photographing by aid of these new objectives, are quite equal to anything he has accomplished with any other firstclass modern objectives. The result of a comparison of the one-eighth oil-immersion with the new one-tenth and onesixth water- and glycerin - immersion objectives of C. A. Spencer & Sons showed that the Zeiss objective, though pressing very closely, was nevertheless somewhat inferior to the Spencer objectives, especially by daylight, and when using very high oculars; still, the manifest advantages of the oil-immersion were so great that the latter firm are now perfecting a system on this plan, and microscopists are under great obligations to Professor Abbe and Mr. Carl Zeiss for this new departure, which promises so much.

Immersion Condensers.

In utilizing the increased angle of aperture obtained by the new immersion objectives, an immersion illuminator is required, and that devised by Professor Abbe is said to have a balsam angle of 138°. The reflex illuminator of Mr. Wenham has also been used with great advantage, also Dr. Woodward's arrangement of prisms, the under surface of the slide being connected with these illuminators by means of glycerin. Mr. Tolles long ago suggested the use of a hemispherical lens thus cemented or attached to the under surface of the slide; and Mr. George Wale, of Paterson, N. J., simplifies the whole matter by attaching a small prism, three eighths by a quarter inch, without any mounting whatever, to the under surface of the slide, by means of glycerin.

A New Cover-adjustment for Microscopical Objectives. The well-known optician, Mr. E. Gundlach, has proposed a new cover-adjustment for objectives, which, he claims, has some great advantages over the old method. These advantages, as he states them, are: (1) The adjustment exerts no deleterious influence on the corrections of the aberrations, and is equally as efficient for any thickness of the coveringglass as for uncovered objects. (2) The working distance is the same for any cover-thickness, except for immersion objectives. For this reason objectives of very short working distance will, with this adjustment, admit of even the thickest covering-glass. (3) The magnifying power is unchanged. (4) The image is placed but slightly out of focus. (5) The adjustment is very sensitive. (6) It can be made to indicate exactly the thickness of the cover. All these advantages, with others, are obtained, he claims, by discarding the old method of adjustment by moving one or more of the systems of lenses, and by placing a movable front of plane glass before the anterior lens of the objective, filling the space between with glycerin. A thinner or thicker stratum of this glycerin, according to the distance between the plane glass and the lens, gives the required adjustment (American Jour nal of Microscopy, June, 1878).

Professor Abbe's Apertometer.

This apparatus is intended to enable an exact measurement of angular aperture of any object-glass, dry or immersion, to be made; and to afford a definition of aperture which is not limited by the maximum air-angle, which is independent of the medium in front of the lens, and which, at the same time, by its theoretical signification, may give a direct indication of the resolving power of an objective. It has long been evident that the expression "angle of aperture is deceptive as an indication of resolving power, since this is proportional, not to the angle itself, but to the sine of the semi-angular aperture; in other words, in large angles the ratio of resolving power can bear no proportion to the mere number of degrees, the sines of such angles having very small progressive increase. Professor Abbe proposes a new name-numerical aperture; and by means of this "numeri

cal aperture" all objectives, dry, water-, or oil-immersion, can be directly compared. The apparatus cannot well be described without a diagram. Suffice it that it is mainly a semicircular disk of thick crown-glass, with polished edges. On one of the faces of the disk two scales are engraved. The inner one reads off the largest possible angle from air into the medium (crown-glass) of which the disk is made, or twice the "critical angle." The other reads off the corresponding "numerical aperture," which is a number that is always the product of the index of refraction of the medium in front of the objective multiplied by the sine of half the angle of aperture. Knowing this number, and also the index of the medium in front of the objective, we can from these get the equivalent angle of aperture for that medium. The internal scale is graduated from 0 in the middle to say 82° (or double the critical angle for crown-glass) on either side; and this, as already said, reads off the air angle. The external scale, concentric with, but outside of, the other, commences with 0 in the middle, and reads 1 on either side, coincident with the 82°+ of the inner scale; but the divisions extend much beyond this-say to 1.3 or 1.4. Suppose, now, the objective to be dry, and, as near as possible, 180° air angle, using the apertometer, the angle would be read 82°+ on the inner scale-i. e., 82°+ in glass, which is equivalent to 180° in air. On the outer scale we would read 1 (=sine 90°, half the air angle). This is the equivalent numerical aperture; and since this is a number which is equal to the sine of the semi-angle of aperture multiplied by the index of refraction of the medium (in this case air=1 nearly), we have 1=sine semi-angle x1, or sine of semi-angle of aperture=1; i. e., semi-angle = 90°, twice which=180°, or air angle, as before; and this is the utmost any objective could read with only air in front. Applying, now, a medium-say water (whose index is 1.33)—the outer scale, with the same objective, would read off-say 1.1. This is the numerical aperture—an aperture which, if taken in air, would be imaginary-i. e., surpass 180°; but to get water angle we proceed as before: sine aperture= 1.1 =0.827 sine 55° 30'; and twice this, 111°, is the water angle of the objective. If, instead of water, balsam (with index 1.5) had been used, and yet the scale had only read

1.33