tion of a bright disk or line of light, which are capable of totally obscuring an object of less diameter than these fringes, varies as the natural sine of the half angle of aperture for the same wave of light. Now the half wave-length for an extreme aperture of nearly 180° is, upon this theory, the 26 of an inch, which approximates very closely with the measurements of that very difficult test diatom, Amphipleura pellucida. It would seem, therefore, that the statements of Mr. Webb as to the minuteness of the letters in the Lord's Prayer, as mentioned above, must be erroneous. Those who are interested in the question of the present limits of vision will find many useful hints in Dr. Royston-Pigott's paper in the above-named journal, and from which we extract the following notice of the application of the formula of Helmholtz. From the experiments of Mr. Broun, F.R.S., who found that a dark-brown hair 0.0026 inch wide and 2.5 inches long was visible by a young eye against a northwest sky at thirty-six feet distance, subtending at that time 14 seconds of arc, it follows that such an eye can actually see lines on glass Too of an inch wide and long; and if such be the power of the naked eye without a lens, it ought to follow that the Tooooo of an inch ought to be seen by the same eye with a power magnifying ten times. Now Nobert's lines, if the interspaces are the same width as the lines, when they are 112,000 to the inch, would have an absolute diameter of 224000 of an inch; and as such a line would at ten inches distance subtend one ninth of a second nearly, a magnifying power of a little over five hundred diameters should make a visual angle thirty times greater than Mr. Broun's result above stated. We must conclude, therefore, with Dr. Pigott, that although theoretically, and for brilliant lines and points, the separable interval may be for the widest aperture half a wave-length, yet when by proper precautions of illumination the diffraction can in a great measure be destroyed, the limit is much smaller than the one assigned.

APPARATUS AND OBJECTIVES.

In the American Naturalist for December, Dr. R. H. Ward briefly reviews the exhibition of microscopes at the Centennial Exhibition. The Continental microscopes were chiefly represented by the exhibit of Nachet, the English depart

ment by that of Ross, so far as attractiveness of appearance was concerned, for R. and J. Beck's exhibit was more complete, but badly displayed; Crouch also exhibited a full series of instruments of excellent workmanship and at moderate prices. The only American display of any note was by Zentmayer, of Philadelphia. Messrs. Bausch and Lomb, of Rochester, however, exhibited a large series of entirely new designs, elaborated under charge of E. Gundlach, formerly of Germany, and chiefly remarkable for excellent workmanship and high optical qualities at greatly reduced prices. The other American exhibitors were T. H. M'Allister, George Wale, and J. W. Queen and Co. Powell and Lealand, Hartnack, Zeiss, Spencer, and Tolles were conspicuous by their absence.

In the American Journal of Microscopy for April, the wellknown optician Mr. E. Gundlach describes two new illuminating-glasses for the microscope. One is a hemispherical lens, which is connected to the object-slide by a drop of water or glycerin, and of such thickness that the converging rays from the mirror undergo no refraction at the first or convex surface, but, on emerging from the plane surface, or, better, the glycerin, are powerfully refracted, but almost without aberration, as the centre of the curve is in the optical axis of the microscope, and the object itself is very nearly in the centre of curvature. The other is an oblique light-projector, distinguished from the condenser by the fact of the lower surface being plane instead of convex, and parallel with the upper one.

In the April number of the Monthly Microscopical Journal, Rev. S. G. Osborne describes a modification of Reade's "kettle-drum" illuminator under the novel name of the "Exhibitor." Like most other devices of this class, the practice and patience required to master it will more than counterbalance its advantages (if, indeed, it really have any) over the simpler and easier modes of illumination.

In the Monthly Microscopical Journal for August, 1877, Surgeon J. J.Woodward, Brevet Lieut.-Col., U.S. A., describes a simple device for the illumination of balsam-mounted objects for examination with immersion objectives whose "balsam angle" is 90° or upwards. A truncated rectangular prism of glass, supported base upwards upon a similar trun

cated metal prism, is connected with the lower face of the glass slide by interposition of a drop of oil of cloves. Rays transmitted through this prism will, of course, not be received into any dry objective, or any immersion one of less than 90° balsam angle when transmitted through balsam-mounted objects. With those objectives, however, that will admit such rays (giving a bright field) and are properly corrected, the Amphipleura pellucida is readily resolved in balsam. In the same journal is a note on a new "Paraboloid Illuminator" for use beneath the microscope stage, by James Edmunds, M.D. It is a modification of the well-known Wenham parabolic illuminator, much smaller, and made with great care—being, in fact, a paraboloid lens, of glass, of low refractive index, carefully cut off at a point about one twelfth of an inch below its latus rectum. The truncated surface is connected with the under surface of the slide by glycerin, and suitable stops, etc., are applied under the lens.

HISTOLOGY.

Dr. J. G. Richardson, of Philadelphia, having obtained specimens of blood from the several individuals of different parts of the world who went to the Centennial Exposition last autumn, after measuring carefully every isolated circular red disk, cautiously avoiding those that manifested even slight departures towards an oval form, arrived at the following results, which we condense from the tabular view given in the American Naturalist, March, 1877: 1400 corpuscles were separately measured; the average size was 3224 (0.007878 mm.), the maximum was 777, and the minimum of an inch. Of these, 1158, or 83 per cent., measured between 3448 and of an inch, a difference of size scarcely discernible with a power of 200 diameters; about eight per cent. were less than 344, and nine per cent. more than 30 of an inch in diameter; the total number of an inch across was six, or less than one half of one per cent.; and the total number 2777 of an inch in diameter was ten, or less than one per cent. Herr Ebner, in a memoir on the histology of the hair, presented to the Academy of Sciences, Vienna, July 12, states that the inner root sheath is essential for hair-formation; and though broken through by the hair, it grows during the whole hair-vegetation, in the lower part of the follicle, with even

greater rapidity than hair. He defends Langer's view that new hairs are formed in the old follicle and on the old papilla.

In the Monthly Microscopical Journal for June is a note, with illustrations, from a paper by W. Blythe, M.R.C.S., on the "Microscopical Active Principle of the Cobra Poison," published in the Analyst. Magnified 250 diameters, it appears as long, slender, prismatic, and radiating crystals; and so terribly active is this substance, for which the name cobric acid has been proposed, that one fifth of a grain injected into a man's veins would be fatal. So that we have here a rival to aconitia, weight for weight, in its power of destruction.

STAINING AND MOUNTING.

The new double staining, by the mixed boracic solutions of carmine and indigo carmine (sulphindogate of potassium), as suggested by Merkel, promises very interesting and useful results, though as yet very unequal, probably owing to the action of the oxalic acid, into a solution of which the preparation must be put after the staining, to fix the indigo blue, but which often destroys the carmine, or changes it to a yellow color. The blue boracic solution of indigo carmine by itself is highly recommended by Mr. Golding Bird. It stains rapidly, is well fixed by the oxalic process, and is of a very agreeable color. An account of the action of these dyes may be found in the American Journal of the Medical Sciences, January, 1877.

Mr. T. S. Ralph, in the Science Gossip of October, proposes chloral hydrate as a medium for mounting, since it will dissolve and unite perfectly with many gums, resins, and alkaloids, so that we may obtain mediums varying from a fluid to a jelly-like or gum-like consistence.

BACTERIA, ETC.

Signori Lauri and Terrigi have been conducting a series of experiments upon the so-called Campagna marsh poison. They find in the endochrome of algae growing on the Campagna and Pontine marshes minute dark granules, belonging to the group of pigmented sphærobacteria of Cohn (Bacterium brunneum of Schroeter), and yielding Monila pencillata of Fries on cultivation, and which appear to be identical

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with the "pigment granules" present in the liver, spleen, and blood of persons who have suffered from malarial diseases; and by cultivating such granules from a human liver Lauri obtained a Zoogloea. On the basis of these observations, the prevalence of malaria at certain seasons is explained by the immense numbers of sphærobacteria in the air, rising from the dead and decaying algæ as the waters sink in the marshy pools, and which, swept hither and thither by the wind, excite malarial diseases when they penetrate into the human body.

The Bacteria of Denmark have recently been studied by Dr. Eugene Warming, and an abstract of his paper upon them is given in the Journal of Botany for December, 1876. All along the Danish coast there is found, during calm weather, a red coloration of the water close to the shore, chiefly due to Bacterium sulphuratum, under which name are united a number of forms, appearing, 1st, as spheres (Monas vinosa, E.); 2d, as roundish bodies with constriction and granules at the ends (Monas Warmingii, Cohn); 3d, like Monas vinosa, but crowded with sulphur grains (Monas erubescens, E.); 4th, long, narrow, cylindrical, and filled with sulphur grains (Rhabdomonas rosea, Cohn); finally, the series is closed by a spiral form. Besides these, many other species are pretty fully described.

In a letter addressed to the Secretary of the Royal Society, London, under date February 14, 1877, Dr. Tyndall states that heat discontinuously applied is a "germicide," and that, even in the midst of a virulently infective atmosphere, it is possible to sterilize all infusions by a temperature lower than that of boiling water. This is effected, however, not by a simple substitution of time for intensity, but depends solely upon the manner in which the heat is applied. The secret of success is to apply the heat for a period not exceeding the fraction of a minute in duration, during the period of latency preceding the clouding of infusions into visible Bacteria, and while the germs are being prepared for their emergence into the finished organism. As they reach the end of this period successively, the heating process must be repeated at intervals, so that the softened and vivified germs on the point of passing into active life are killed as they arrive successively at this stage. After a number of repetitions