off 1.1, we should have for balsam angle: sine 1.1 1.5 aperture= =0.733 sine 47° 15'; twice which, or 94° 30', balsam = = angle. If the objective had, instead of 1.1, given 1.25 as numerical aperture, its balsam angle would have been 113°. We have, then, in these two cases of 1.1 and 1.25 a resolving power in the first 10 per cent., and in the last 25 per cent., greater than the possible limit of a dry lens (sine 90°=1) (Journal of the Royal Microscopical Society, March, 1878). New Form of Micrometer. Mr. G. I. Burch has described a micrometer based upon a comparison of the reflection of a scale with the image of the object, and which he claims is equal in accuracy to all other micrometers except the Cobweb. It consists of a cap, fitting over the eye-piece, containing a piece of neutral-tint glass (or looking-glass, with the amalgam removed in the centre), set diagonally, so as to reflect to the eye the image of a scale, which is carried by an arm ten inches long, attached to the cap, the object being observed in the usual way, through the eye-piece, but looking through the diagonal glass. To adjust the scale so that it may read decimals of an inch, etc., it is moved on the arm nearer to, or further from, the eye, till, on adjusting the focus so that the apparent distance of the two images may coincide, every tenth division on the scale shall cover the T or the T of the stage micrometer, according to the power used (Journal of the Quekett Microscopical Club, No. 37, 1878). New Test-object. Professor Ranvier recommends as a test for objectives intended for histological work ("which are required, not for flat bodies presenting only fine striæ, but for objects of irregular and varying forms-rough, concave, or convex") the isolated muscular fibrillæ of the wings of the Hydrophili. With a power exceeding 300 diameters, the alternately thick and thin dark disks which characterize the fibrillæ may be seen. Although Professor Ranvier, from whose book on "Practical Histology," just published, the preceding is taken, is the leading histologist of the day, yet he seems to be quite ignorant of the principle involved in the con struction of the binocular microscope, since he states, in his book, that in the binocular microscope there are not two different images, but the same image, presented to each of the eyes of the observer. However true this may be of Powell & Lealand's "binocular for high powers," it is not true either for the Wenham, Nachet, or Stephenson binocular, or for the Tolles binocular eye-piece. Indeed, with the Nachet form, which has some advantages, the instantaneous conversion of a relief into a depression, or vice versa, according to the manner in which the images are made to enter the eyes, is quite striking; and the very fact of orthoscopic or pseudoscopic vision thus produced, by alteration of the position of the prisms, proves that the images are not similar. M. Ranvier states that he has found the penetrating power or focal depth of the binocular superior to that of the monocular. The reason for this is, that each eye uses only its own half of the objective; and any objective, when half the front is covered, will, even with the ordinary monocular, have increased focal depth. A moist chamber of very simple construction is described by Dr. Strassburger. It consists of a ring of card-board soaked in water, on which the covering-glass is placed. The drop of water containing the Spirogyra, the copulation of which was to be observed, must be suspended from the under surface of the covering-glass, and may then be preserved for several days; whereas, if placed under the covering-glass in the usual way, the plants will invariably die. Self-centring Turn-table. A self-centring turn-table is described by Mr. C. F. Cox, in which the slide is grasped at the two opposite corners by right-angled clutches, which are moved simultaneously by one milled head, turning a right- and left-handed screw. As this device centres the slips only lengthwise, Mr. Cox proposes, when it is necessary to make several cells in the same slip, to hold the slide between two right-angled triangles of brass, which are grasped by the clutches, the slide being between them, and thus allowing several cells, if necessary, to be made on one slide, by simply slipping it along. (American Journal of Microscopy, Jan., 1878). Theoretical Limit of Aperture. In an able paper on the Theoretical Limit to the Apertures of Microscopical Objectives, Professor G. G. Stokes, D.C.L., etc., after alluding to Professor R. Keith's elaborate computation relative to Tolles's one-sixth microscopic objective, which is given in full in the Journal of the Royal Microscopical Society, July, 1878, and fully endorsed by him, states that the reason for scepticism as to the results of such calculations seems to be a notion derived from a priori considerations, that it is impossible to collect into a focus a pencil of rays emanating from a radiant immersed in water or balsam of wider aperture than that which in such a medium corresponds to 180° in air, or, in other words, than twice the critical angle; and this he disproves by showing that in certain particular instances it is untrue, so that the aperture in the case of a medium with a refractive index 1.525 may, at the extreme, exceed the supposed limit by over 16°. MICRO-ORGANISMS, BACTERIA GERMS, SPORES, ETC. M. Gunning, in a note read July 1, at the French Academy, states that he has repeated his experiments on Anaerobiosis, or life without oxygen, under conditions to which no exceptions can be taken, and proves conclusively that such life is impossible. Admitting the practical impossibility of obtaining spaces where the absolute absence of oxygen could be proved, he used glass flasks, hermetically sealed, in which as large quantities as possible of putrescible matter were placed in contact with the smallest possible quantities of oxygen. The details of his mode of doing this are published in the Annals of the Academy of Sciences, Amsterdam, vol. xii., 1878, and in the sixth part of the Journal of Practical Chemistry. He found that when the flasks were sealed and exposed to a temperature of 38° to 40°, putrefaction was immediately established-to be definitely arrested, however, in all the flasks after a longer or shorter period; often very short, but always sensibly proportioned to the quantity of oxygen supposed to be present. After two years some of these flasks had lost Q little or nothing of their primitive freshness. When the flasks containing the putrescible matters terminated in tubes provided with cotton-wool, or which were recurved many times upon themselves, and whose tapered points were hermetically sealed-so that at any given moment, by breaking the points, their contents could be exposed anew to contact with the air deprived of germs-he found that, by waiting until the contents had arrived at a state of complete inertia before establishing this contact, the air no longer produced the least phenomenon of putrefaction or appreciable alteration; proving that the Bacteria, as well as their germs, were not only dead, but that the organic matters are not susceptible of spontaneously producing others. Organisms Suspended in the Atmosphere. M. P. Miquel, after alluding to the statements of M. Charles Robin, that the atmosphere contains (besides all kinds of débris) spores, pollen, skins of insects, and (rarely) eggs of Infusoria, and also to the experiments of Drs. Maddox and Cunningham, describes his own newly contrived" aeroscopes," by means of which he collected from 500 to 120,000 organized cellules per cubic meter of air; thus showing the atmosphere to contain at least 100 times more germs than Drs. Maddox and Cunningham have stated. M. Miquel states two general facts as applicable to all organized corpuscles of the atmosphere whose diameter is greater than the of a mil limeter: 1. The average number of Microbia of the air, small in winter, augments rapidly in spring, remains nearly stationary in summer, and diminishes in autumn. 2. Rain always provokes the recrudescence of these Microbia. The increase brought about by rain is not simply sensible, it is often surprising. For example, in summer, when to great heat succeeds a storm, or a rain somewhat sustained, the instruments, which the day before recorded 5000 to 10,000 germs, record more than 100,000 the next day. Temperature and moisture seem the principal causes of variation in micro-germs in our atmosphere. The cellules most diffused in the air are undoubtedly spores of the Mucedina; then fructifications of certain fungi; then come pollens of variable size and color; and, lastly, green algæ, voluminous quantities of which are sometimes transported in the air (Comptes Rendus, vol. lxxxvi., p. 1552). Schulze's Mode of Intercepting the Germinal Matter of the Air. In 1836 Schulze described an experiment which has obtained considerable celebrity. Placing in a flask a mixture of vegetable and animal matters and water, he inserted in the cork closing the flask two glass tubes, air-tight, but bent at right angles above it. The infusion was boiled, and while steam was issuing from the two tubes, he attached to each a group of Liebig's bulbs, one filled with a solution of caustic potash, the other with concentrated sulphuric acid. Applying his mouth on the potash side, he sucked air daily through the sulphuric acid into the flask. But though the process was continued from May till August, no life appeared. The germs diffused in the atmosphere are supposed to have been destroyed by the acid in this experiment, but others have failed to obtain the same results. Professor Tyndall has recently stated that the success of the experiment depends upon passing the air-bubbles so slowly through the acid that the floating matter, up to the very core of every bubble, must come into contact with the surrounding liquid; and that water may be substituted for both acid and potash. His crucial experiment was as follows: Two large test-tubes, each about two-thirds filled with turnip infusion completely sterilized, were so connected together that air could be drawn through them in succession. Two narrow tubes passed through the cork of each test-tube in the same manner as in Schulze's flask; and it was so arranged that the tube which delivered the air should enter near the surface of the fluid, the exit tube in each case ending immediately under the cork. Two series of Liebig's bulbs, charged with pure water, were attached to the two tubes of this arrangement, one being connected with a large receiver of an air-pump, the other left open to the air. The connection between the receiver and the adjacent bulb being first cut off by a pinchcock, the receiver was exhausted, and, by carefully loosening the pinch-cock, a very slow passage of the air through the test-tubes was secured. The rate of transfer was, however, such that the air above the infusions was renewed twenty or thirty times in twenty-four hours. At the end of twelve |