the indices are adjusted, as before described, so that their sharp points just touch the margin of the circular field limited by the contour of the back lens of, or any diaphragm in c. If the aperture is great enough, the indices should be brought to the disk in such a position as to touch the margin from within. The position of the straight edges of the indices on the internal scale of the disk will give the semi air-angle of the objective a, as stated before. The external scale will give another definition of the aperture which is more abstract, and may be applied to those immersion lenses the angular aperture of which, taken in air, would surpass 180°; i. e. would be imaginary. This external scale will give the value of the product a = n. sin. w, n denoting the refractive index of any medium in front of the objective, and w the angle of semi-aperture belonging to the same medium. This quantity a, which Professor Abbe calls "numerical aperture,' gives an absolute definition of aperture, which will not depend on the nature of the medium, supposed in front of the lens-air, water, or balsam; and by which lenses of every kind are directly comparable. This value, taken note of as above described, the middle of the readings of both the indices considered, will afford the angular semi-aperture w of the lens for any definite medium, by the formula a denoting the number observed, n the index of the supposed medium. For instance, the immersion lenses of Zeiss will give approximately, a = 1,1; calculated for water (n = 1·333), for balsam (n = 1·50), The internal scale gives the semi air-angle from 5-5 degrees, the external scale the value of a from 5-5 units of the second decimal; by estimate the single degrees and the units of the second decimal of a are easily deduced. The exactness of the observation does not depend either on a very exact focussing to, or an exact centering of the hole in the silvered cover (the centre of which forms the geometric centre of the scales). It is sufficient if any point whatever of the hole is in the field of vision of the objective x, the possible difference of centering being eliminated by taking the mean of the reading of both indices. It is important in this method of measuring that, by the arrangement described, all "false rays," i. e. all rays which do not act in the formation of the ordinary microscopic image, which the objective would produce, are excluded. If the observation is made with the naked eye, the pupil of the latter (which by its central position at the end of the tube will coincide with the ordinary image as seen in the eye-piece) acts as a diaphragm for this purpose. In the case of the auxiliary microscope the same effect is produced by a diaphragm above the achromatic lens, belonging to the apparatus. În observing the indices on the disk by the auxiliary microscope, this diaphragm excludes all rays besides those which would form the ordinary microscope image in the middle part of the field of vision. Therefore this diaphragm forms an essential part of the apparatus, and must be specially adjusted to the objective B, in diameter and position, in order to fulfil its task. In the two scales the position of every line has been calculated, the calculation being based on the measured index of the crown glass forming the disk. FOREIGN MICROSCOPY. On the Orthonectida, a new Class of Animals parasitic on Echinodermata and Turbellaria. By M. A. Giard.*—The little Ophiuran, Ophiocoma neglecta, contains a singular parasite which may serve as the type of a whole group of animals of very curious organization and hitherto almost unknown. The following are the circumstances under which this parasite is met with. Ophiocoma neglecta is an Ophiuran with condensed embryogeny, or viviparous. The incubatory cavity, situated in the aboral part of the disk, communicates freely with the exterior; for the most advanced embryos contained in this cavity frequently present upon their arms a pretty Vorticella, which occurs almost always upon the arms of the parent animal. On tearing open the disk in order to extract the embryos from it, we find it, in certain individuals, filled with a multitude of animals like large ciliated Infusoria, which traverse the field of the microscope in a straight line, and with the rapidity of an arrow. The animals occur of two forms, which I shall name provisionally the elongated and the ovoid form. In both they are simple planule, that is to say, organisms composed only of two layers of cells-an exoderm or outer layer of ciliated cells, and an endoderm consisting of larger cells bounding a linear central cavity with no buccal aperture or anus. Notwithstanding this low organization, the body is metamerized, and the metameres even present remarkable differentiations. The first ring terminates anteriorly in a blunt cone, and bears a tuft of rigid setæ. It is followed by a cylindrical ring of the same length, the whole surface of which is roughened with papillæ, apparently disposed in ten longitudinal rows; this is the only part of the body which does not present vibratile cilia. The third ring is larger than the first two taken together; it widens gently towards its posterior extremity. The fourth metamere is of the same dimensions as the papilliferous ring, it is followed by a terminal ring, furnished with longer cilia at its posterior extremity, conical and subdivided into two metameres less distinct than the preceding ones. Such is the elongated form. The last rings form a sort of club with which the animal beats the water, independently of the movement of the cilia, and by sudden blows, which one might think due to the action of muscular elements. The ovoid form differs from the elongated form only in its less length and greater breadth; but I have ascertained that it is not the result of a contraction of the animal. Perhaps it is a sexual form, perhaps also a young state of the parasite. I give this strange animal the name of Rhopalura ophiocoma.† Intracellular Fermentation.-In a note communicated to the French Academy by M. Muntz, reference is made to experiments of MM. Lechartier and Bellamy showing that fruits, roots, and leaves From the Annals and Magazine of Natural History' for February, 1878. + Comptes Rendus,' October 29, 1877. removed from the action of oxygen became the seats of alcoholic fermentation with evolution of carbonic acid, and without the appearance in their tissues of any alcoholic fungi. These observations confirm the statements of Pasteur in 1861, that if plants continued to live in an atmosphere of carbonic acid they became ferments for sugar and behaved like beer yeast. M. Fremy thought that the true explanation was that yeast cells were formed, and to settle this question M. Muntz made his experiments, and found that the plants he grew in air produced no alcohol; that those grown in nitrogen afforded appreciable quantities; and the plants continued to develop. He did not search for mycoderms, but assumed none were present, because the plants began in a few hours to produce oxygen and preserved their vitality, which he considers they would not have done had they been invaded by fungi. To detect minute quantities of alcohol he employed Lieben's method, which depends upon the action of iodine and an alkali at a slightly elevated temperature upon alcohol. It gives rise to iodoform (a yellow solid), the production of which he watched under the microscope. He supports the conclusions of Pasteur that the living cells of the higher plants can in the absence of oxygen act like fungus cells and produce a true alcoholic fermentation.* The Inversion of Sugar by Fungi.-M. Gayon states to the French Academy, as the result of his observations, that Penicillium glaucum, Sterigmatocystis nigra (Aspergillus niger) rapidly invert sugar solutions, but other Mucors, such as M. spinosus, M. mucedo, M. circinelloides, Rhizopus nigricans, leave them intact. The unicellular plants Pasteur calls torulas, act also as inverting ferments. When the mucors are obliged to live without free oxygen in the must of beer or wine, their mycelium becomes chambered and develops ferment cells, which reproduce themselves in the same form while the conditions are unchanged, but develop in the normal state when replaced in very aerated liquids. The ferment cells of Mucor circinelloides are spherical, and remarkable for activity of pullulation. In solutions of levulose, or glucose, the alcoholic fermentation proceeds as in beer must, but in cane-sugar solutions no such action occurs, as the sugar is not inverted by the mucors mentioned. M. Trécul, commenting upon these observations, concluded that those observers were right who affirmed that P. glaucum could pass into the form of beer yeast and return back to its original form, which M. Pasteur denied. M. Pasteur, in reply, referred to his 'Études sur la Bière' as confuting this idea.† Formation of Blood Fibrin.-M. Hayem described to the French Academy microscopical studies on this subject. He states that the bodies he calls hæmatoblasts, which can be recognized in living animals, experience great alterations when they pass out of the vessels. He states that when a preparation of coagulated frog's blood has a current of iodized serum passed through it the "hæmaties may be seen disposed in rosettes around masses of hæmatoblasts, fixed in their positions by filaments springing from the centre of the * Comptes Rendus,' January 7, 1878. + Ibid. rosettes. This mode of treating the blood displays the hæmatoblasts transformed into irregular, angular, stellate corpuscles, with extremely fine delicate fibrils springing from them, branching and forming a network not easily seen, except they are coloured with iodine. Human blood exhibits these changes very plainly. The hæmatoblasts of the ovipora, like those of the higher vertebrates, experience rapid modifications. A few minutes after the preparation is made they become much changed, and may be seen in the interspaces between the hæmaties as little corpuscles, mostly spinous, isolated, or grouped in chaplets; afterwards in small irregular masses. These corpuscles are in general more highly refractive than the hæmatoblasts that form them, and are often of a greenish yellow colour. If blood is taken from a living animal and diffused through enough iodized serum to hinder coagulation, the hæmatoblasts appear isolated and in their normal shape, but after some hours they exhibit small prolongations that seem formed of their own substance. In defribrinated blood neither hæmatoblasts nor their corpuscles are found, and this is the case with blood taken from a dead body after post mortem coagulation. The hæmatoblasts, as well as being destined to become adult red globules, possess special properties, and may be considered as a third species of blood elements. Are they the determinating cause of coagulation? This seems probable. At any rate, three factors are concerned in coagulation; a substance proceeding by exosmose from the hæmatoblasts, and which perhaps represents paraglobulin; isolated or grouped corpuscles formed by them in the process of cadaveric change, and from which the network of fibrils springs; and a substance primitively dissolved in the plasma, modified in the presence of the matter exuded by the hæmatoblasts, and forming by precipitation nearly all the fibril network. In their normal state the smallest hæmatoblast corpuscles are about 1 μ in diameter, and the largest rarely more than 8p. In intense anæmia, especially when allied to a cachectic condition, we voluminous masses formed by the hæmatoblasts 60 or 70 μ in their largest diameter, but usually the network springing from them is less than in the normal state. In acute maladies, the hæmatoblasts are less abundant, but, contrary to what is observed in cachexies, the fibrin forms a rich and thick network.* see Action of very Low Temperature on Bacteria.-While the action of high temperature on bacteria has been frequently studied, few observations have been made as to their behaviour at low temperatures, but it has been found that they stiffen at 0° (C.), and are not killed at 18° to 25° (C.). Herr A. Frisch by means of solid carbonic acid and ether exposed some putrefactive fluid bacteria and some forms of coccus and bacterium in the morbid products of living organisms to - 87.5°, and allowed them in the course of 2 hours to rise to 0°. The result was that the bacteria in the fluid withstood this low temperature, and was able to grow rapidly when transferred to a suitable nutritive fluid. Further information is to be given concerning the resisting power of Coccus, Bacterium, and Bacillus.† * Comptes Rendus,' Jan. 7, 1878. Der Naturforscher,' 5, 1878. |