In addition to this, the instrument has been made larger (it measures 18 cm. in length), by which means the object to be cut may be considerably longer.

The most important drawback hitherto, however, was not being able to alter easily the direction of the section when the object was once screwed down. The whole object had to be taken out every time, and the body in which it was imbedded differently shaped. Those who have had to make longitudinal sections through hairs, &c., know what that is. Besides this, with the old arrangement it was quite impossible to make oblique sections through their whole length of embryos which were at all curved. It was generally the case that the sections gradually took another direction with respect to the longitudinal axis, and not unfrequently the oblique section passed finally into a frontal section. In the modified instrument the clamp for the object is now fastened to a round socket; by this arrangement it becomes possible to alter with great rapidity the inclination of the section in every direction, at least within certain limits, more extensive however than most objects require. The great advantage of this arrangement is self-evident. The round socket ought to be kept oiled with good machine oil, and the clamp screw which acts on a lever to fix the round socket must never be screwed too tight. The foot is not made of cast iron, as before, but is a heavy brass plate. It is best to have four knives, two straight and two angular. With these improvements this instrument places even a novice in a position to produce excellent sections in the course of a short time.*

The Movement of Microscopic Particles suspended in Liquids.Professor Stanley Jevons records in the " 'Quarterly Journal of Science' for April, under the above title, the result of the investigations he has made on this subject. He objects to the names "molecular movement," "Brownian movement," or Dujardin's " titubation," and suggests "pedesis," from the Greek andŋois, leaping or bounding. The best possible exhibition of the motion is to be got by grinding up a particle of pumice-stone in an agate mortar, and mixing it with distilled water. The minute angular particles will be seen under the microscope to leap about with an incessant quivering movement, so rapid, that it is impossible to follow the course of a particle. The substance most convenient for experiments, he considers, however, to be fine pure china clay or kaolin, a small quantity of which shaken up with pure water makes a milky liquid, a drop of which will show the motion in great perfection. He considers that he has completely disproved the suggestion that the motion is excited by rays of light or heat falling upon the liquid, or that it is connected with the shape of the particles; and from the observations he made on the length of time during which the motion will continue, he disagrees with the opinion recently expressed by Professor Tyndall that it is due to surface tension. He then proceeds to point out the intimate connection between pedesis and suspension of particles in liquids. In the absence of pedesis, suspended particles attract each other and become aggregated together into little groups, which then acquire sufficient weight to force their way down through

* Dr. H. Reichenbach, Assistant to the Zoological Institute of Leipzig University, in the Archiv für Mikroskopische Anatomie,' xv. 1.

the resisting liquid; pedetic motion prevents the formation of groups, and keeps the minute particles apart so that each encounters the separate resistance of the fluid.

Pure water exhibits pedesis in the highest perfection, even the air and carbonic acid usually dissolved in it producing a perceptible difference. If, however, instead of mixing china clay with pure water, it is mixed with a very dilute solution, say one part in a thousand of sulphuric acid, the pedetic movement is almost entirely destroyed, the same effect being produced by almost any mineral acid, and as a general rule by all salts and other soluble substances. To this general rule, however, there are certain remarkable exceptions, such as pure caustic ammonia (but not its compounds), boracic acid, and silicate of soda-gum arabic even possessing the power of increasing the motion. Comparing the substances which do not prevent the motion with those which do, it becomes apparent that, with some doubtful exceptions, they differ widely in the power of making water a conductor of electricity. Faraday found that some acids, such as the sulphuric, phosphoric, oxalic, and nitric, increase the conducting power of water enormously, whilst others, as the acetic and boracic acids, produce no change; gum and ammonia producing no effect, whilst its carbonate does, and sulphate of soda and many soluble salts producing much effect. The argument in the case of pedesis is, the Professor considers, exactly analogous to that which Faraday employed in his inquiry into the production of electricity by the Armstrong electrical boiler, which he found must be supplied with pure distilled water to yield much electricity. The smallest drop of sulphuric acid or a little crystal of sulphate of soda prevented the evolution of electricity, as also did the addition of any of the saline or other substances which give conducting power to water. As ammonia increases the conducting power of water only in a small degree, Faraday concluded that it would not take away the power of excitement, and accordingly, on introducing some to the pure water, electricity was still evolved, but the addition of sulphuric acid by forming sulphate of ammonia took away all power. The analogy of these circumstances to that of pedesis is so remarkable," the Professor writes, "that little doubt can be entertained that the same explanation applies. It is perfectly pure water which produces electricity and pedesis; almost all soluble substances prevent both one and the other phenomenon, but ammonia is one of the few exceptions—it allows both of electric excitation and pedesis. Boracic acid is another exception, and gum a third. . . In spite of some discrepancies and failures, I still think the analogy between pedesis and Armstrong's electrical machine so strong, as to leave little doubt that pedesis is an electrical phenomenon.' In attempting to explain the exact modus operandi, we can only speculate that the action upon a minute irregular fragment will never be exactly equal all round. In order that a particle shall rest motionless in a non-conducting fluid, it must be in exactly equal chemical and electric relation to the fluid on all sides. That this should happen is almost infinitely improbable, and a condition of unstable equilibrium within limits is the result. The Professor concludes by pointing out that there is probably a close connection between pedesis and the phenomena of osmose. M 2

A Test Object for Histologists.-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 fibrille 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.

On the Rhizopoda of the Salt Lake of Szamosfalva.-Dr. G. Entz has described the Rhizopoda obtained by him from a salt pool at Szamosfalva, near Klausenberg, in Hungary. He procured in all twelve species, five of which, all shelled species, are described as new, and two of them as the types of new genera. These are Pleurophrys helix, Plectrophrys (g. n.) prolifera, Euglypha pusilla, Microcometes tristrypetus, and Orbulinella (g. n.) smaragdea; the other forms noticed are Ciliophrys infusionum, Cienk., Podostoma filigerum, Clap. and Lachm., and five species of Amœba.

The majority of the Rhizopods belong to forms which are very common in fresh water, but which must probably be referred to the category of organisms which occur indifferently in both fresh and salt water; and, so far as this supposition applies to the Amaba, Dr. Entz furnishes a confirmation of it in a subsequent short note, in which he states that he found Amoeba limax and A. radiosa very abundantly in sea-water from Cuxhaven. (He regards the marine forms A. marina, Duj., A. polypodia, F. E. Schulze, and possibly also Prot mœba polypodia, Hack., as probably identical with A. radiosa.)

Of the forms peculiar to the Szamosfalva salt-pool, two (namely, Euglypha pusilla and Microcometes tristrypetus) find their nearest relations in fresh-water organisms. Pleurophrys helix, on the contrary, belongs to a marine type. Of the two new genera, Orbulinella is the most nearly related to the marine perforated Foraminifera, and Plectrophrys is referred to the neighbourhood of Pleurophrys, Plagiophrys and Chlamydophrys, and may be either a marine or a freshwater type. As a negative character bearing on the marine or fresh water affinities of the Rhizopodal fauna of Szamosfalva, the author remarks on the total absence of Arcella and Difflugia, both of which are so abundant in, and characteristic of, fresh water.

*

The Conversazione of the Royal Society on May 1.-The following objects of interest to microscopists were exhibited :-Dr. Woodward's rectangular prism illuminator, to be used with immersion lenses; and Dr. Edmunds' immersion paraboloid (exhibited by Mr. J. Mayall, jun.). A new form of micro-spectroscope, exhibited and made by Mr. Adam Hilger on a plan suggested by Mr. Sorby. Instead of placing the lens used to focus the slit below the prisms, it is placed just above them, and a cylindrical lens is fixed below, in order to correct the astigmatism. The advantage of this arrangement is that the whole apparatus is very greatly reduced in length; in fact, to about half the usual size. The lens used to focus the slit also serves to focus a graduated scale seen by reflection over the spectrum, which enables the observer to measure at once the position *Hungarian Naturhistorische Hefte,' 1877, 3 and 4; Ann. Nat. Hist.,' May, 1878.

of any absorption band. An improved form of micro-spectroscope (designed and exhibited by Mr. F. H. Ward, M.R.C.S.), the improvements being (1) quick movement of the slide carrying the slit, (2) scale for registering the position of the slit, (3) arrangement for comparing three spectra, or for splitting a single spectrum and inserting a second spectrum between the halves, and (4) new form of comparison stage.

The Soirée of the Chemical Society.--At this soirée, given by the President, Dr. Gladstone, F.R.S., at Burlington House, on May 30, there were exhibited :-A new arrangement of polarizing apparatus for the microscope, in which both the polarizing and analyzing prisms can be readily shifted out of the field, thus allowing the object to be viewed by direct illumination. Apparatus for photographing plates of crystals (ordinary-sized microscopic objects enlarged to 3 inches) by means of polarized light (both exhibited by Messrs. Murray and Heath).

A French view of the Binocular Microscope. The difference in the appreciation of the binocular microscope by microscopists in England and America on the one hand, and those of France and Germany on the other-in which latter countries Wenham's prism, if not unknown, is at any rate almost wholly unused-is a phenomenon not easily to be accounted for. The following is the view taken of binocular microscopes by Professor Ranvier, of Paris, the leading histologist of the day, in his book, just published, on 'Practical Histology':-"The binocular microscopes give, it is true, the sensation of relief, but they cannot properly be called stereoscopic. What gives the notion of relief is that our two eyes do not see exactly the same image of an object; it is this principle which has been utilized in the stereoscope, in which a different ima re of an object is placed before each eye so as to produce a single impression. In the binocular microscope, on the contrary, there are not two different images; it is the same image which is presented to each of the eyes of the observer. The sensation of relief is the result of an illusion founded on habit, and consequently this kind of microscope cannot be considered stereoscopic. Moreover, these instruments have the inconvenience of diminishing the clearness of the image, and it is not possible to use them with the higher objectives. The only way of obtaining with the microscope a notion of the relief of objects and of their superposition is by employing the ordinary microscope with objectives of large angle of aperture. As these only allow extremely limited portions of the objects to be seen, we get a complete knowledge of the latter by varying the focus by means of the fine adjustment. The respective situations of two points of the same object and of two different objects will be determined by the impression conveyed by the alteration of the fine adjustment necessary to see them successively. In short, the binocular or stereoscopic microscope, which is a good instrument for demonstration, cannot be employed in histological researches."

The views held by microscopists in this country are, it is needless to say, widely different from the foregoing, and agree with those expressed by Dr. Carpenter in 'The Microscope and its Revelations.'

On p. 59 (fifth edition) he says, "It is easily shown theoretically, that the picture of any projecting object seen through the microscope with only the right-hand half of an objective having an even moderate angle of aperture, must differ sensibly from the picture of the same object received through the left-hand of the same objective; and further, that the difference between such picture must increase with the angle of aperture of the objective."

"The stereoscopic binocular is put to its most advantageous use when applied either to opaque objects of whose solid forms we are desirous of gaining an exact appreciation, or to transparent objects which have such a thickness as to make the accurate distinction between the nearer and their more remote planes a matter of importance" (p. 69).

At page 79 the writer draws attention "to two important advantages" he has found the binocular to possess. "In the first place, the penetrating power or focal depth of the binocular is greatly superior to that of the monocular microscope;" and in the second place, when employed on objects suited to its powers, "the prolonged use of it is attended with very much less fatigue than is that of the monocular microscope."

The Revivification of Diatoms.-Mr. Habirshaw, of New York, states that "in 1871, Captain Mortimer brought from San Francisco in his ship a large bottle of diatoms (from fresh water), intending to study them during the voyage. When he arrived in England they were still alive, but afterwards dried up and remained in that state in his cabin until the summer of 1877-a period of six years. Having found the old bottle, which we knew very well, we refilled it with water, and on examining it several days later we found some living specimens in it. At first this phenomenon inspired us with some doubts, but after a subsequent examination we came to the conclusion that these diatoms really were alive. The vessel has gone to sea again, and we await its return to verify anew the facts which we observed."

A Method of Staining Rapidly.—I have long known that carmine staining acts quicker when the watch-glass containing the carmine and the section was not covered. But there is the disadvantage in this method, that particles of dust settle on the surface of the fluid, and are apt to adhere to the section on its being taken out. I recently tried warming the fluid, in order to overcome this drawback, and also because I thought that the more rapid evaporation of the carmine solution accelerated the colour being taken up, and I arrived by this means at surprisingly favourable results.

After the first attempt had proved successful, I modified the procedure as follows:-Over a water-bath (only partly filled) with a large opening, a wire netting is placed, upon which, as soon as the water begins to boil, the section is put in a watch-glass containing the carmine solution, and exposed to the action of the steam. In the course of from two to five minutes the sections are completely stained. They are then washed twice in distilled water, placed for a few minutes in common alcohol, and for the same time in absolute alcohol,

*Journal de Micrographie.'