wide ("plus 180° ") angle of aperture, and in support of his view refers to an experience in which a friend with an English low-angled glass (50° to 70°) showed "Nasmyth's membrane" in a section of human tooth, which the glass of higher aperture proved to be an optical illusion only. "We often hear the remark," says the Professor, "that wide-angled glasses are just the thing for the display of lined objects, surface markings, diatoms, &c., but that, owing to their short focal length and limited working distance, the trouble attending the adjustment of collar, and in general the difficulties pertaining to their use, they are unsuited to the purposes of the histologist; while, on the contrary, low-angled glasses of greater working distance, requiring no skill in management, are the tools with which the real work of the microscope has been and will continue to be done; and such are fondly termed good, honest, and reliable 'working glasses.' I can never listen to this line of argument without entertaining the suspicion that sloth and inactivity lie at the bottom. We never hear astronomers complain of the care they are compelled to use in instrumentation; on the contrary, they pride themselves on the accomplishment of being able to work instruments requiring a great amount of skill and precision in manipulation. The objectives of short working distance originated years ago, when German pathologists were in the habit of using common windowglass to cover their mounts, and at that time the extremely thin glass now so easily procurable was unknown.

"A vast amount of work has been done with these 'honest and reliable working glasses,' and will have to be done over again; and this revising work is now in progress."

The Professor also refers to a case in which a sample of urine, from a child supposed to be suffering from a disease of the kidneys, appeared, when examined with a low power (}, of 70° aperture), to be in every respect healthy, "but on further examination with a wideangled glass, and with an amplification of nearly 4000 diameters, it was found to be literally swarming with vibriones."

New Process of Colouring Microscopic Preparations with a Picroaniline Solution.*-Whilst engaged on the normal structure of the general lymphatic system, I had occasion to communicate to my colleagues of the Société medico-physique of Florence the good results which I had obtained in my investigations (better than by any other colouring matter) by a solution of aniline blue and of picric acid. I was unable at that time to enter much into details concerning this new and most simple method of staining, having only experimented upon a single tissue, that of the lymphatic system. Now, however, after having, together with Dr. Brigidi, made a large number of observations upon almost all the different tissues, I can speak about the method with more detail, and recommend it to those who are occupied with histological studies.

The two substances which I recommend for producing a very beautiful green colour have been already employed for some time in

*Dr. A. Tafani, in the Journal de Micrographic.'

histology to colour tissues, both normal and pathological. Yet I do not find that anyone, until we did so, made use of these two reagents simultaneously, so as to obtain a composite colour different from that which each produced separately. Everyone knows, indeed, how soluble aniline blue is used as a matter of preference, and to save time, to colour elements and tissues which have been previously submitted to the hardening action of alcohol and chromic acid, although more time is required for those which have been submitted to the action of the latter reagent.

It is also known that certain tissues, such as those of the spleen and the lymphatics, and the cerebral and spinal nervous tissues, retain their colour better and with more elegance when aniline blue is used; that the preparations thus coloured do not lose the tint which they have acquired by the addition of the acids, whilst alkaline solutions and even glycerine affect them in time. A colouring matter possessed of such advantages is, however, but little employed compared with others, such as carmine, hematoxyline, &c.; and I believe the reason to be that preparations coloured by means of aniline blue, although very elegant, do not show all their details so well differentiated and so plainly as can be done with other processes, for example, with picro-carminate. Blue staining, in general, but particularly that produced by aniline blue, will not allow histological forms to be defined in all their details. I might almost say that the contours fail to be recognized, which prevents our distinguishing in a tissue rich in cellules the limits of the various elements.

It is besides well known that picric acid (in a saturated solution) colours the morphological elements and not the amorphous substances (Robin). It follows from this that the tissues which have been submitted to its action take a beautiful yellow sulphur tint, and do not in any way lose the distinctness of their outlines. This is owing to the fact that picric acid is a reagent which does not precipitate in a granular form the substances forming the tissues or elements on which it is made to act, whilst the contours of the nuclei, the nucleoli, the granulation, and the cell-walls do not disappear. Moreover, the action of picric acid is not like that of chromic acid, which enters into combination with the substances upon which it reacts (Ranvier); and it also constantly happens that coloured preparations, after being hardened by the latter acid, are completely deprived of their colour by repeated washings with water. The action of picric acid on tissues is therefore much less detrimental than that of chromic acid.

Whilst then the colouring properties of aniline and picric acid, when they act separately, are sufficiently well known, no one has until now (at least so far as I know) employed these two substances at the same time and on the same tissue, so as to obtain a different tint by their reciprocally modified action, and giving rise to some important peculiarities, especially in certain special tissues.

The idea of using picric acid in combination with another substance to obtain a third, unlike it, and whilst partly possessing the properties of the component substances, also some other new properties resulting from the mixture, is certainly not new, if we refer

to Schwarz, who advised the mixing of picric acid with carmine in studying the unstriated muscles of the intestines, or to the picro-carminate of Ranvier, now so well known. I may add also that it has been suggested that a green stain might be obtained with picric acid dissolved in glycerine by the addition of a certain quantity of a decoction of logwood and the neutral chromate of potash, in the proportion of 1 part to 1000.

I avail myself, however, of picric acid mixed with soluble aniline blue to obtain a green tint of considerable delicacy, homogeneous as far as the eye can detect, and which serves to bring out in relief the smallest details which are presented by the tissues and their elements. This green stain is easily obtained, in a comparatively short time, either by subjecting the preparation to be stained to a solution in water of soluble aniline and picric acid, or first to a solution of aniline and then to another of picric acid. In whichever way these colouring matters are employed, an effect is obtained equally quick and satisfactory. The solutions, whether of picric acid or aniline, ought to be saturated, which can be done without difficulty by leaving an excess of each substance at the bottom of the vessels in which the materials are placed to dissolve. In this way we are always sure of employing only saturated solutions. When it is wished to make use of the picroaniline solution, 100 cubic centimetres, for example, should be taken of the saturated aqueous solution of picric acid, and into it should be poured 4 or 5 cubic centim. of the blue liquid also saturated. The resulting solution admirably stains a preparation of the lymphatic glandular system in the space of a few minutes. If it is desired to employ the two substances separately, keep the preparation in the aniline solution for a few minutes, and afterwards place it in the picric acid. In working thus we can see that the preparation is not stained too much by the aniline, and to this end it is well to take it out as soon as it has acquired a light sky-blue tint. By taking it out at this point one is always sure that it will show the nuclear elements sufficiently coloured, whilst the protoplasmic parts and others will be only very slightly stained. By waiting, on the contrary, till the preparation has taken a dark blue tint, the nuclei will be very deeply stained and the other parts rather deeply also, so much so that on submitting it afterwards to the staining action of the picric acid it becomes confused and obscure. The preparations which have been treated with the aniline solution, with the precautions indicated above, and then placed in picric acid, pass in the course of about fifteen minutes from sky blue to a delicate green. After this treatment the tissues show the nuclei both free and cellular sufficiently stained with green, the protoplasm and the fibres coloured pea-green, though faintly and with a delicate shade. Since the staining by the aniline produced by the first treatment was less in these parts, the yellow colour predominates over the blue, whence there results a lighter and more delicate tint.

Similar results are obtained by making a picro-aniline solution act on the different tissues. It is possible to stain with great advantage not only fresh tissues, but also such as have been subjected to the action

of different hardening reagents, such as alcohol, chromic acid, bichromate of potash, &c.

Microscopical preparations obtained by these processes may be preserved like others in fluids or balsam. It should be observed, however, that picric acid, being, as I have said, soluble in water and in alcohol, might easily be removed from the preparations upon which it has been made to act. To prevent this inconvenience it is important that the glycerine used to preserve the preparations should be slightly tinged with picric acid, and if balsam is used it is necessary to dehydrate the preparations in alcohol containing also a small quantity of the same acid in solution. In the latter case, after this treatment, the preparation may at once be placed in oil of cloves or turpentine without fear of the staining suffering from it. I would likewise observe here that if it is intended from the first to mount the preparation in balsam, the operation may be abridged by transferring the preparation immediately from the solution of aniline blue, in which it has acquired the tint I have indicated above, to a bath of alcohol to dehydrate it, the alcohol containing per cent. of picric acid in solution.

With the picro-aniline solution not only may different tissues be stained according to the ordinary process, i. e. by plunging the preparation into it, but interstitial injections may be made with it, and small artificial oedemata produced with Pravaz's syringe. By operating thus, for example, on a lymphatic gland, the colouring matter can be made to penetrate into the cavernous system where the endothelial cellules may be recognized lightly coloured with green, the characteristics of which, already well described by Professor Bizzorero, are seen with more clearness than with any other reagent. If a small oedema be produced under the skin of the groin in a rabbit or guineapig, the connective cellules and the fibres between which they are situated may be studied to perfection, as Renaut has done, by means of eosine, which is soluble in water. The picro-aniline solution, finally, may be very well employed in interstitial injections intended to show the relation between the cellule connectives, especially when the picric acid instead of being dissolved in water is dissolved in one-third part of alcohol.

The preparations thus obtained are not affected by the weak acids, acetic phenic, diluted chlorohydric &c., whilst alkaline solutions rapidly destroy their beautiful outline. The preparations, mounted in fluids or dry, with the precautions mentioned, preserve their stain for a very long time, for I have some which have undergone no alteration for more than a year.

The picro-aniline solution (which is specially to be recommended for the study of the lymphatic glandular system and for the retina) does also very well for other normal or pathological tissues. Thus have preserved some complete sections of the medulla oblongata stained by this method, which for clearness and elegance leave nothing to be envied in those produced by carmine.

Amplifiers.-In an article on the " Amplifier," by Dr. Devron, in the 'American Journal of Microscopy,' he says:- "The Tolles' amplifier used on a large compound microscope, with lenses of various makers, of

ro, e, and, caused resolutions on the Nobert 19th band-plate, which without it at the same time and with the same illumination could not be seen with the same objectives; with that increased resolution the loss of light was not appreciable. Its value was made quite apparent by two photographs showing the resolution of Amphipleura pellucida, one by a and the other by a objective, the latter with the amplifier. The resolution into transverse striæ in both photographs and their appearance is so similar, that were they not numbered I could not tell which was obtained by the objective."

Penetration. The following note from 'Le Microscope,' by Dr. J. Pelletan, p. 55, states a fact that is generally unknown, but which ought to be known to everyone who works with the microscope: "From a purely theoretical point of view an objective with penetration is in reality a defective objective." My experience is that there is not one in a thousand of the users of the instrument—especially the histologists-but have the idea, and their opinions are governed by that idea, that an objective that will show at one sight (or focussing) all the strata or planes in a section of a tissue or 10 of an inch thick, or, in other words, one that has great penetrating power (better defined by the optician's term, "depth of focus "), must be the best and most important objective that can be made. Whereas it can be demonstrated that such depth of focus is optically incompatible with the best definition, which is the quality above all others desirable in a lens.

Depth of focus is a quality desirable in certain cases and for some purposes; the objectives for such use should be made expressly for that property, and the purchasers of such ought not to expect them to be capable of the higher histological work of modern microscopy.

There is, of course, a wide difference in definition of objectives that possess depth of focus. On the other hand, all objectives that are without that quality do not have the maximum defining power.*

The Genus Ligula.-M. Donnadieu holds that all the species described by authors as forming this genus are only the different phases of development of the same species, or the same parasite found in different animals, the so-called genus being simply a species of the genus Dibothrium—the Dibothrium ligula.

Nutriment of Bacteria.-MM. Dupont and Hoogewerff, of Rotterdam, have investigated the chemical constitution of the materials that nourish bacteria. Test tubes, like those used by Cohn, were filled with 20 c.c.m. of the nutrient fluid, and two drops of bacterium fluid, made with decomposing beans or peas, were added; the tubes having been first deprived of any atmospheric dust by hot water. Care was taken that the distilled water employed contained no organisms. Following Mayer, they prepared a normal nutrient fluid with 1 per cent. acetate of ammonia, 0.5 per cent. phosphate of potash, 0.5 per cent. sulphate of magnesia, and 0.05 per cent. phosphate of lime. As Mayer stated, they found the most important ingredient to *Carl Reddots, in American Journal of Microscopy.'