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paper concludes with some notes by Professor Maskelyne, on the connexion between molecular phosphorescence and crystalline structure.

The crystals experimented on have been the diamond, emerald, beryl, sapphire, ruby, quartz, phenakite, tinstone, hyacinth (zircon), tourmaline, andalusite, enstatite, minerals of the augite class, apatite, topaz, chrysoberyl, peridot, garnet, and boracite. Of these, the only crystals which give out light are diamond, ruby, emerald, sapphire, tinstone, and hyacinth. The light from emerald is crimson, and is polarised, apparently completely, in a plane perpendicular to the axis. Sapphire gives out a bluish-grey and a red light polarised in a plane perpendicular to the axis. The ruby light exhibits no marked distinction in the plane of its polarisation,

Among positive crystals tinstone glows with a fine yellow light, polarised in a plane parallel to the axis of the crystal. So far the experiments accord with the quicker vibrations being those called into play, and therefore in a negative crystal the extraordinary, and in a positive crystal the ordinary, is the ray evoked. Hyacinth, however, introduces a new phenomenon, being dichroic, the colours, in three different crystals, being pale pink and lavender-blue, pale blue and deep violet, and yellow and deep violet-blue, polarised in opposite planes.

The only conclusion arrived at is, that the rays, whose direction of vibration corresponds to the direction of maximum optical elasticity in the crystal, are always originated where any light is given ont. As yet, however, the induction on which so remarkable a principle is suggested, cannot be considered sufficiently extended to justify that principle being accepted as other than probable.

VI. “Note on a Direct Vision Spectroscope after Thollon's

Plan, adapted to Laboratory use, and capable of giving exact Measurements." By G. D. LIVEING, M.A., Professor of Chemistry, and J. DEWAR, M.A., F.R.S., Jacksonian Pro

fessor, University of Cambridge. Received April 3, 1879. Having seen in the “Journal de Physique" for May, 1878, the account of M. Thollon's ingenious direct vision spectroscope, it occurred to us that by a little modification we could adapt his plan so as to produce an instrument well fitted for the work in which we were engaged, combining the advantage of excellent definition, which his plan secures, with the means of getting exact measurements with the least possible chance of errors of adjustment or inequalities in the working of the automatic system. The principle consists in having two prisms only (half prisms as M. Thollon calls them), of which one is fixed, and receives the light from the collimator by a reflecting

prism and transmits it in a plane at right angles to the axis of the collimator to the second prism.

This second prisim is moveable about an axis parallel to its edge and to the axis of the telescope, and has a right angled reflecting prism attached to it, so that the light after traversing this prism twice passes the second time through the fixed prism and so by reflection into the telescope. The lever carrying the second prism with its reflecting prism is moved by a micrometer screw, by the head of which the movement of the prism is read.

We placed the design in the hands of Mr. Hilger, some time since, and we now exhibit the instrument to the Society.

In the last number of the “Journal de Physique,” M. Thollon describes some modification of his instrument, but it does not seem that his modified plan is so well adapted to the ordinary use of a chemical laboratory as ours.

The accompanying diagram represents a section through the prisms at right angles to the axis of the collimator and telescope.

April 24, 1879.

THE PRESIDENT in the Chair.

The Presents received were laid on the table and thanks ordered for them.

The Right Hon. Richard Assheton Cross, Secretary of State for the Home Department, was admitted into the Society.

The following Papers were read :

1. “On the Nature of the Fur on the Tongue." By HENRY

TRENTHAM BUTLIN, F.R.C.S. Communicated by J. BURDON
SANDERSON, F.R.S., Professor of Physiology in University
College, London. Received March 26, 1879.

[PLATES 10—13.]

The fur on the tongue is generally stated to consist chiefly of epithelial cells, usually sodden and granular. But several observers have described fungi as existing in it, or in the buccal mucus. Robin, for instance, describes a form of Leptothrix (L. Buccalis) in the mouth, and particularly in and between the teeth. Kölliker mentions, as of constant occurrence, masses or dark-brown bodies (which had previously been described by Miquel and Neidhardt, as occasionally present) having a granular aspect, which he believed to be of the nature of a fungus, similar if not identical with the fungus affecting the teeth. Billroth speaks of finding in the white fur of himself and of several patients, exquisite palmelloidal forms of Ascococcus and Glococcus colonies.

The object of this paper is to show that schizomycetes form the essential constituent of the far, and to explain, as far as possible, some of the laws which govern the formation of fur.

The tongue is kept clean by free movement and by being rubbed against the interior of the mouth, the gums, and the teeth ; but fur almost always exists upon its surface, both in health and in disease. The fur is generally thickest in the morning before food is taken, and during illness, when the necessary cleansing is not properly performed. It occurs, too, most abundantly in the centre and back part of the tongue, covering a triangular area immediately in front of the circumvallate papillæ, for this part of the tongue is most difficult to keep clean. It occupies the papillary surface of the tongue, scarcely ever extending beyond it, and is, therefore, not found posterior to the circumvallate papillæ. It does not form a continuous layer unless it is exceedingly thick, but lies upon the tops of the filiform and some of the fungiform papillæ. In children the fungiform papillæ are usually quite free from fur, but in adults the difference between the fungiform and filiform papillæ is not nearly so well marked, and, with the exception of those situated near the apex of the tongue, the fungiform papillæ are frequently coated. Far forms upon the filiform papillæ, because these papillæ are rough and possessed of longer or shorter epithelial processes, to which foreign matters cling readily, and from which it is very difficult to dislodge them. The fungiform papillæ, on the contrary, are usually smooth and rounded on the summit, and even when large are easily kept clean.

The accompanying tables refer to the constancy of the presence of fur, to its thickness in health, and to its relation to the papillæ.

Analysis of Cases examined. On 68 healthy tongues—fur on all except one.

On 178 tongues of persons suffering from disease or accident-fur on all except two.

Table showing relation of fur to papillæ on 62 healthy tongues, with remarks on the

age
of the
persons

and the characters of the papillæ.

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Table of thickness of the fur on 68 healthy tongues, with remarks

on the papillæ and the nature of the tongue.

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When thin the fur can only be scraped off with difficulty, and always brings with it numeroas fragments of the hair-like processes which form the terminations of the filiform papillæ. But, when thicker, soft, and moist, it can be removed in considerable quantity with ease.

Microscopical examination of the results of such scraping gives, in nearly every instance, the same results.

1. Débris of food and bubbles of mucus and saliva. 2. Epithelium.

3. Masses which appear at first to consist of granular matter, but which are the glæa of certain forms of schizomycetes. When large and closely packed they are of yellow or yellowish-brown colour, but when smaller and more loosely held together are almost colourless. They are generally attached to portions of the hair-like processes which have come away with them, on account of the tenacity with which they adhere to the processes. Vertical sections of hardened tongues show the relation of these masses to the filiform papilla better than mere scrapings of the surface of the tongue. The filiform papillæ, instead of exhibiting fine, clean, tapering processes, terminate in processes which are uneven, tuberculated, or beaded, and blunted at their ends, owing to the presence of these bodies. Around the masses float free fungi, often exhibiting very active movement. The relative proportion of the three constituents of fur varies under certain conditions. The quantity of débris of food and bubbles is much greater during or immediately after eating than during fasting, although there is no corresponding increase of the fur at such times. The epithelium is much more abundant in thin fur than in thick fur, its quantity depending rather upon the vigour with which the tongue is scraped than upon the amount of fur present. It can be obtained in just as great quantity where no fur is present, provided the tongue be closely scraped. The schizomycetes are found in every case in which there is for upon the surface of the tongue, and I have even found a little of the glæa where no fur was perceptible to the naked eye. The quantity of gloea depends roughly upon the quantity of fur. The position of the gloa corresponds with the position of the fur. The fur dots the tops of the filiform papillæ, and the glæa is attached to the processes of these papillæ. Fresh scrapings of fur show this relation of the glæa to the filiform papillæ, but vertical sections of hardened tongues show more than this. They show that the filiform papillæ are the sole seat of the glæa, which does not exist between the papillæ, and seldom upon the fungiform papillæ. Again, the colour and appearance of the thin grej

fur

corresponds with the colour and appearance of the thin grey pellicle which forms upon the surface of Bacterium-producing fluids, and as the latter becomes whiter and more opaque as it becomes thicker, so does the fur become wbiter and more opaque with increased thickness. A modification of colour is, however, frequently produced by the yellow or brownishyellow tint of the gloa.

In order to ascertain the true nature of the glæa, and to obtain it in a much purer form than that in which it exists naturally upon the

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