both of phenacite and of beryl is more closely allied to that of chrysoberyl and corundum than the received theory of their chemical constitution would indicate.

We would not convey the impression that in all these crystals the appearances we have described are strongly marked, or that they have passed wholly unnoticed hitherto. Every one who has become familiar with the optical properties of crystals must have noticed that, with many always regarded as uniaxial, there is not unfrequently in some positions a small separation of the cross into the hyperbolas which are characteristic of biaxial structure. But these irregularities, although long known, have never been satisfactorily explained. They have been hitherto residual features not accounted for by the received theory of crystalline structure, which explains so satisfactorily the general order of the phenomena observed with the polariscope. We have endeavoured in this paper to trace their true significance :-first by showing that the appearances we are discussing are precisely similar to the effects which can be obtained by known means with mica plates; and secondly by observing, on different specimens of various minerals, every intermediate stage between the unmistakable effects of twinning on plates of mica or vermiculite, and the delicate phases of the same phenomena seen with sections of crystals of tourmaline, corundum, or phenacite. One other illustration of our theory.

The rhombic angle of Witherite (native baric carbonate) is 118° 30'; and the all but universal hexagonal macling of crystals of this species is a well-known fact*. The rhombic angle of Aragonite (the corresponding form of calcic carbonate) is 116° 10′; and the much greater divergence of this angle from 120° determines, as is also known, a style of macling which is usually quite different from that of Witherite. In the isomeric calcite, however, we have the type of all hexagonal forms. Hitherto the crystalline forms of calcite and Aragonite have been regarded as being as widely separated as possible; and a comparison of these two well-known mineral species has furnished one of the most striking illustrations of dimorphism. But may not, after all, the comparatively small physical differences between these two minerals correspond to a crystallographic difference no greater, fundamentally, than the difference between the rhomb of 116° 10' and the rhomb of 120° ?

The macles of chrysoberyl and Witherite are illustrations of a general truth, fully recognized in mineralogy, that all rhombic crystals whose angles approach 120° tend to form hexagonal macles. The optical phenomena described in this paper certainly suggest the theory that a perfect hexagonal form and structure * See figures, Dana's 'System of Mineralogy,' p. 697.

may be the result of a more fundamental and molecular macling, which results when the angle is exactly 120°.

Observations of Sénarmont.-The only previous observations which we have been able to find bearing on the subject of this paper are those of the late eminent mineralogist, H. de Sénarmont, of Paris. In a well-known paper (Ann. de Chim. et de Phys. 3rd ser. vol. xxxiii. p. 391), Sénarmont showed that salts which were both geometrically and chemically isomorphous might have very different optical relations-for example, that, while the biaxial crystals of two such salts might have the same bisectrix, the plane of the optical axes in the crystals of one might be perpendicular to the corresponding plane in those of the other. He further proved, by crystallizing together two salts so related, that in the crystals of the isomorphous mixtures thus obtained the optical angle varied with the varying proportions of the constituents between the extreme conditions in the crystals of either salt; and by trial he succeeded in forming from two biaxial salts crystals which, in monochromatic light at least, appeared uniaxial. In a later paper (Ann. de Chim. et de Phys. 3rd ser. vol. xxxiv. p. 171) Sénarmont applied the principles which he had thus experimentally verified, to explain the variation of the optical angle of the micas. In this paper he seeks to prove, first, that all micas may be referred to a right rhombic prism with angles of approximately 60° and 120°; secondly, that while in some micas the plane of the optical axes is parallel to the shorter diagonal of the rhomb, as in fig. 13, in others it is parallel to the longer diagonal, as in fig. 14. Interpreting these facts by the results of his experiments on isomorphous salts, he draws the inference that there are, crystallographically at least, but two species of mica which are geometrically isomorphous but optically distinct, that these are represented by the varieties which have the widest optical angle between the axes in either plane, and that all other varieties with optical angles varying from 0° to 70° in either direction are isomorphous mixtures of the two optically distinct conditions of the mineral.

The observations described in this paper, although they prove that another cause may also determine the variation of optical angle in micaceous minerals, do not necessarily invalidate this beautiful theory of Sénarmont. The variations observed with other minerals, not only on different specimens, but with the same specimen at different temperatures, and which are so beautifully seen with the orthoclase from Wehr in the Eifel, and with crystals of selenite, indicate that such variations may be determined by conditions of molecular structure wholly independent of the macling here described. We have shown that the macling

does produce the variation in certain cases; and it must remain for future investigation to assign the limits of the influence which this cause may exert. We would only remark in conclusion that, although in the fifty-seven varieties of mica examined by Senarmont he did not note a single instance in which the position of the plane of the optical axes (with reference to the diagonal of the rhomb) was different on different parts of the same specimen, or even on different specimens from the same locality, he does describe and figure several remarkable macles of Muscovite mica similar to those of vermiculite described above and represented by fig. 10. In one instance the plane of the optical axes is parallel to the shorter, and in the other to the longer diagonal of the rhombic prism; but in both cases it has the same relative position in the several individuals of the macles, the three planes forming with each other angles of 60°. On the plate of another macle a difference of optical angle was observed on different portions of the plate; and this effect was probably similar to that we have been studying in this paper. On another plate from the same crystals, he observed a superposition of the lamina of.the different individuals of the macle; and the following language by which this phenomenon is described, and which is the only reference made to it, shows how closely he came to the results recorded in this paper :

"Une partie seulement du cristal maclé est commune aux trois lames; mais ici certaines plages (pointillées sur la figure) ont un caractère optique tout particulier; elles ne cessent jamais de développer des couleurs dans la lumière polarisée, quelle que soit d'ailleurs l'orientation de la lame cristallisée. Il est évident que dans ces plages il y a superposition de lames appartenant à des cristaux orientés, les uns comme le petit, les autres comme le grand cristal, de façon que quand les unes ont leurs sections principales dans le plan de polarisation, les sections principales des autres font avec ce plan des angles de 60 degrés.

"Cet exemple prouve donc, non-seulement que les micas peuvent se grouper latéralement sans que leurs clivages cessent d'être parallèles, mais que des feuillets superposés peuvent même appartenir à des cristaux dont l'orientation diffère de 60 degrés. Un pareil mode de groupement, qui ne trouble ni la régularité de la cristallisation, ni la transparence, semble indiquer que l'arrangement moléculaire de ces prismes rhomboïdaux diffère très-peu de celui qui conviendrait au prisme hexagonal régulier."

XXXII. On the Sensibility to Light of Bromide of Silver with respect to the so-called Chemically Inert Colours. By HERMANN VOGEL*.

WITH regard to certain colours (red, yellow, and green) the

photographic action, as we are aware, is either very limited or altogether wanting. This circumstance not only throws difficulties in the way of copying coloured objects (oil paintings), but also with regard to taking portraits, in which coloured clothes and likewise a yellow tint of the complexion, light hair, or rosy cheeks are reproduced in abnormal conditions. What is of a light colour, when of a yellowish tint, comes out bright; and this drawback is only to be got over to a certain extent by a subsequent retouching of the negative.

This abnormal insensibility to colour, as regards photographic plates, is pronounced in the most marked manner with respect to the colours of the spectrum, wherein beyond the violet a powerful action is manifested, which in the visible spectrum (according to the researches hitherto made) does not extend deeper than to the line E in the green. (See Dr. Schultz-Sellack, 'Reports of the German Chemical Society,' 1871, p. 211). Researches recently instituted by me by means of bromide of silver, however, have shown that the sensibility of this preparation not only embraces a considerably wider extent of the spectrum, but even that, by the employment of certain accessories, its sensibility may be carried as far as into the red, or, in other words, to regions where hitherto for photography dark night reigned.

I received recently from England some dry bromide-of-silver plates which Wortley prepares in that country for sale, by a process which is only in part divulged. On exposing these plates to the spectrum, I found to my astonishment that they proved to be more sensitive in the green (that is to say, near the line E) than in the light blue (that is to say, near the line F). Here was an instance of sensibility not in accordance with our previous experience-namely, that a colour the chemical action of which was held to be weak, proved more energetic than that of one which was looked upon as powerful. This circumstance induced me to enter at once upon a more intimate examination of the behaviour of bromide of silver with regard to the colours of the spectrum.

I formed my spectrum by the use of a photographic camera and a Steinheil lens, which I adapted to the battery of prisms of a direct-vision spectroscope. The width of the slit was 0.25

* From Poggendorff's Annalen, vol. cl. p. 453. Kindly communicated by W. G. Lettsom.

Phil. Mag. S. 4. Vol. 47. No. 312. April 1874.

T

millim. The solar rays were thrown on to it by means of a Foucault's heliostat, for the use of which I am indebted to the great kindness of my friend Dr. Zenker. The length of the spectrum from D to G was 35 millims.

When desirous of comparing my experiments, I chose the time from 11 till 2, and then operated only when there was a cloudless sky, which it is true is but seldom the case at this period of the year. The time of exposure lasted generally ten minutes. The plates were developed with protosulphate of iron. Even in my earliest experiments I found that the sensibility of the bromide of silver extends considerably further than is stated by Dr. Schultz-Sellack, who obtained therewith an action only from the ultra-violet as far as nearly to the line F in the blue. In my experiments the sensibility extended in all cases beyond the line F, more or less far beyond according to the transparency of the atmosphere. Upon this latter point I purpose entering more at length in a separate communication.

I made a trial of the bromide of silver in two forms :-first, as a so-called wet plate, that is to say, when moist from an adhering solution of nitrate of silver derived from the silver-bath wherein the plates were sensitized; secondly, as a dry plate, obtained by washing off the solution of silver and then drying the plate. (For further details see Reports of the German Chemical Society,' 1873, p. 89.) The behaviour of the two classes of plates was different.

The result was that dry bromide of silver exhibits a more extended sensibility for colours than does the bromide which lies beneath a silver solution; when an acid development was employed, the latter manifested sensibility up to the middle point between D and E-that is to say, nearly up to the yellow; the former, however, exhibited sensibility 2 millims. beyond the line D, or up into the orange.

The action of the two plates was moreover very different in a qualitative sense. With moist bromide of silver an extremely energetic action is seen between G and F (in the indigo and blue); near F, however, it declines suddenly, and nothing but a faint indication could be traced as far as the other side of E. Dry plates, on the other hand, exhibited a far less decided action in the blue than the wet plates did; this action, however, declined only gradually, and it extended, as has been remarked above, as far as beyond D.

Hence dry bromide of silver is more sensitive for the rays of lesser refrangibility in the visible solar spectrum; moist bromide of silver is most sensitive for the blue rays of greater refrangibility.

For ordinary photographic plates a solution of silver acts as a