FIG. 17.

B

D

angles to the horizon, and making a right angle with each other, the horizontal vibrations of the cord A B, will give rise to two other vibrations, parallel respectively to E F and GH. And if we assume the two branches B C and BD to be unequally tense, the waves produced by the vibration of A B will be propagated along them with unequal velocity. So that this illustration, which I have adopted from Sir John Herschel's able treatise on light, serves to explain not only how a vibration in one plane may be resolved into vibrations in two other planes, but also why the two resulting waves are propagated with unequal velocity.

Though a thin plate of Iceland-spar or of any other doubly refracting crystal serves, when placed in the polariscope, for the production of colour; yet certain crystals are preferable to others on account of the facility with which they may be split into thin laminæ. Selenite and mica are especially convenient for this purpose; and the former of these is extensively employed by opticians in the preparation of a variety of beautiful and ingenious polariscope illustrations. On this account a brief notice of it is

requisite.

Selenite, or sparry-gypsum, is the native crystallized hydrated sulphate of lime (Ca O. SO,+2 Aq.) It occurs imbedded in London clay. It is found also at Shotover Hill, near Oxford, where the labourers call it quarry glass, and likewise at the Isle of Sheppy. Very large crystals of it are found at Montmartre, near Paris. The crystalline forms in which it occurs belong to the oblique rectangular prismatic system. Haüy and the late Mr. William Phillips describe its primary form as a right obliqueangled prism; so that the lateral faces of the crystal are regarded by them as the terminal planes. But the optical characters of the crystal prove the incorrectness of the description of these celebrated mineralogists: and here, I would observe, is an excellent illustration of the great value of polarized light to the crystallographer. In this particular instance it enables him to distinguish a lateral face, from a terminal plane, of a prism.

The crystals of selenite which are most frequently met with, are oblique rectangular prisms, with ten rhomboidal faces, of which two are considerably larger than the others (fig. 18 A). They are very easily slit into thin laminæ (a b, fig. 18 B), parallel

to these larger lateral faces (terminal planes of Haüy and W. Phillips).

Macles or hemitrope crystals of selenite are very common. By hemitrope, a word derived from the Greek (from μ half, and rpéro I turn), is meant a figure produced by cutting the primary crystal in two, causing one of the fragments to make half a revolution, and then uniting the sides actually in contact. The most singular and common hemitrope variety of selenite is that called arrow-headed selenite (fig. 18 C), and which is so called because the crystal is formed like the barbed head of an arrow. Its nature may be easily explained. Cut a card or thin board in a rhomboidal form to represent one of the lamina taken from lateral faces of the prism (fig. 18 D). Then divide

it in the direction of its greater diagonal (a b), and transpose the separated parts in such a manner, that two of the alternate angles, produced by the diagonal division, shall make the point -the other two, the barbs-of the arrow-head.

The optical structure of films or thin plates of selenite, having a thickness of from th to the th of an inch, is very curious. In two rectangular directions they allow perpendical rays of polarized light to traverse them unchanged: these directions are called the neutral axes. In two other directions, however, which form respectively angles of 45° with the neutral axes, these films have the property of double refraction. These directions are usually denominated depolarizing axes; but they might be more correctly termed doubly refracting axes.

In order to render these properties more intelligible, suppose the structure of the film to be that represented by fig. 19, in which the film is seen to be crossed by two series of light lines, or passages, the one perpendicular to the other. These are to represent the neutral axes. We may imagine, that in these directions only can the ethereal molecules vibrate. A ray of incident polarized

FIG. 19. Imaginary Structure of a Plate of Selenite.

light whose vibrations coincide with either of these lines, is transmitted through the film unchanged. But a ray of incident polarized light whose vibrations form an angle of 45° with these lines, or, in other words, which coincide with the diagonals of the square spaces, suffers double refraction; that is, it is resolved into two vibrations, one parallel with a b, the other parallel with cd, and, therefore, the directions of the diagonals of the squares are called the doubly refracting or depolarizing axes. But the two resulting vibrations are not propagated, in these two rectangular directions, with equal velocity, the one suffering greater retardation than the other, so that the waves, at their emergence, are in different phases of vibration, though they do not interfere so as to produce colour, because their planes of vibration are rectangular. By the analyzer, however, their planes are made to coincide, and colour is produced; and on rotating the analyzer on its axis, the colour changes and becomes complementary.

To illustrate these statements, place a film of selenite, of uniform thickness, in the polariscope. On rotating the film (the analyzer and polarizer remaining still), a brilliant colour is perceived at every quadrant of a circle, but in intermediate positions it vanishes altogether. We observe, however, that the tint does not change, but only varies in intensity. If, now, the film be fixed and the analyzer rotated, we also observe colour at every quadrant of a revolution; but the tint changes and becomes complementary at every quadrant-the same tint reappearing at every half revolution: so that when the film alone is revolved one colour only is

seen, but when the analyzer alone is revolved, two colours are

If we employ, as the analyzer, a double refracting prism, we observe two complementary disks of colour, and these may be made to cross and produce white light as before shown.

The thickness of the film of selenite determines the particular tint. If, therefore, we use a film of irregular thickness, different colours are presented by different thicknesses. A wedge-shaped piece will produce parallel bands of colours, and two wedges crossed present diagonal bands. A plano-concave film, as well as a plano-convex film, gives concentric rings of colour, the former with a black, the latter with a white, centre.

Two films superposed, do not give the colour which would arise from the mixture of the two colours, but either the colour which corresponds to the joint thickness of the films, or that which belongs to the difference of their thickness. When the two films are put together, as they lie in the crystal, the resulting colour depends on the sum of the thicknesses. But when the two films are crossed, so that similar lines in the one are at right angles to similar lines in the other, the resulting tint depends on the difference of the thicknesses. These facts admit of very beautiful, curious, and interesting illustrations. Thin films of selenite of uniform thickness are so arranged as to slide over figures also formed of films of selenite. The changes of colour effected in the tints are most striking, and to unphilosophical minds almost magical.

In the opticians' shops are met with a great variety of devices prepared with films of selenite of different thicknesses, and which constitute philosophical toys illustrative of the beforementioned facts. Gothic windows, stars, flowers, fruits, animals (butterflies, parrots, dolphins, and chameleons), and theatrical figures (Jim Crow, harlequin, &c.), are some of the ingenious, and often laughable illustrations contrived by Mr. Darker.

(To be continued in our next.)

SECOND ANNIVERSARY

OF THE

PHARMACEUTICAL SOCIETY,

Held at No. 17, Bloomsbury Square, May 16th, 1843.

WILLIAM ALLEN, F.R.S., PRESIDENT, IN THE CHAIR.

THE Chairman, after having expressed the pleasure which he felt in meeting his brethren, called upon the Secretary to read the

REPORT OF THE COUNCIL.

IN presenting to the Members the Second Annual Report, the Council think it right again to advert to the general objects of the Society, in order to point out the progress which has been made, and what still remains to be done.

The Society was established for the purpose of ameliorating the condition, and raising the character of those who practise Pharmacy in Great Britain. It is proposed to attain this end, first, by uniting all the Chemists and Druggists into one body; secondly, by introducing a system of pharmaceutical education; thirdly, by claiming for the body thus organized and educated, such protection and privileges as the qualification of the Members would entitle them to possess.

The first of these measures has been, to a considerable extent, carried into effect by the formation of a large and influential Association, which comprises within its ranks the leading members of the trade in the metropolis, as well as in most of the cities and towns in other parts of the empire. This union has been sanctioned and ratified by a ROYAL CHARTER OF INCORPORATION, recently obtained and announced. The Association has ramified throughout the country, and in most of the provincial towns there are local secretaries. Thus the Pharmaceutists of this country are now recognised and represented; their affairs are conducted by a Council, annually elected by the Members; and although there still remain a considerable number who have not yet joined the Association, the Council trust that this number will gradually diminish, as the promulgation of the advantages of the institution induces fresh candidates to apply for admission.

The second of the above measures, which relates to education, has been progressing in like manner. A School of Pharmacy is established in London, and provincial schools have been projected by the Members resident in many large towns. The examination of candidates for admission has come into operation;