N. M.-No. 1 appears to be a fair sample.

B. CHASTON. We publish an Index at the conclusion of each volume. "OMICRON."-We know of no work devoted exclusively to the purpose mentioned. Such information is generally to be met with in modern works on Chemistry. For the preparation of chloride and oxide of silver, see page 724 of this number.

W. H. (Glasgow).-See Vol. I., No. 4, page 140, of this Journal. W. SUNDERLAND.-The following is the formula for the Mistura Creasoti" of the Edinburgh Pharmacopoeia: -Take of Creasote, Acetic Acid, of each, M. xvj.; Compound Spirit of Juniper, Syrup, of each fi; Water, fxiv. Mix the Creasote with the Acid, add gradually the Water, and lastly, the Syrup and Spirit. The Acetic Acid ordered in this formula is the Glacial Acid of the Edinburgh College. This mixture is said to have been found useful for allaying sea-sickness.

"H. B. M." Blue Ink is made by dissolving Prussian Blue in an aqueous solution of Oxalic Acid. Red Ink is generally an infusion or decoction of Brazil Wood in Vinegar, thickened with Gum, and sometimes a little Alum. W. N. T.-We know of no method of testing the genuineness of tobacco, excepting by its physical characters, or by examining it for the presence of the peculiar proximate principles of the plant. The unmanufactured leaftobacco should be used in medicine.

"AN ASSOCIATE." (Leominster).-Smith's Compendium.

W. C. I.-A formula for Tincture of Hybiscus Abelmoschus was published by Dr. Reece; we believe the proportions were 2oz. to 16oz. A Platinum Capsule is ordered in making Acidum Phosphoricum dilutum, P. L., because the acid is liable to act upon other materials at the high temperature required. W. E. W.-Selecta è Prescriptis, price 5s.

"DELTA." See the notice inside the cover of Vol. II., No. 9. The diploma will be issued to the founders as soon as the requisite number are printed and signed, and to the other members as soon as possible afterwards.

"INQUISITOR."-See the answer to Delta. We cannot notice any further remarks on the subscription, except upon STATISTICAL DATA. We have already proved by CALCULATION, that the amount cannot be reduced at present without altering the constitution of the Society. Frequent repetitions are fatiguing. See the leading article of our last number, and the passages referred to in a note, page 618. Inquisitor complains that the articles in the Journal are too scientific and not sufficiently practical. We hope to improve. A. A. having attended lectures on Chemistry, wishes to know what further qualifications he requires in a situation as a brewer, a soap-boiler, or a sugarrefiner-Ans. Practical experience.

"TIRO" inquires what course he ought to pursue in order to become acquainted with botany? Ans. If in London, attend Dr. Thomson's lectures; if in the Country, study Nature, and Lindley's Elements.

"OBADIAH," being an Associate, is not liable to the fine for non-registration of his indentures, and need not pass the examination.

R. H.-State the case to Mr. Churchill.

MR. MASON complains that he has written more than once without receiving We have sent him two letters by post. One of his questions is answered in this number.

answers.

B. V. W. S.-The PHARMACEUTICAL SOCIETY is a voluntary association, and could not be made compulsory without an Act of Parliament.

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Advertisements to be addressed to Mr. CHURCHILL, Princes Street, Soho.

Other Communications to THE EDITOR, 338, Oxford Street (before the 20th, if answers be desired in the ensuing number).

VOL. II.-No. XII.-JUNE 1st, 1843.

TRANSACTIONS

OP

THE PHARMACEUTICAL SOCIETY.

LECTURE II.

ON THE POLARIZATION OF LIGHT, AND ITS USEFUL APPLICATIONS.

BY JONATHAN PEREIRA, M.D., F.R.S.

HONORARY MEMBER OF THE PHARMACEUTICAL SOCIETY.

3. COLOURED POLARIZATION.

WHEN an excessively thin film of a doubly refracting crystal is placed in the polariscope, that is, between the polarizing and analyzing plates, the most gorgeous colour or colours appear, and when the analyzer is rotated on its axis they change to complementary tints. If the film be of uniform thickness, the colour is uniform; but if the film be of irregular thickness, different colours are perceived.

In order to produce colour, it is necessary to use, first, a polarizer, as a tourmaline, or a reflecting plate; secondly, a film of a doubly refracting crystal, called the depolarizer; and, thirdly, an analyzer or test, as a tourmaline, a reflecting plate, or a doubly refracting prism.

The office of the polarizer is indicated by its name; it polarizes the light. Without this no colour is perceived, for a reason which will be hereafter explained.

The doubly refracting film, called the depolarizer, receives the light thus polarized, and doubly refracts it. That is, a system of waves, constituting the incident ray, entering the crystalline film, is resolved into two systems of equal intensities within it. These form respectively the ordinary and extraordinary rays (fig. 14, O and E). They are polarized in planes +45° and -45° to that of the incident system, so that the plane of polarization of the ordinary system forms an angle of 90° with that of the extraordinary system.

Now, the two systems of waves thus produced traverse the crystal in different directions and with different velocities; but as the film or plate is excessively thin, they emerge superposed. One set proceeds through the crystal more slowly than the other; or, in the language of a distinguished writer on this subject, one

set lags behind the other: so that at their emergence they are found to be in different phases of vibration.

By the analyzer each of the two systems (O and E) is resolved into two other systems (Oo Oe and Ee Eo), so that now four systems or two pairs are produced.

But the vibrations of these four systems are made in two planes : that is, two in one plane, and the other two in a second plane, which is rectangular to the first. Now, as the two vibrations which are made in the same plane, are not in the same phase (the one system having suffered a greater retardation than the other), the waves interfere and produce colour (if the incident light be white). But the two vibrations of the one plane conspire, while those of the other plane are opposed. Hence the tint or colour produced by the interference of the waves, in one plane, is complementary to that produced in the other plane. So that if the analyzer be a doubly refracting prism, both complementary colours are seen by transmission; but if it be a reflector, one is reflected and the other transmitted; whereas, if it be a tourmaline, one is transmitted, while the other is suppressed, extinguished, or stifled.

Production of Complementary Tints.

A. A ray of common or unpolarized light incident on B.

B. The polarizer (a plate of tourmaline).

C. A ray of plane polarized light incident on D.

D. The doubly refracting film or depolarizer.

E. The extraordinary ray} produced by the double refraction of the ray C.

O. The ordinary ray

G. The analyzer (a doubly refracting prism).

Eo. The ordinary ray

Ee. The extraordinary ray} produced by the double refraction of the extraordinary

Oo. The ordinary ray

ray B.

Oe. The extraordinary ray }produced by the double refraction of the ordinary ray 0.

To render somewhat more intelligible the cause of the colours being complementary, and, therefore, to explain what is meant by the conspiration and opposition of vibrations, let us suppose the vibrations of the polarized light (C, fig. 14) to be made in the plane, CP, fig. 15; and to give more precision to our ideas, let us further suppose that the molecule C is, at a given instant, moving from C towards P.

The doubly refracting film resolves this motion into two other motions, performed at right angles to each other, one in the direction CO, the other in the direction CE. The waves pro

duced by the vibrations in the plane CO, we shall suppose to constitute the ordinary system, while those in the plane C E form the extraordinary system. But the plate is much too thin to have produced between these two systems any sensible separation.

Each of these motions is resolved, by the analyzer, into two others at right angles to each other. That is, the vibration Co is resolved into the vibrations C Oo and C Oe; while the vibration CE is resolved into the vibrations C Eo and C Ee. Now,

it is obvious, that the two motions C Oo and C Eo act in the same direction, or, in other words, they conspire, or strengthen each other; while the motions C Oe and C Ee, though performed in the same plane, oppose or destroy each other.

By rotating the analyzer the reverse happens: C Oo and C Eo oppose or destroy each other, while C Oe and CEe conspire, or strengthen each other, as in fig. 16.

Thus, then, the original polarized ray(C, fig. 14) has been resolved into four rays, two polarized in one plane, and the other two polarized in a plane rectangular to this. The two rays which interfere and destroy each other, differ by half an undulation. The colour produced by the interference of the conspiring rays, corresponds to the difference of the routes of the two polarized rays in the plate or film, while that which results from the interference of the opposing rays, is that which is due to the same difference augmented or diminished by half an undulation. In the case 3 G 2

above noticed, in which CO e and CE e (fig. 15) are opposed, the colour corresponds to the difference plus half an undulation.

But it may be asked, What is the use of the polarizing plate? What is the reason that no colour is perceived if the light which is incident on the double refracting film be common or unpolarized? To explain this, let us suppose that a ray of common or unpolarized light consists of two rays rectangularly polarized. Each of these rays will suffer the same series of divisions, subdivisions, and interferences as the former; but the tints produced by the one ray will be complementary to those of the other; so that we shall thus obtain two pairs of complementary tints, and as the tints of each pair will emerge superposed, they will neutralize each other, and the resulting light will be of uniform whiteness.

For red and green are complementary tints, and produce by their union white light, as I have already demonstrated.

The office of the doubly refracting film, called the depolarizer, is to doubly refract the polarized light. It prepares the rays for the changes which they have ultimately to undergo and by which colour is to be produced. The thickness of the film or crystalline plate determines the tint; but the actual thickness required to produce a given tint depends on the nature of the crystal. By this plate or film two rectangularly polarized systems of waves are produced, which traverse the plate in different directions and with different velocities, and emerge in different phases of vibration. Now as they are superposed, and as the retardation amounts only to a few undulations and parts of an undulation, it might be supposed that colour would be produced by their interference. But I have already stated that two rectangularly polarized rays do not interfere, so as to produce colour. In order, therefore, to make them interfere, their planes of polarization must be made to coincide; and to do this is the function of the analyzer.

In order to assist us in comprehending how a polarized ray may be resolved into two others polarized in different planes, we may take, as an illustration, a stretched cord, fig. 17 A B, dividing at B into B C and B D, making a small angle with each other at B, and having either equal or unequal tension. Let us suppose the extremity A of the single cord to be made to vibrate regularly in either a horizontal or vertical plane; now, by means of two polished guiding-planes, E F and G H, inclined at different