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laminated structure of the agate, and to indicate that the phenomena of double refraction are produced by an alternation of laminæ of two separate refractive and dispersive powers. In Iceland spar, one set of the laminæ may be formed

. by a combination of oxygen and calcium, while the other set is formed by a combination of oxygen and carbon. In chromate of lead, the chromium and oxygen may give one image, while the oxygen and lead give another. In like manner the carbonate of lead, the carbonate of strontites, jargon, and other crystals may give double images, in virtue of similar binary combinations. Of the simple inflammable bodies, sulphur is the only one which has the property of double refraction, but it will probably be found that it holds a metal or some other ingredient in its composition, which chemists have not been able to discover.

If the explanation which has now been given of the polarising power of the agate should be confirmed by future experiments, this property will be considered as a case, though a

a very curious one, of double refraction; but if these conjectures should be overturned, the phenomena which we have described must be ranked among the most singular appearances in the wide

range of optical science.

2. On ihe double Refraction of Chromate of Lead. In the course of my experiments on refractive powers, I discovered a double refraction in this metallic salt of such enormous magnitude, that the deviation of the extraordinary ray is more than thrice as great as that produced by Iceland spar. The ratio of the sines, for both refractions, and the other MDCCCXIII.

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properties of this extraordinary mineral will be noticed in the next article.

Index of Refr.

2.224

3. On Substances with a higher refractive Power than the Diamond.

Since the time of Sir Isaac Newton, who first measured the action of the diamond upon light, its refractive power has been regarded as superior to that of every other substance; but, in the course of my researches, I have found that realgar and chromate of lead exceed the diamond in refractive power, and that this high refraction, in both these substances, is accompanied with dispersive powers greater than those of any

other body. The following are the measures which I have obtained for these, and a few other substances.

Refractive Powers.

Index of Refr. Chromate of lead

Phosphorus (gr. refr.) - 2.926 Sulphur, native

2.115 Ditto, least refraction 2.479 Cryolite

1.344 Realgar

1.307 Diamond (according to NEWTON)

2.439 Dispersive powers or values of R Chromate of lead

Phosphorus

0.128 (gr. refr.)

0.400 Flint glass (highest) 0.052 Ditto, least refraction 0.262 Diamond

0.038 Realgar 0.255 Water

0.035 Oil of cassia

0.139 Fluor spar Sulphur

0.130 Cryolite It appears from the first of these tables, that phosphorus

2.510 Ice

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0.022

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is next to diamond in refractive power, and that the three simple inflammable substances have their refractive powers in the order of their inflammability. Dr. Wollaston has placed phosphorus below horn and Aint glass,* but I am confident that this distinguished philosopher, to whom the physical sciences are so deeply indebted, will find, upon making the experiment with prisms or lenses, that I have assigned the right place to that remarkable substance. The difference between the extreme dispersive powers in the second table is very remarkable, and the result for oil of cassia indicates in that body the existence of some ingredient, which chemical analysis has not been able to detect.

4. On the Existence of two dispersive Powers in all doubly

refracting Crystals. It has been long known, and it is indeed obvious, from a simple inspection of the images formed by a prism of Iceland crystal, that the one image is more coloured than the other, or that the actual dispersion of the one refraction is greater than the dispersion of the other, in the same manner as the dispersion of a prism of Alint glass with a refracting angle of 12 degrees, is greater than the dispersion of a prism of the same glass with an angle of only 10 degrees.

Dr. WOLLASTON, who was the first person that examined the subject of dispersive powers with philosophical accuracy, makes the dispersive power of Iceland spar considerably above water, and even above diamond. Upon repeating this experiment, with the least refracted image, I found the dispersive

* Dr. WOLLASTON is satisfied that his original estimate was erroneous, and that Dr. Brewster's determination is very near the truth. H. D.

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dR power, or the value of

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to be 0.026 very considerably below water, which stands at 0.035 of the scale, and I therefore concluded that Dr. WOLLASTON had examined the greatest refraction, while I had examined the least, and that the vast discrepancy between our measures arose from the existence of a double dispersive power. This conclusion was confirmed by determining the dispersive power of the greatest refraction, which coincided exactly with the order assigned to it by Dr. WOLLASTON.

The dispersive powers, which I have obtained for other doubly refracting crystals, such as carbonate of strontites, carbonate of lead, and chromate of lead, have confirmed this result, and establish the general law, that each refraction of crystals which give double images is accompanied with a separate dispersive power. The double dispersive powers of these bodies are given in the following table. Chromate of lead (gr. refr.) estimated at 0.400 Ditto Ditto must exceed

0.296 Ditto (least refr.)

0.269 Carbonate of lead (gr. refr.)

+ 0.091 Ditto (least refr.)

0.066
Carbonate of strontites (gr. refr.)

0.046
Ditto
(least refr.)

0.027
Calcareous spar (gr. refr.)

0.040 Ditto (least refr.)

0.096 In a table of refractive powers, published by the late Mr. Cavallo, he has given, from other authors, the dispersions, or the dissipations as he calls them, of a few substances, and he has annexed a different dispersion to the two refractions of

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Iceland crystal; but it is obvious, from a simple inspection of the table, that these are measures of the dispersion or quantity of colour, and not of the dispersive power of the substances. The measures in the table alluded to, with the exception of one or two, are so completely incompatible with those taken by Dr. WOLLASTON and myself, that I can scarcely believe that the experiments were ever made.

The singular property of a double dispersive power, while it seems to exclude some of the theories by which the double refraction has been explained, adds another to those numerous difficulties with which philosophy has yet to struggle, before she can reduce to a satisfactory generalization those anomalous and capricious phenomena which light exhibits in its passage through transparent bodies.

I have the honour to be,

dear Sir,

your most obedient humble servant,

DAVID BREWSTER.

Edinburgh, 23, Duke-street,

December 19, 1812.

To Sir H. Davy, LL.D.F.R.S.

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