mission of a beam of light into a dark room, through apertures of different forms; and we are much inclined to suspect that they depend on the twofold transmission of the light to the eye, perhaps after repeated internal reflections, from the different points in the lateral surfaces of the substances employed. The effects were most conveniently observed in cubes of 12 inch, and better in white than in yellowish glass: in cubes of an inch only, they were indistinct nor were they produced by fluor spar, rock salt, or by any kind of liquids: they were modified, and sometimes inverted, by the interposition of a plate of mica: and ice acted in a similar manner in depolarising the light transmitted through it. We find in these researches a full confirmation of the experiments which Mr. Malus had made some time before his death, to show, that the polarised light, which falls on a transparent medium at such an angle, as not to be reflected, is transmitted, with no material diminution of its intensity. Dr. Seebeck's language is a little enveloped in the mysticism of the school to which, by some singular caprice of fancy, he has thought proper to attach himself: but we cannot hesitate to believe, that as he continues his examination of the phenomena of nature, he will by degrees be persuaded of the futility of the objections which Mr. von Goethe has advanced against the Newtonian doctrine of the composition of white light, and of the inaccuracy of the assertions on which some of those objections are grounded.

While the optical philosophers of France and Germany have been engaged in these researches, Dr. Brewster has been very laudably employed, in this country, in experimental investigations relating to the same interesting department of physical science. He has found that the agate, cut by a plane perpendicular to its laminae, transmits one only of the polarised portions of light: that the polarity of light may be destroyed by transmitting it in a certain direction through almost all mineral substances, and through korn, tortoise-shell, and gum-arabic; while in certain other directions its properties rema in unaltered, whence he has distinguished, in these substances, different depolarising and neutral axes; and that the light reflected from the oxydated surface of polished steel is so modified, as to prove, in his opinion, that the oxyd is a thin transparent substance. His observations on the colours, sometimes exhibited by crystals of Iceland spar, seem to be identical with those of Martin and Malus.

Dr. Brewster has very ingeniously exercised his inventive. powers in the contrivance of a variety of micrometers, goniometers, microscopes, and telescopes, several of which may very possibly be found useful in particular circumstances, although to others there appear to us to be many material objections: but, without referring to the test of experience, it would be of little

utility for us to discuss their particular merits. Some detached remarks, however, we shall take the liberty of submitting to our readers, on passages of the work which appear to require correction. The advantage which Dr. Brewster attributes to the use of a transparent fibre for a micrometer, (p. 71,) is merely imaginary; since, although it is true that the central rays suffer no inflexion,' this circumstance affords us no assistance whatever in judging when the rays are actually 'central;' and the light transmitted by such a fibre, whenever the luminous object is in its neighbourhood, could only create confusion. In speaking of a telescope for the measurement of angular positions, Dr. Brewster observes that the line, which joins any two stars, forms every possible angle with the horizon in the course of 23 hours and 56 minutes;' (p. 128,) but this is obviously a mistake; for at the poles of the earth the 'angle would not vary; and in other latitudes only within certain limits. The table of the variation of the focal length of a telescope, (p. 218) is wholly erroneous, from the employment of linear feet and square inches in different parts of the same formula. Dr. Brewster has misunderstood Professor Robison and Mr. Wilson, where they observe, that the focal length of an achromatic telescope must be lengthened, when it is directed to a star towards which the earth is moving, (p. 221): it was not from the different distances of the stars, but from the difference of the relative velocities of light, that they argued, according to the general opinions respecting light, the necessity of the occurrence of such a minute variation. In p. 424-5, the magnifying power is miscalculated, and we must read 4.9 for 5.6.

The most useful part of the whole work appears to be the series of experiments on the refractive powers of fluid and soft substances performed by interposing them between the object glass of a microscope, and a plane glass nearly in contact with it, and then measuring the joint focal length of the combination. The comparative distances, thus obtained, are exhibited in several extensive tables: but we cannot help feeling some surprize, that the author has not attempted to deduce, from any one of his numbers, the direct refractive power of the substance concerned, as he certainly would have done if he had been aware how easily it might have been accomplished, after a preparatory investigation, dependent on the common laws of dioptrics. From such an investigation we have obtained formulae for each of the two series of experiments; for the first (pp. 258, 268, 270,) f being the focal length expressed by the number in the table, and r the index of refraction, r = 1.887 —; and for the second, (p. 264) r = 2.31 — .8f f Thus we obtain for phosphorus 2.125, sulfur 2.008, aloes 1.643,

1.31.

balsam of Tolu 1.636, oil of cassia 1.625, guaiacum 1.609, and pitch 1.589. Dr. Wollaston's Table gives for phosphorus 1.579, and for pitch 1.53; and there can be no doubt that the accidental presence of some phosphoric acid, and some oil of turpentine, on the surfaces of these substances occasioned an error, in these instances, in Dr. Wollaston's determinations, however excellent his method may be in other cases; for we cannot agree with Dr. Brewster, in thinking that the acknowledged exhibition of the index appropriate to the extreme red ray is an objection to the method. It is remarkable, as our author has justly observed, that the assignment of so high a refractive density to phosphorus restores the inference of Newton, respecting the relation between refractive powers and inflammability, to its original univer sality and importance.

Dr. Brewster's mode of ascertaining the refractive powers of solids, by immersing them in a mixture of fluids of equal refractive density, is perfectly unobjectionable; and he observes that it is easy to discover, in this manner, the internal flaws and other irregularities of gems, without the labour of polishing any part of their surface. He does not, however, appear to have followed this method in determining the indices of refraction which are contained in his table, (p. 283,) having employed for this purpose 'the same prisms in which the dispersion was corrected,' and probably in the same manner: hence, from an erroneous mode of computation, his numbers are almost uniformly too large thus we have phosphorus 2.224, sulfur 2.115, carbonate of lime 1.665 and 1.519, oil of cassia 1.641, and guaiacum 1.619, all of which exceed the more accurate determinations which we have already mentioned. In the same manner we find for diamond 2.487 to 2.470, instead of 2.439, the density assigned by Newton; and it is probable that the chromate of lead and realgar, both of which Dr. Brewster finds more dense than the diamond, are also rated somewhat too high at 2.974.. 2.503, and 2.549: the former appears to have a double refraction more distinct than any other known substance.

For a similar reason, we can place no dependence whatever on the table of dispersive powers, which is calculated according to a coarse approximation, wholly inapplicable to the circumstances of the experiments. The mode of inclining a prism of a greater density, until it caused the image of a right line, viewed through it and in conjunction with a prism of smaller density, to be colourless, would be a very good one, provided that the apparatus were so arranged, that the rays should be perpendicular to the common surface of the prisms; but even then Dr. Brewster's mode of calculation would be only applicable to prisms with very small

refracting angles. In the only experiment which is related with precision (p. 306,) the result implies an impossibility; for if we trace a ray of light through its intricate progress from the water to the glass, the angle of incidence upon the last surface will come out 41° 5', while the utmost obliquity, at which it could have been transmitted is 38° 14', consequently the index of refraction assigned to the prism, 1.616, must be extremely erroneous, if the angular measurements were correct. And since various errors of this kind may have affected the different results in different degrees, we cannot depend on the tables, even for the order of the different dispersive powers.

Dr. Brewster appears, however, to have been more successful in confirming and extending the observations of Dr. Blair on the different proportions in which the prismatic spectrum is divided, according to the diversity of the substances which afford it. He has shown very clearly, both from theory and by experiment, that the violet rays must be proportionally more expanded by a prism with a large angle than by a smaller one of the same substance; while he has found, on the other hand, that a smaller prism of a more dispersive substance almost always expands the violet rays more than a larger prism of a less dispersive substance; and that when two such prisms are combined, they exhibit a green fringe in the usual place of the red, and a 'wine coloured' fringe in that of the violet. The substances most expansive of the violet are oil of cassia and sulfur; the least expansive, sulfuric acid and water, although water has not quite so low a dispersive power as fluor spar. It seems to follow from Dr. Brewster's estimate, that the proportions of 2 red, 3 green, 4 blue, and 3 violet, which are nearly those of Dr. Wollaston's determination, are changed, when sulfuric acid is employed, at least as much as to 4 red, 3 green, 3 blue, and 2 violet; but we feel great difficulty in believing that so great a variation as this could have escaped the notice of any attentive observer. We have no doubt, however, that if Dr. Brewster continues to pursue his ingenious investigations, he will by degrees acquire a habit of introducing greater accuracy into his measurements, and what is of still more importance, more mathematical neatness into his calculations; and, with these improvements, we doubt not that his future labours may be productive of material benefit to those departments of physical science which have engaged his attention.

ART. IV. Letters on the Nicobar Islands.

don. 1813.

8vo. pp. 64. Lon

THIS little book is another proof of the zeal with which the

Moravian missionaries have laboured in the vineyard; even when their benevolent exertions have produced no other fruit, they have contributed to our knowledge of remote countries and savage tribes.

The Nicobar islands are a small cluster situated at the entrance of the Bay of Bengal. Navarrete says that there is a spring in one of them which gilds iron, copper, and wood; but he knew not whether the gilding were permanent. If this account have any foundation in truth, it seems to indicate good copper mines. Such a report was easily improved. The Portugueze assured Gemelli Carreri, that this water had the property of transmuting iron into gold; and that the Dutch, ambitious of possessing a country where nature thus effected what the alchemists of Europe had so long laboured in vain to discover, endeavoured to conquer it; but lost about eight hundred men in the attempt. If the Dutch made such an attempt they have not recorded it; at least we have sought in vain for any notice of it in the great historian of their exploits and discoveries in the east. Long since the days of Jason and the golden fleece, wilder expeditions have been undertaken. Juan Ponce de Leon, the discoverer of Florida, sent a ship in search of the island of Bimini, where the Spanish conquerors as well as the Indians firmly believed there was a fountain which possessed the virtue of Medea's kettle, and restored to youth whoever bathed in it. The Indians of Cuba made a voyage to Florida, in quest of a river of the same marvellous quality. But the most remarkable of all voyages of discovery was that which the kings of the Maldives repeatedly undertook to a certain island called Pollovoys, for the purpose of attempting its conquest from no less a personage than the devil; not metaphorically, by a spiritual warfare like that of the missionaries they believed that the devil was in actual possession of the island; and they sent an expedition of conjurors to propose terms to him, and negociate for a cession on his part.

There may have been another motive for the Dutch expedition, if it were really made. The two largest islands Nancauwery (from which the whole group is sometimes denominated) and Comarty form a harbour which is sheltered to the westward by the island of Katsoll, and to the east by Trikut, a long, narrow, flat island, abounding with cocoa trees. Both entrances have a clear deep channel, through which the largest ships may pass, both with a N. E. and S. W. monsoon: the harbour is capacious