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, if 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 universality 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 re D 4 fracting fracting 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. ART. IV. Letters on the Nicobar Islands. Svo. pp. 64. London. 1813. 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 and excel lent; lent; Mr. Fontana says it is one of the safest in India. 'Ships may ride there in perfect security about half a mile from shore, sheltered from all winds. This natural advantage did not however induce the Dutch to make a second attempt, and the next adventurers who tried their fortune in the Nicobars, were of a very different character. In 1711, P. Faure, a Jesuit, and P. Bannet his companion, were landed here with a sack of rice, and their religious utensils in a little box. As soon as they were set on shore they knelt and prayed, and kissed the earth, pour en prendre possession au nom de Jesus Christ, says the editor of the Lettres Edifiantes. In 1756, the Danes from Tranquebar attempted to form a commercial settlement here, and in that spirit of gratuitous innovation, of which geographers so often have cause to complain, they new named the group, after the reigning king, the Frederic Islands; and Kar-Nicobar, on which their settlement was placed, New Denmark. The court of Copenhagen was at that time distinguished by a spirit of literary and religious zeal; and the Ordinary of the Moravians (or United Brethren as they call themselves) was officially informed that it would give the king singular pleasure if some brothers would settle on these islands, and endeavour to bring the inhabitants to the knowledge of Christ. The invitation was readily accepted by a society of whom it may truly be said, that they possess the zeal of the Jesuits, unalloyed by any worldly motives. While they were preparing to send out their colony, tidings arrived that the settlement had been given up, almost all the colonists having fallen victims to the climate. Disheartening as this was, the brethren informed the government that they were still willing to undertake the mission, but that as it would be neither advisable nor feasible to settle a colony in so distant and wild a country immediately from Europe, it was necessary that they should previously have an establishment at Tranquebar, in order to support the mission in the islands from thence. There were no persons in Denmark who thought that the Hindoos might as well worship Jaganaut as Christ, and that christianity was not calculated for the latitude of India: the Danish Asiatic Company therefore granted them a settlement, with permission to preach the gospel to the heathen, and to embody them into the christian church by baptism, according to the laudable example of their brethren in Greenland and the West Indies. The colony arrived at Tranquebar in 1760, carrying with it those orderly and industrious habits which have made the Moravians respected wherever they are known. Their artificers and their physician found abundant employment; they cultivated the land with success, and excited much surprise by planting a vineyard. Such indeed was the good repute which they obtained while waiting there for an opportunity to begin their settlement in the Nicobars, that their historian Crantz assures us the English governor of Bengal wished them to form an establishment at Chatigan on the Ganges, but they did not think it right to change their original purpose. In 1768, the Danish government made a second attempt at settling in the islands; six brethren accompanied the establishment, and fixed their residence in Nancauwery. The same deadly climate which had frustrated the former attempt, proved fatal to this. The servants and soldiers of the company died so fast that, in the year 1771, two Europeans and four Malabars were all who survived. The missionaries had not suffered in equal proportion; their temperance probably rendered them less susceptible of the diseases of the country, yet they lost a third of their number. The commercial settlement was now, as might be expected, finally abandoned, but the brethren persevered under the most difficult and disheartening circumstances. They were even dependent for subsistence upon supplies from Tranquebar. An Englishman, by name Holford, who resided in that city, rendered the Moravians the most essential service, by joining them for several years in chartering a small vessel, which carried out necessaries for the mission, and returned with produce; the sale whereof however fell far short of the expenses of the outfit. A French privateer searched one of their ships; a few old English newspapers were found in a trunk belonging to an English gentleman on board, who had escaped from Hyder-ally, and this, says M. Haensel, was pretence sufficient for a Frenchman to seize upon a neutral Danish vessel! It reduced the missionaries to the greatest immediate distress, nor were they ever able to obtain restitution. The mission was continued till 1787; the expense of life and money which it required was then properly thought too great to be afforded longer; and its only fruits are to be found in the little book before us, the recollections of the last surviving missionary, John Gottfried Haensel. We are indebted for it to the venerable Mr. Latrobe, to whom literature, as well as his own community, is beholden for many and useful services. In consequence of the inquiries of Mr. Wilberforce concerning the mission, he prevailed on Mr. Haensel to embody and preserve the knowledge which had been so dearly purchased; and this translation of Mr. Haensel's original letters is his work. By combining the information here contained with that which other writers have communicated, a connected view may be given of the state of these islands. - The fullest accounts are those which have been given by Mr. Hamilton, Mr. Fontana, and Lieutenant Colebrooke, in the Asiatic Researches. There is little to be found in earlier writers, and that |