Nov. 6, 1879 J 1 



NATURE 



23 



indicated by small red dot?. In spare spaces to the north of the 

 chart, small inset maps have been drawn to give the completion 

 of the geography iu the Polar areas, and upon these will be 

 found, indicated by colour, the average summer limit of open 

 water as far as known. The curves of equal magnetic variation 

 are also shown upon these small maps, and the spots known as 

 the magnetic poles are named. The northern limit of woods, 

 beyond which trees are unknown, is shown upon the small map 

 of the Arctic regions. The principal ocean mail routes are 

 shown by broken black lines, and upon the longer lines the 

 names of ports of departure and arrival are named. The number of 

 <lays, the average of numerous voyages is noted on each line, and 

 the distances in nautical miles from port to port are also given. 

 The submarine telegraph cables are shown by strong black lines 

 with dots at short intervals, and the various cables to the United 

 Slates are identified by having their dates attached. The land 

 is coloured politically giving the most recent territorial divisions, 

 and a bright red colour is reserved for British possessions, which 

 enables the reader to see easily how frequent are the stepping 

 •t jnes of British territory over the face of the earth. Altogether 

 it will be seen this map is well calculated to serve a great variety 

 of useful purposes; its execution is all that could be desired. 



Dr. Nachtigal has received a telegram from Malta to the 

 effect that Herr Gerard Rohlfs's expedition, having reached and 

 explored the Kufara Oasis, was there set tupon and plundered. 

 Herr Rohlfs and Dr. Anton Stecker were consequently com- 

 pelled to return to Benghazi, though they hoped to receive help 

 and compensation from the Turkish Provincial Government. 



Trl'bner and Co. will shortly publish a new work on 

 Madagascar, under the title of "The Great African Island: 

 Chapters on Madagascar," by the Rev. James Sibree, jun The 

 work will contain a popular account of recent researches in the 

 physical geography, geology, and exploration of the country, 

 and its natural history and botany ; and in the origin and divi- 

 sions, customs and language, superstitions, folk-lore, and reli- 

 gious beliefs and- practices of the different tribes. It will contain 

 physical and ethnographical maps. 



Geologists will be glad to learn the appearance of a trust- 

 worthy map of mines in Russia in Europe by Prof. W. M oiler, 

 " Carte des Gites rniniers de la Russie d' Europe." 



We notice in the last number of the Bulletin of the Belgian 

 Geographical Society a paper on the colour of eyes and hair in 

 Belgium, by M. Vanderkindere, with maps : on the Zambeze, 

 by M. Wauters ; and the quarterly report on the demographical 

 and medical statistics. 



The Church Missionary Society a short time back entertained 

 the idea of establishing a sanatorium on the west coast of Africa, 

 and the matter, it may be remembered, caused some discussion 

 between their adviser, Capt. R. F. Burton, and the Rev. T. J. 

 Comber, a Baptist missionary, at one of the Geographical 

 Society's meetings last session. It was proposed to place the 

 sanatorium on Mount Cameroons, which rises to a height of over 

 13,000 feet, just in the angle of the Gulf of Guinea, opposite 

 Fernando Po. Two agents of the Society accordingly proceeded 

 thither in the missionary steamer Henry Venn, and ascended the 

 mountain to the highest peak. Their report was favourable to 

 the suitability of a spot some 7,500 feet high, known as Mann's 

 Spring, but to build a residence there and cut a road to it would, 

 it appears, cost more than the Society can afford in order to 

 recruit the health of their missionaries. 



Under the heading of ethnography, a paper by Pere Petitot, 

 on the Asiatic origin of the Indinns of Arctic America is com- 

 menced in the current number of Les Missions catlioliques. 



The great work undertaken by the Russian Geographical 

 Society under the title of " Works of the Ethnographical and 

 Statistical Expedition to South- Western Russia " is now com- 

 pleted. The whole work consists of seven volumes, in nine 

 fascicules, or nearly 4,800 pages, and it contains abundant most 

 useful information as to those countries which afford so great an 

 interest by the variety of their population. 



We notice the appearance of the following important works 

 recently published by the Russian Geographical Society: — (1) 

 The eighth volume of its Memoirs (Zapioki), which contains 

 a " General Sketchjof a Theory-of Constant Marine Currents," by 

 Colonel Schilling, and a "Note on the New Map of Persia," 

 by General Stebnitzky, with the map itself, which is one of the 

 most important acquisitions to the exact cartography of Asia 



during recent years. — (2) The fourth volume of the translation 

 of Ritter's "Asia," being the description of the Altay and Sayan 

 Mountains' within the limits of the Russian Empire, with a very 

 important appendix (far larger than the original work itself), by 

 MM. Potanin and Semenoff, being a resume of all new informa- 

 tion acquired from 1832 to 1875. — (3) "The Kashgar Eand " 

 (Kashgaria), an historical and geographical sketch, of the 

 country, of its military forces, industry, and trade, by M. 

 Kuropatkin, with additions of General Stubendorff and M. 

 Sreznevsky. — (4) " A Journey to the Holy Land of the Prince 

 Radzivil-Sirotka during the Years 1582 to 15S4," published and 

 annotated by M. Hildebrandt ; and (5) The two first volumes of 

 a " Catalogue of the Library of the Geographical Society," con- 

 taining books on mathematical, physical, and general geography. 

 The importance of this cataloguejwill be realised by all those who 

 know what a number cf works appear in Russian on the 

 geography of Russia and Asia, and how difficult it is to know 

 them. We notice with pleasure that the catalogue contains 

 detailed indexes of all papers that have appeared in the publica- 

 tions of the Geographical Society. An important work, being 

 the description of M. Potanin's journey to north-western 

 Mongolia is already in the press. 



CELESTIAL PHOTOMETRY 

 "TPHE volume of the annals of the Harvard Observatory just 

 A issued is one of great importance to astronomical science, 

 as the new director, Prof. Pickering, has included in it the 

 photometric observations which have lately been carried on with 

 so much vigour. The first chapter is devoted to a description of 

 the forms of instruments— many of them new — which have been 

 employed, and in this notice we shall limit ourselves to an 

 analysis of this part of the volume. 



The first instrument employed was constructed by attaching a 

 Nicol to a double-image prism in such a way that it could turn 

 freely around its axis. By a graduated circle and index, the 

 angle could be measured to tenths of a degree. When two 

 bright objects were viewed through this instrument, two images 

 of each were formed by the double-image prism, either of which, 

 by turning the Nicol, could be made as faint as was desirable. 

 Whatever their relative light, the faint image of the brightest 

 could thus always be reduced to equality with the bright image 

 of the faint object. The true relative brightness is then deduced 

 from the angle through which the Nicol is turned. 



This form of photometer may be used without a telescope in 

 the comparison of bright stars which are sufficiently near each 

 other, but the loss of light is large. By Fresnel's formula for the 

 reflection of light, each of the four surfaces of the prisms will 

 reflect four per cent. The amount they would transmit, were 

 there no other losses, would therefore be (■96)'' = '849. This 

 supposes that the faces of the Nicol are perpendicular to its 

 axis. If made of the usual form, the loss would be still greater. 

 The unavoidable defects of the surface, dust, absorption, and 

 the reflection at the surface of the balsam cementing the prism, 

 reduce still further the transmitted light. About "So will remain 

 under favourable circumstances. Since the prism forms two 

 equal images, only one half or '40 can pass into each, and when 

 the two images are reduced to equality, their brightness will be 

 only - 20 or "40 of that of the fainter object. For any but the 

 brightest of the heavenly bodies, it is accordingly necessary to 

 increase the light by means of a telescope. 



The following general remarks occur on this form of instru- 

 ment. 



"Since the relative positions of the Nicol and double- 

 image prism are unimportant, either might be placed in 

 front of the object-glass, between the object-glass and the 

 field-lens, between the field-lens and eye-lens, or between the 

 eye-lens and the eye. Unless the double-image prism is placed 

 in front of the object-glass, two images of the latter will, 

 in general, be formed, giving two emergent pencils, both of 

 which must pass without loss into the eye. There is danger 

 that on moving the eye one or other of these pencils will be 

 partially cut off, thus reducing the brightness of one of the 

 objects. If the two images to be compared are brought very 

 near together, this is less likely to occur. On the other hand, 

 at least one of the images of a double-image prism is not 

 achromatic; and, if the prism is placed in front of the object- 

 glass, the colour becomes very marked. In this case, also, it 

 becomes difficult to obtain a prism having such flat surfaces that 

 the images will not be distorted, since any irregularities are 



