Feb. 19, 1885] 



NA TURE 



37i 



Moon (at First Quarter at nil.) rises, ioh. 37m. ; souths, 

 18I1. 19m. ; sets, 2h. 8m.* ; decl. on meridian, 17° 8' N. 

 Planet Rises Souths Sets Ded. on Meridian 



t Occurs on the following day. 

 It may be mentioned that times of disappearance and reap- 

 pearance for the occultation of Aldebaran for various other 

 positions in the United Kingdom will be found in Nature, vol. 

 xxxi. p. 322. 



Phenomena of Jupiter's Satellites 



GEOGRAPHICAL NOTES 



Gen. Gordon, when Governor of the Soudan in 1874, sent 

 home to a friend a map of the route between Suakim, Berber, 

 and Khartoum, drawn by himself. Mr. Stanford has reproduced 

 this map in facsimile by permission, and it will probably be of 

 great interest at the present juncture. 



Mr. STANFORD lias recently issued two maps of the Soudan, 

 in connection with the military operations which are at present 

 being carried on in that region. These maps are most excellent, 

 and must prove highly serviceable to all who wish to follow the 

 course of events. 



A \ OLUME on New Guinea, which should be of great interest, 

 is about to appear in Holland. The former Dutch Resident at 

 Ternate, Mr. van Braam-Morris, in the course of his official 

 tours on the Amberno or Rochussen rivers, succeeded in going 

 a considerable distance to the south. His report, with the 

 accompanying map, is now being prepared for publication by 

 Mr. Robidee van der Aa, who is himself a high authority on 

 New Guinea. 



Mr. A. M. Skinner, Vice-President of the Straits branch of 

 the Royal Asiatic Society, has published at Singapore, a Geo- 

 graphy of the Malay Peninsula and the surrounding countries, in 

 three parts, containing almost all that is know 11 regarding the 

 physical and political geography of these regions. The idea of 

 the work was suggested by the Council of the Royal Colonial 

 Institute, applying to the various Colonial Governments for 

 school-books which might be used in schools at home for the 

 instruction of pupils in the position, resources, and general 

 progress of the Colonies. 



It is announced that Mr. Stanley's new work on "The 

 Congo " will be published by Messrs. Sampson Low and Co. in 

 April next. 



Under the title of Explorador (the Explorer) a Portuguese 

 journal commenced its appearance with the new year at Lisbon. 

 It will appear twice a month, and will chronicle the advance of 

 science in all its branches, but especially that of geography and 

 travel. 



At the meeting of the Geographical Society of Paris on the 

 6th inst. a letter was read from the French consul at Zanzibar 

 describing recent events of geographical interest in Eastern 

 Africa. Lieut. Gouin, Resident of France at Nam-Dinh, in the 

 delta of the Red River, gave some information on the naviga- 

 tion and commercial resources of the southern mouths of the 

 Red River. The most southern of all, the Cua-Day, is said to 

 be navigable for sea-going junks, and to give immediate access to 

 the richest rice-producing provinces of the delta. M. Leon 

 Rousset read an account of a journey of eight months in Turkey. 

 He dealt chiefly with the junction of the Turkish with the 

 European railways. 



ON A MODIFICATION OF FOUCAULT'S AND 



AHRENS'S POLARISING PRISMS 

 TN tracing by the usual methods the course of rays through 

 one of the polarising prisms recently devised and constructed 

 by Mr. C. D. Ahrens (described in the Journal of the Royal 

 Microscopical Society for September 1884, and in the Philosophi- 

 cal Magazine for last month), I found that, in the case of a ray 

 incident in a direction parallel to the axis of the prism, that 

 component of it which passes through the middle spar-prism as 

 the ordinary ray falls on the second surface of that prism at an 

 angle of 42 35'. 



This is greater than the critical angle (37° 12') for ordinary 

 rays passing from calc-spar into air. Hence, if a film of air (as 

 in Foucault's prism), instead of a film of Canada balsam (as in 

 Ahrens's prism), is placed between this spar-prism and the next, 

 the ordinary ray will be totally reflected, while the extraordinary 

 component will still emerge and be available as a plane-polar- 

 ised ray for experiments, as in Foucault's prism. 



This' extraordinary ray, however, is n it only deviated on 

 emergence, but also over-corrected for colour ; the deviation 

 from the direction of the original incident ray being — 



For Fraunhofer's line F 12° 20' 



C 12° 35' 



(as determined by using the light of a hydrogen vacuum-tube). 



Both the deviation and the dispersion can be almost entirely 

 corrected by passing the ray through a prism of crown glass 

 combined with a prism of very dense flint glass, as shown in the 

 drawing given below. 



flint glass. 



What is said above respecting the ray incident parallel to the 

 prism-axis applies to all rays incident at angles not greater than 

 14° with the axis ; and thus the combination forms a polarising 

 prism with an angular field of 28 , about equal to that of an 

 ordinary Nicol's prism, and far greater than that of a Foucault's 

 prism (which is only 8°). 



The following points, among others, appear noteworthy in the 

 above prism : — 



(I) Its length is scarcely more than twice its breadth, the pro- 

 portion between the two dimensions being rather greater than in 



