i8o 



NATURE 



\_Dec. 26, 1878 



the source. But it is a curious fact that the two errors 

 occurring on the undulatory theory (" aberration " and 

 ''equation of light ") happen precisely to counteract each 

 other ; so that therefore the luminous source is seen in 

 its true position, i.e., in the saine position as by the cor- 

 puscular theory. The fact of the two errors compensating 

 each other therefore prevents this occurrence of error 

 from servang as a visual test between the two theories. 

 There is, however, a distinct objective difference between 

 the two theories in this case, as regards the position of 

 the telescope (previously referred to) ; but before recurring 

 to this, we may consider more closely the mode in which 

 the above compensation of the two errors is brought 

 about. 



. 8. It is well known that the effect of the error termed 

 "aberration" is to make the luminous source appear 

 forwards of the position it occupied when the wave left 

 it, and this by an absolute amount equal to that traversed 

 by the telescope during the time the light took to pass 

 from the source to the telescope. But from the fact that 

 the luminous source is in motion, the latter is actually at 

 the time of observation situated forwards of the position 

 it occupied when the wave left it, and by precisely the 

 above amount (since the source is moving at the same 

 velocity as the telescope). Hence the two errors will 

 precisely compensate each other, and the luminous source 

 will be seen through the telescope in its true position, i.e., 

 in the same position as by the corpuscular theory. But it 

 is important to note that on the undulatory theory, the 

 telescope, in order to receive the parallel beam emitted by 

 the luminous source, must be placed not opposite the 

 source, but somewhat backwards, to make up for the 

 slanting track of the beam attendant on the motion of 

 the luminous source. On the corpuscular theory, on the 

 other hand (where the track of the beam of light is normal 

 to the line of motion of the source), the telescope must be 

 placed opposite the source in order to catch the beam. 

 The /^^///<7« of the telescope in the two cases therefore 

 constitutes a distinct physical difference between the two 

 theories, which might serve as a test.^ 

 • 9. The above considerations may suffice to show in a 

 simple manner that important differences exist in the 

 effects attendant on the motion of a luminous source on 

 the corpuscular and undulatory theories of light, and that 

 these differences would be in principle cB-'pahlt (supposing 

 practical difficulties surmounted) of constituting a simple 

 and decisive test between the two theories, 



S. ToLVER Preston 



GEOGRAPHICAL NOTES 



The Paris Geographical Society held last Friday its 

 annual December meeting, at its hotel, un^er the presi- 

 dency of Admiral La Ronciere le Nourry. The report of 

 the progress of geography was read by M. Maunoir, the 

 Secretary. M. de Ujfalvy delivered an address on the 

 region of Central Asia which he visited, and which may 

 be termed the extreme frontiers of the Russian empire, 

 dnd which are just now attracting so much notice. This 

 traveller will soon come to London to give the same 

 address before the Royal Geographical Societj'. His 

 exploration was made at the expense of the French 

 Government. On the following Saturday a banquet 

 took place at the Continental Hotel. The usual 

 tbasts were given. After dinner M. de Lesseps, who 

 is very likely to be nominated president of the 

 Society for 1879, ^t ^^^ April meeting, gave some 

 account of his visit to Tunis with Capt. Roudaire, in 



^ The fact pointed out by Sir John Herschel in regard to the difference in 

 the velocity of light on the undulatory and corpuscular theories, in the case 

 oiF a luminous body moviug directly towards or from the observer, would 

 appear alsD to be worthy of remark. It is evident, for example, that in the 

 case of a luminous body moving towards the observer, the velocity of light 

 would add itself to that of the body on the corpuscular theory ; but the 

 velocity would be unaffected on the undulatory theory. 



order to investigate the conditions of the creation of the 

 new Saharan Sea. M. de Lesseps described the whole 

 scheme as being easily practicable for a sum of not more 

 than 60,000,000 of francs. He said that the French 

 extension telegraph system was extending all over Tunis 

 and Tripoli, and that Arabs were accustomed to follow 

 the telegraphic line as their camels travelled at a quicker 

 rate when following its track. He intimated that the 

 Egyptian telegraphic system was extending to the equator, 

 and that he advised M. Cochery, the Director of French 

 Postal Telegraphy, to extend it all orer Sahara up to 

 Senegal. 



Petermann' s Mittheilungen (for so it will continue to 

 be named) for December contains a long and careful 

 account of Chartography at the Paris Exhibition ; 

 Dr. Carl Martin contributes a paper, based on Chilian 

 sources, on the Chonos Archipelago, which is accompanied 

 by a map. An excellent paper on the present condition 

 of Afghanistan (with map), is contributed by Herr F. 

 von Stein. This number, besides the usual table of 

 contents to the volume, contains a complete alphabetically 

 arranged index to Dr. Behm's useful monthly summaries. 



We have received a specimen of the first number of 

 one of those stupendous geographical works of which 

 the French are so fond, and the best example of which is 

 Elisde Reclus' "Gdographie Universelle." Indeed, the 

 new work announced seems to have pretty much the 

 same object as that of Reclus, though the method is 

 different. The new work is to be issued by the Librarie 

 des Connaissances Utiles, and the author is M. Charles 

 Hertz. Its title is " La Gdographie Contemporaine 

 d'apres les Voyageurs, les Emigrants, les Colons, les 

 Commergants." It will consist of ten series of from 

 three to five volumes each. We trust the publishers 

 will find subscribers patient enough to wait for the end. 

 There will be from 600 to 800 maps and hundreds of 

 illustrations, and the work will be issued in weekly parts. 

 We calculate it will take fifteen years to complete. 

 Judging from the specimen, a good deal of narrative 

 and adventure will be introduced into the work, and that 

 it will be at least as entertaining as instructive. The 

 first series will deal with polar and maritime expeditions, 

 the second with Africa, the third with Asia, the fourth 

 with Australia, and the fifth with means of communica- 

 tion, geographical societies, &;c. The other five series 

 will be devoted to a description of the nations of European 

 origin and their enterprises over the globe. It is a grand 

 scheme. 



Dr. TholozAN, physician to the Shah of Persia, is 

 organising a scientific exploration of the province of 

 Khuzistan, the southern province of Persia. The expe- 

 dition will start from Bassorah on February i next. 



NOTES 



The Corporation of the City of London having determined 

 to identify themselves with the movement by the Livery Com- 

 panies of London for the advancement of technical education, 

 on Thursday last elected the following to serve on the Board of 

 Governors of " The City and Guilds of London Institute for the 

 Advancement of Technical Education " : — The Lord Mayor 

 ex officio ; the Recorder, ex officio ; Aldermen : Sir Thomas 

 Dakin, Knt. ; Sir Robert W. Carden, Knt. ; Mr. William 

 Lawrence; Sir Francis W. Truscott, Knt.; Sir Wm. A. 

 Rose, Knt., F.R.S.L., F.R.G.S.; Simeon C. Hadley ; Com- 

 mon Council: Mr. Joseph Beck, F.R.A.S.; Mr. W. Bas- 

 singham; Mr. J. L. Shuter, F.R.A.S. ; Sh C. Reed, Knt., 

 LL.D., F.S.A., Dep. ; Mr. George Shaw; Mr. J. Edmeston; 

 H. Lowman Taylor, J.P., Dep. ; S. E. Atkins, Dep. ; Sir 

 Jno. Bennett, Knt. ; Mr. Henry Greene ; ]Mr. John Faulkner ; 

 Mr. Thomas Waller. 



