430 G. K. Gilbert — Congress of Geologists. 



There is a slight difficulty to be rioted in consequence of the 

 fact that there are two series of waves in sodium light. The result 

 of this superposition of these is that as the difference of path 

 increases, the interference becomes less distinct and finally dis- 

 appears, reappears, and has a maximum of distinctness again, 

 when the difference of path is an exact multiple of both wave- 

 lengths. Thus there is an alternation of distinct interference 

 fringes with uniform illumination. If the length to be meas- 

 ured, the centimeter for instance, is sach that the interference 

 does not fall exactly at the maximum — to one side by, say, 

 one-tenth the distance between two maxima, there would be 

 an error of one-twentieth of a wave-length requiring an arith- 

 metical correction. 



Among other substances tried in the preliminary experiments, 

 were thallium, lithium, and hydrogen. All of these gave 

 interference up to fifty to one hundred thousand wave-lengths, 

 and could therefore all be used as checks on the determination 

 with sodium. It may be noted, that in case of the red hydro- 

 gen line, the interference phenomena disappeared at about 

 15,000 wave-lengths, and again at about 45,000 wave-lengths : 

 so that the red hydrogen line must be a double line with the 

 components about one-sixtieth as distant as the sodium lines. 



Art. XLYIII. — The work of the International Congress of Geolo- 

 gists ; by G. K. Gilbert. 



[Address delivered before the Section of Geology and Geography of the Ameri- 

 can Association for the Advancement of Science, at the New York meeting, 

 August 10, 1887.] 



Eleven years ago this Association met at Buffalo. It was the 

 year of the Centennial Exhibition, and we were honored by the 

 presence of a number of European geologists. This naturally 

 opened the subject of the international relations of geology, and 

 the proposition to institute a congres's of geologists of the world 

 took form in the appointment by the association of an Interna- 

 tional Committee. The project thus initiated found favor else- 

 where, and there resulted an international organization, which 

 up to the present time has held three meetings. It convened 

 first at Paris, in 1878, then at Bologna in 1881, and at Ber- 

 lin in 1885.* Its next meeting will be held in London next 

 year, and an endeavor will be made to secure for the United 



* The report of the Paris meeting, entitled Congres international de Geologie is 

 a document of the Exposition universelle Internationale de 1878, d Paris, and cov- 

 ers 313 pages, 8°. The report of the Bologna meeting was printed by the Con- 

 gress: "Congres geologique international, compte rendu de la 'Ime session, Bologna, 

 1881;" pp. xv + 663, 8°. The official report of the Berlin meeting has not yet 

 appeared ; but its proceedings are abstracted in " The work of the International 

 Congress of Geologists, and of its Committees. Published by the American 



