﻿76 Intelligence and Miscellaneous Articles. 



Arago had no absolute confidence in the measurements obtained by 

 his then method, and sought a more delicate process for the obser- 

 vation of such delicate phenomena than the one he had used. This 

 opinion gains in plausibility when we consider that it was by a sub- 

 sequent discovery that he became possessed of a method of observa- 

 tion (that of interference) which has never been equalled in delicacy. 

 I have been able to apply this process to the measuring the dis- 

 persive power of gases by two different methods. In both cases the 

 interference-fringes were produced by means of M. Billet's semi- 

 lenses. 



First method. — When the ordinary apparatus are used for produ- 

 cing fringes, the experimenter finds that the determination of the dis- 

 persion is impossible if, working with white light, a central zone is 

 discriminated, in reference to which everything is symmetrical, and 

 which can be sighted. The number of fringes becomes considerable 

 when a decidedly homogeneous light is used ; a central region is no 

 longer distinguished ; the fringes of the system resemble each other ; 

 every point of comparison has disappeared, and we no longer know 

 which of the fringes to stop at on the cross wires of the telescope. 

 To measure dispersion it is necessary, therefore, to obtain with ho- 

 mogeneous light a system of fringes limited in number — a condition 

 which has been realized by very simply modifying the semilenses. 



To reduce the number of fringes, we cover by any suitable method 

 (by a small plate or by the removal of the interposed tubes) the in- 

 terior edges of the semilenses. (A lens which, cut, has undergone a 

 loss of matter produces this result in a third manner.) Then the 

 part which is in common, instead of extending indefinitely, may 

 become defined, and the region which succeeds the plane of the maxi- 

 mum of the fringes may terminate in only giving one fringe, as 

 takes place in the region preceding this plane of the maximum. 

 But to observe these fringes the lens alone cannot be employed ; 

 the screen of ground glass must be used, through which they are 

 viewed. This was Fresnel's old method. To compensate the retar- 

 dation, I used M. Billet's compensator, graduated for each colour 

 by a very simple method, which consists in determining for each of 

 them the zero of the instrument, which is variable. 



Two differential tubes containing dry gases were placed in front 

 of the semilenses, behind a collimating lens which had the slit for 

 focus. 



In a special memoir I shall publish the numerous precautions to 

 be taken in the arrangement of the apparatus. In the case of air, 

 the two tubes were filled with dry air under the same pressure, and 

 the air in one of the tubes was gradually rarefied. It was merely 

 necessary to measure (1) the pressures H and H' of the gases, 

 (2) the temperature, (3) the graduation of the compensator. 



For one of the colours investigated the index of the gas was given 

 by the formula 



N =^7IfI±pM (1) 



