S86 Profs. Trowbridge and Wood on Groove-Form . 



centimetre the radius of curvature was 12*9 cm. Thus, 

 using electromagnetic units, 



,2 



™ =2-lxl(F, 



e 



and taking the result obtained from the photographic method 

 of measuring the magnetic deflexion, we have 



mv C K 1M 



— =6'DX 1(K 

 e 



Hence v = 3*23 X 10 7 centimetres per second and — =49*7. 



1 in 



Now since for the hydrogen ion in electrolysis — = 9*6 x 10 3 



the result of these experiments gives the value for the 

 atomic weight of radium B as 194. Considering the diffi- 

 culty of the experiments this number is in good agreement 

 with the theoretical value 214. Also it will be noticed 

 that the velocity of the particles has very nearly the value 

 3*27 x 10 7 centimetres per second, calculated on the assump- 

 tion that the momentum of the recoil-atoms of radium B is 

 equal to that of the a-particle causing it to recoil, and that 

 the atomic weight of radium B is 214. 



It is with pleasure that we take this opportunity of 

 thanking Professor Rutherford for the facilities he has 

 afforded us for carrying out these experiments in his labora- 

 tory and also for many valuable suggestions during the course 

 of the work. 



XCVIII. Groove- Form and Energy Distribution of Diffrac- 

 tion Gratings. By Augustus Trowbridge, Professor of 

 Physics, Princeton University, and B. W. Wood, Pro- 

 fessor of Experimental Physics, Johns Hopkins University* . 



PRACTICALLY no rigorous investigation has ever been 

 made of the distribution of energy among the spectra 

 of different orders formed by a diffraction grating, as a 

 function of the wave-length of the light and the form of 

 the grooves. The chief obstacle in the way of such a 

 study is the difficulty of obtaining an exact knowledge of 

 the nature of the furrow cut by a diamond point upon a 

 surface of glass or speculum metal. Microscopical examina- 

 tion teaches us very little or nothing in the case of such fine 

 markings, and it is not safe to infer that the groove will 

 conform at all to the ruling point. 



* Communicated by the Authors. 



