On the Anomalous Dispersion of Carbon. 405 



measure one of the vapour-pressures than the other. Sup- 

 pose pi to be measured, then 



He 



, C x d\ogpi 7 



nee 



and the numerical solution becomes practicable. 



XXXVI. The Anomalous Dispersion of Carbon. y 

 By Prof. E. W. Wood *. 



IT was suggested to me some time ago by Professor Ames 

 that the rapid decrease of amplitude on a wave-front, 

 resulting from its passage through a prism of some strongly 

 absorbing substance, such as cyanine, might not be without 

 influence on the direction of propagation of the transmitted 

 ray. 



In Huygens's construction a constant amplitude is assumed 

 over the wave-front, and it is quite conceivable that varying 

 the amplitude might shift slightly the position of the 

 ' ; effective-point." 



There are obviously two ways of attacking the problem : 

 the mathematical and the experimental. I have been unable 

 to treat the case by any of the geometrical methods, and the 

 simplest way seems to be to determine the form and position 

 of the intensity curve, treating the transmitting edge of the 

 prism as a narrow aperture, the amplitude decreasing across 

 its width according to some linear function, and solving by 

 the method employed in the case of the Praunhofer or 

 telescopic diffraction phenomena. I have not yet attempted 

 the solution in full, but a cursory examination leads me to 

 anticipate that the central maximum will not be symmetrical 

 with respect to the centre of the system. The highest point 

 of the intensity curve will undoubtedly be at its usual place, 

 in the line normal to the centre of the aperture, but the slope 

 may be steeper on one side than on the other. In all deter- 

 minations of the refractive index of strongly absorbing 

 prisms, when working near the absorption-band, the slit 

 image is broadened by diffraction. By setting the cross-hair 

 of the eyepiece on the centre of this broadened image, we 

 assume that we have determined the centre of the system. 

 If, however, the central maximum of the pattern is unsym- 



* Communicated by the Physical Society. 



