Tin; 

 LONDON and EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



FEBRUARY 1833. 



XVI. On the Modification of the Interference of two Pencils 

 of Homogeneous Light produced by causing them to pass 

 through a Prism of Glass, and on the Importance of the 

 Phenomena which then take place in determining the Velocity 

 with which Light traverses refracting Substances, By R. Pot- 

 ter, Jun. Esq.* 



[With Figures : Plate II.] 



T^HE investigation which is the principal subject of the 

 -■■ present paper, arose from my meeting with a peculiar 

 phenomenon in interference, whilst repeating, in a new mode, 

 an experiment first described by Professor Powell in a former 

 Number of the Philosophical Magazinef . The experiment 

 consists in placing a prism of glass in the direction of two 

 pencils of light which interfere; these pencils arising from 

 the reflection of the light which diverges from the image of 

 the sun, given by a lens of short focus, by two mirrors slightly 

 inclined to each other. Professor Powell, by using a prism 

 with a refracting angle of only a few degrees, and common 

 light, believed that he found the same parts of the luminous 

 pencils to interfere, after refraction by the prism, which would 

 have interfered if it had not been interposed, and that the 

 only alteration was in the direction of the pencils, produced 

 by the refraction. I saw that this experiment merited a more 

 rigorous examination than Professor Powell had given to it; 

 and I adapted to an apparatus for trying the experiment of 

 M. Fresnel by two mirrors slightly inclined to each other, a 

 piece carrying a small prism having its smallest refracting 



* Communicated by the Author. 



f See Phil. Mag. and Annals, N.S. vol. xi. p. 1 ; and the preceding 

 volume of the present Journal, p. 433. — Edit. 



Third Series. Vol. 2. No! 8. Feb. 1833. M 



