90 INTERFERENCE OF LIGHT. 



(107) The phenomenon of interference is displayed in a 

 striking manner by the mutual action of direct and reflected 

 light ; and the experiment in this form is more manageable 

 than that of Fresnel. We have only to take a piece of plate 

 glass, or a metallic reflector, and to place it in such a posi- 

 tion that the rays diverging from the luminous origin shall be 

 reflected at an angle of nearly 90. A screen placed on the 

 other side of the mirror will receive both the direct and re- 

 flected pencils ; and as they meet under a small angle, and 

 have traversed paths differing by a small amount, they are in 

 a condition to interfere. The phenomenon is better seen, 

 however, by receiving the two pencils upon a lens, behind 

 which the eye is placed.* It will be readily seen that the sys- 

 tem of bands formed in this manner is but half of that pro- 

 duced in Fresnel's experiment. 



(108) There is yet another mode of studying the funda- 

 mental phenomenon of interference, which is in some respects 

 more convenient than any of the former. It is obvious that 

 the original beam may be separated by refraction, as well as 

 reflexion ; and if the inclination of the two refracted pencils 

 be small, similar results will take place. For this purpose it 

 is only necessary to procure a prism with a very obtuse angle, 

 and to allow the beam of light to fall perpendicularly on the 

 opposite face. It is evident that this beam will be differ- 

 ently refracted, at emergence, by the two faces which con- 

 tain the obtuse angle ; and that it will thus be divided into 

 two beams, which will be slightly inclined. These beams 

 then proceed from one common origin, and meet under a 

 small obliquity, and therefore fulfil all the conditions neces- 



* The details of this experiment are fully described in a Paper by the 

 Author, published in the seventeenth volume of the Transactions of the Royal 

 Irish Academy. 



