COLOURS OF THIN PLATES. 75 



uncertainty in the results. Where luminous objects are so small 

 that the eye cannot readily distinguish parts, the absolute quan- 

 tity and the intensity of the light are confounded. I am not 

 aware how far this may have been the case in Mr. Potter's instru- 

 ment ; but it is remarkable that, if we suppose the quantities of 

 light reflected from the two glasses to have been taken as the 

 terms of comparison, the calculated results will accord very closely 

 with theory.* 



When a beam of light falls upon two plates superposed, some 

 of the many portions into which it is divided by partial reflexion 

 at the bounding surfaces are often in a condition to interfere and 

 exhibit colour. Thus, when light is transmitted through two 

 parallel plates, slightly differing in thickness, the colour produced 

 will be that corresponding to the difference, and will be indepen- 

 dent of the interval of the plates. This phenomenon was ob- 

 served by Mr. Nicholson,! and was shown by Dr. Young to arise 

 from the interference of two pencils, one of which is twice reflected 

 within the first glass, and the other twice reflected in the second. 

 Sir David Brewster observed a similar case of interference pro- 

 duced by two plates of equal thickness, slightly inclined, the thick- 

 ness traversed in the two plates being altered by their inclination. 

 In both these cases, however, the interfering pencils are mixed 

 up with, and overpowered by, the light directly transmitted, and 

 some contrivance is necessary to make the fringes visible. The 

 phenomena are much more obvious in the light reflected by both 

 plates, which, on account of their inclination, is separated from 

 the direct light. It is obvious, in fact, that the direct image 

 of a luminous object seen through the glasses will be accom- 

 panied by several lateral images, formed by two, four, six, etc., 

 reflexions. These images Sir David Brewster observed to be 

 richly coloured. The bands are parallel to the line of junction of 

 the two glasses, and their breadth is greater the less the inclination 

 of the plates. J The colours in the first lateral image are produced 

 by the interference of the pencils which have undergone two re- 

 flexions, one of them being reflected internally by the first plate 

 and externally by the second, while the other is reflected inter- 



See Phil. Mag. vol. v. p. 441. 



f Nicholson's Journal, vol. ii. p. 312. 



I Edinb. Trans., vol. vii. p. 435. 



