346 FOURTH REPORT — 1834. 



the case of crown glass. But, independently of the uncertainty 

 connected with the empirical law which is taken by Mr. Potter 

 as the basis of his computation in these deductions, the photo- 

 metrical method itself seems to be open to objection. It appears 

 to be assumed, in the application of that method, that where the 

 quantity of light incident upon an irregularly reflecting surface 

 is given, the quantity of reflected light will be the same in its 

 entire amount, and in all directions, whatever be the incidence. 

 This seems to be contradicted by obvious facts. There is yet 

 another difficulty in the application of this method which appears 

 to leave room for some uncertainty in the results. AVhere lu- 

 minous objects are so small that the eye cannot readily distin- 

 guish parts, the absolute quantity and the intensity of the light 

 are confounded. I am not aware how far this may have been 

 the case in Mr. Potter's instrument ; 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 cal- 

 culated results mil accord very closely with theory*. 



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

 of the many portions into M'hich 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 pro- 

 duced will be that corresponding to the difference, and will be 

 independent of the interval of the plates. This phenomenon 

 was observed by Mr. Nicholson f, and ^vas 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 produced by two plates of equal thickness, slightly 

 inclined, the thickness traversed in the two plates being altered 

 by their inclination. In both these cases, however, the inter- 

 fering pencils are mixed np with, and overpowei'ed 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, and which, on account of their 

 incluiation, 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 accompanied by several lateral images, formed 

 by 2, 4, 6, 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 



• See Phil. Mag. vol. v. p. 411. 



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



