12 EIGHTH REPORT — 1838. 



sible, that a very intense heat might discharge the blue tint altogether, 

 but it is difficult to obtain satisfactory results with a mineral which de- 

 crepitates by the action of heat, and thus prevents the observer from 

 comparing the tints under circumstances exactly the same. 



An Account of certain New Phenomena of Diffraction. 

 By Sir D. Brewster. 



The phenomena of the inflexion or the diffraction of light observed 

 by Sir Isaac Newton, Fresnel, and others, were those which are visible 

 at a greater or less distance behind the diffracting body, and according 

 to the undulatory theory they are produced by the secondary waves 

 which fall converging on the points where the fringes appear within and 

 without the geometrical shadow. These fringes are all calculable by a 

 formula given by Fresnel, depending on the relation of the two quan- 

 tities a and b, a being the distance of the place where the fringes are 

 formed from the diffracting body, and b the distance of the diffracting 

 body from the point from which the beam of light diverges. In the 

 phenomena hitherto studied, the quantity a is always positive. The 

 new phenomena discovered and described by Sir David Brewster are 

 those in which a is negative ; and they may be represented by a formula 

 differing from Fresnel's only in the sign of a. These new phenomena 

 are rendered visible by bringing lenses of different foci in contact with 

 the diffracting body, and the fringes seen in any case are those belong- 

 ing to a value of — a equal to the focal distance of the lens. The fringes 

 are in this case produced by the secondary waves, which proceed 

 diverging from the main wave, from a point between the diffracting body 

 and the luminous centre, whose distance from the former is a. When 

 — a is equal to b, the fringes are formed in parallel rays ; and when the 

 diffracting body is placed between the lens and the eye, they are formed 

 in converging rays. Hence, in studying these phenomena, we may use 

 a telescope with a micrometer, and obtain accurate measures. These 

 phenomena were illustrated by diagrams. 



An Account of an Analogous Series of Neio Phenomena of Diffrac- 

 tion lohen produced by a Transparent Diffracting Body. By Sir 

 D. Brewster. 



These phenomena, when carefully produced by the various methods 

 which he explained, exhibited a series of splendidly coloured bands of 

 light, sometimes perfectly symmetrical and sometimes unsymmetrical, 

 accordingly as the diffracting body was regular or irregular in its section ; 

 and the author remarked, that an instrument could thus be constructed 

 for giving new patterns of ribands of all forms and colours. The theory 

 of the phenomena he considered quite simple and obvious, but he stated 

 that a comparison of the results of theory and experiment would be 

 difficult, from the difficulty of ascertaining the exact form of the dif- 

 fracting body. 



I 



