REPORT ON PHYSICAL OPTICS. 339 



discovery of the laws of the phenomena, will ev-er be considered 

 as a model of experimental inquiry. A convex lens of glass 

 being laid upon a plane surface of the same material, after the 

 manner of Hooke, the bands of the same colour are arranged 

 round the point of nearest approach in concentric circles ; and 

 the diameters of these circles will be obviously as the square 

 roots of the thicknesses of the plate of air at the points at which 

 they are exhibited. In order to investigate the relation between 

 the colour and the thickness, then, it was only necessary to 

 measure the diameters of these rings in the different species of 

 simple light ; and taking similar measurements when the other 

 circumstances of the phenomena were varied, Newton deduced 

 their laws, as they depended on the substance of the reflecting 

 plate, and on the obliquity of the incident pencil. Newton ob- 

 served, moreover, that there was a second system of rings formed 

 by transmissio7i. The transmitted rings were found to observe 

 the same laws, — with tliis remarkable exception, that the colour 

 transmitted at any particular thickness of the plate was alwa3's 

 complementary to that reflected at the same thickness ; so that 

 in homogeneous light, the bright transmitted ring is always 

 found at the same distance from the centre as the corresponding 

 dark one of the reflected system. 



The observations of Mariotte*, Mazeasf , and DutourJ have 

 added nothing essential to the laws discovered by Newton. 

 Most of these observations, in fact, related to the colours ex- 

 hibited by the plate of air inclosed between two plane glasses ; 

 and m circumstances, thei-efore, much less favourable to the 

 analysis of the phenomenon than those selected by Newton. 

 Perhaps the most interesting of the facts noticed by Mazeas are 

 the effects produced on the coloured bands by the application 

 of heat to the glasses, the colours retreating to the edges of the 

 plates, and the bands diminishing in breadth as the temperature 

 was increased. The same author also found, that no. sensible 

 change took place in the phenomenon when the air was flith- 

 drawn by the air-pump. 



In the observations of Dutour, the reflected and the transmitted 

 tints were observed at the same time, the latter being reflected 

 from the second surface of the lower glass, and returning to the 

 eye through the entire system. This latter set of rings is ren- 

 dered more distinct, when the shadow of an opaque body is passed 

 over the upper surface. In this manner the phenomenon was 

 observed by Sir William Herschel ; and it was found that ad- 



* Traite de la Lumicre et des Coideurs. 



t Memoires presentes, torn. ii. + Ibid., tom. iv. v. %i. 



z 2 



