338 



Continuation of Experiments for investigating the Cause of coloured 

 concentric Rings, and other Appearances of a similar Nature. By 

 William Herschel, LL.D. F.R.S. Read March 23, 1809. [Phil. 

 Trans. 1809,^.259.] 



In the former part of this paper the author had pointed out various 

 methods of producing coloured rings between surfaces in contact, and 

 had proved that no other surfaces are concerned in their formation ; 

 and he now proceeds to show, that, by varying the figures of the 

 surfaces, a corresponding change of the form of the rings will be ob- 

 served. As a spherical surface applied to a plane surface produces 

 circular rings, so, when it is applied to a cylindrical surface, the 

 rings become ellipses ; and when the cylinder is applied to a plane, 

 the ellipses become straight lines, and irregular curves occasion irre- 

 gularity in the form of the rings. But Dr. Herschel is of opinion, that 

 plane surfaces, applied to each other at any extremely small angle, 

 cannot exhibit any such appearances of colour unless they are dis- 

 figured by unequal pressure. 



The author next examines the circumstances of the prismatic blue 

 bow observed by Newton, as the limit to the perfect reflection that 

 occurs in the interior of a prism when light is much inclined to its 

 surface ; and Dr. Herschel calls this the critical separation of the 

 colours. He also examines the prismatic red bow observable at the 

 same critical inclination, when the transmitted light prevails over that 

 which is reflected ; and remarks the change from red to blue, occa- 

 sioned by changing the direction of the light. 



The next phenomena observed are the coloured streaks that are 

 produced adjacent to these bows when the reflecting surface of the 

 prism is in contact with another surface ; the streaks being straight 

 when the surfaces are plain, or curved, according to their different 

 kind or degree of curvature. In the experiments which follow, this 

 effect of a surface in contact with the prism is employed as a crite- 

 rion for distinguishing more clearly the acting surface. 



Various more complicated appearances are next examined; in 

 which, by reason of second reflection, the origin of the bows is less 

 distinct, or in which two bows may be seen at once from the parti- 

 cular form of the prism. 



In the formation of bows, it is shown, that only one surface of the 

 prism is concerned by roughening other surfaces, and by applying to 

 them various glasses, without preventing the appearance of the bows. 



Since the streaks which are seen beneath the blue bow, when a 

 plane surface is in contact with a prism, contain all the prismatic 

 colours, it is evident that the less refrangible, after having been 

 transmitted, are reflected back by the contiguous surface, and then 

 re-enter the prism. 



Dr. Herschel next proceeds to delineate, from strict computation, 

 the course of the several rays ; and, by a diagram drawn upon an 

 enlarged scale, shows, that when light falls upon a pair of surfaces 

 nearly in contact, and when single rays or small pencils are separated 



