267 



He next examines which are the reflecting surfaces, by means of 

 certain scratches, and other defects. 



In the 25th and 26th section he finds, by means of similar defects, 

 that the surfaces in contact are alone concerned in the formation of 

 rings ; and in the 24th and 28th section he discovers, by various ir- 

 regular surfaces which he employs, as 1st and 4th surfaces of two 

 glasses in contact, that these are not concerned in the production of 

 rings. And in the 27th section he observes, that the colour of the 

 under glass does not affect the primary set of rings. 



The results of the foregoing experiments are, 



I. That only two of the surfaces are essential to the formation of 

 concentric rings. 



II. That these two must be of a certain regular construction, so as 

 to form a central contact. 



III.' That rays, from one side or the other, must pass through one 

 of the surfaces at or near the point of contact to the other surface, 

 and be reflected from it. 



And IV. That in all these cases a set of rings will be formed, hav- 

 ing their common centre in the point of contact. 



The cause of these phenomena, Dr. Herschel says, must be 

 either in the nature of the rays themselves, or in the surfaces ; and 

 if it can be shown that the disposition to fits of easy transmission and 

 reflection does not exist, a proposition of accounting for them by 

 modifications occasioned by the surfaces, he thinks, will find a ready 

 admittance. 



In section 30, he shows that the word transmission will not apply 

 to the case where rings are produced by placing a lens upon a me- 

 tallic surface, and wishes to substitute the word absorption. 



In section 31, Dr. Herschel contends that a plate of air, of the 

 thinness which is supposed sufficient, will not give coloured rings, 

 because in a case of circumferential contact, where a concave surface 

 was applied to one that was convex, of very little larger radius, he 

 could not perceive any appearance of colour. 



In section 32, he places a piece of plain glass, four tenths of an 

 inch square, on a concave glass mirror of 1 feet focus, but could 

 observe no rings or colours. 



In section 33, he does not find that a secondary set of colours, 

 produced in the usual way, is altered by being seen through a wedge 

 of air, occasioned by the interposition of card between the edges of 

 two slips of glass. 



And finally, in section 34, Dr. Herschel could discern no colours 

 when two slips of plain glass, two inches long, were in contact at 

 one extremity, and distant only -nrWth of an inch at their other ex- 

 tremities ; although in the first half-inch from their contact, the se- 

 veral distances which Sir Isaac Newton considers as capable of pro- 

 ducing ten successions of colours, must have occurred. 



Dr. Herschel therefore infers, that the rays of light have no dis- 

 position to be alternately reflected and transmitted at certain intervals 

 of space ; but the examination of the various modifications that light 



