Dr. Gilly in Reply to Mr. Varey. 361 



©f the light is, besides, veflecteil perpendicularly by the second 

 surface, and emerges at the place wliere it entered ; but the 

 portion of light reflected irregularly at this second surface ra- 

 diates in ali directions from the point of reflection. The lu- • 

 minous molecules, therefore, of which it consists, traverse the 

 o-lass a second time, but in a different direction ; and thus the 

 fength of their fits changes both from the diflferent extent of 

 then- ])aths, and from the obli(juity of their directions to the re- 

 flecting surfaces. Hence it follows that, in returning to the first 

 surface of the mirror, some of these particles are in fits of 

 transmission, and others in fits of reflection. Knowing therefore 

 the length of their primitive paths, and also that which they 

 describe in their return, and the ratio in which the, fits of the 

 particles are enveloped by obliquity, it is easy to calculate in 

 what points each colour ought to emerge, and in what points 

 the other colours ought to be reflected anew. By following the 

 light emerging into the air according to the ordinary law of 

 refraction, one can calculate the diameter of the rings whicii ought 

 to be formed upon a white surface at any distance from the mir- 

 ror. This is what Newton has done, for plates which had a 

 thickness of nearly a quarter of an inch; and the results were ex- 

 actly conformable to the calculations even when the particles ill 

 first traver:.in.g the plate had suffered more than 34o8G fits. 



In t!ie experiments of x\i. Brewster, hou-cver, the equal thick- 

 ness of the two plates :uid the small iiiclinatiou of their surfaces 

 appear to me to act the part of the equal curvature of the two 

 reflecting surfaces in Newton's experiments, the inclination of 

 the plates having the same influence as the sphericity in chan- 

 ging the length of their paths. It therefore appears to me pro- 

 bable that the two results may be calculated by the same for- 

 mula:, and I propose in a short time to verify this supposition. 

 But, whatever be the result, I have thought that natural philo- 

 sophers would peruse with pleasure these details relative to ex- 

 periments which will probably lead to a thorough knowledge of 

 the manner in which the coloured rings are produced. 



LX\'II. Dr. GiLisY in Rtpli/ to Mr. Farey on the Slratificd- 

 liun of Grtat Britain, 



To Mr. Tllloch. 



Sir, — X OUR readers will (I doubt not) be heartily tired of a 

 coutroversv which is neither productive of interest or instruc- 

 tion, and which consists of little else than recrimination bandied 

 irom one side to the other. 1 foresaw that a few expressions 



in 



