15© On the JfeS?ions and Prcperttes of Light. 



and oppofite effedt produced by their different inflexibilities; and the fame thing may in like 

 niaiiner be fliewn to happen in the return of the rays from the body after reflcxioiu But 

 let the rays be fo refledted that they fliall pafs by the body without entering any more than 

 one fphere of flexion ; then thej will be feparated by their flexibilities, as we before defcribed. 

 It appears, then, that if the rays of light were not differently reflexible, flexion could never 

 produce the coloured images, by feparating the compound light. And indeed, this may be 

 eafiiy proved by fad. At 144 feet from the bending body, the greatefl: fringes by flexion 

 are only half.an inch in length, whereas the fourth or fifth images by reflexion are above 

 half an inch at one foot from the reflecting furface: the one fort is therefore more than 

 144 times more diftended than the other, whereas the flexion could, at the very farthel't, 

 only double them. AUo the diftin£lneff, and brightnefs, and regularity of the colouring, 

 are quite diflerent in the two cafes ; the fuppofed caufe would neither account for the order 

 of the colours, nor for their abfence in common fpecular reflexion, and refraction through 

 two prifras joined together with their angles the contrary ways. Laftly, it we fuppofe the 

 images to be produced by flexion, and then reflefted from the body, it would follow that 

 light incident on a prifm (hould be decompounded, formed into feveral coloured images, 

 and then refratSled, the violet being leaft, and the red mod bent ; all which is perfectly the re- 

 verfe ofwhatadlually happens. I have multiplied the proof of this propofition, perhaps beyond 

 what is neceffary; but its great importance to the whole theory will, I hope, plead my excufe; 

 Let us now fuppofe that a homogeneal beam pafiTcs through the fpheres of flexion, it will 

 follow that no divergence can lake place from the bending power of the body ; fo that we 

 have only to eftimate the effedl produced by the reflexion, and to enquire whether the dif- 

 ferent reflexibilities of the rays can caufe the images to vary their fizes according as they 

 are formed by different rays. In fig. 3. let AB be the body, CD the limit of its fphere of 

 reflexion, and IP a beam of homogeneal rays, as red, incident at P and refleiSted to R, form- 

 hig there the image Rr. It is evident that the greater reflexibility of the rays IP can only 

 alter the pofition of the centre of Rr, making it nearer the perpendicular than the centre of 

 an image formed by any other rays would be. But the greater length of Rr fliews that a 

 greater quantity of rays is refle£ted, or that the fame quantity is fpread over a greater fpace, 

 and that in the following way. Let IF f i be a beam of violet-making rays entering AbCD, 

 and refle£led fo as to form the image Rv. The force exerted by AB decrealing according to 

 fome law (of which we are as yet ignorant) as the diftance increafes, is not fufBcient to turn the 

 rays back till they have come a certain length within ABCD. But for the fame reafon it turns 

 back all that it does refleft before they come nearer than a certain diftance : between thefe 

 two limits, therefore, the rays are turned back. But the limits are not the fame to all the 

 rays ; fome begin to be turned at a greater diftance from the body than others, and confe- 

 quentlyare refleded to a greater diftance from the middle ray of the incident beam. Thus 

 if IF fi be changed to a red-making beam, it begins to be turned back at f, and the rays 

 farthcft from AB are reflefled to r inftead of to v, where they fell when IF fi was violet- 

 making ; not but that the fame quantity of rays is reflected : the only difference is, that the 

 moft reflexible are reflefled fartheft from the body by their greater reflexibility, and farthefl: 

 from each other by this other property. Exadlly the fame happens in the cafe of refra£lion, 

 mutatis mutandis ; but there fecms to be a flight variation in the manner in which the dif- ' 

 ferent rays are difpofed into images of different fizes by flexion. In this cafe alfo the bend- 

 ing 



