OF NEWTON'S OPTICS. 



63 



Fig. 55. 



Newton now repeated the first ex- 

 periment described in page 219; but, in- 

 stead of using compound solar light, 

 he used homogeneous light, obtained by 

 a prism placed behind the hole through 

 which the light was transmitted. He 

 found the shadows of all bodies held in 

 this light bordered with fringes, not as 

 before of different colours, but of the 

 colour of the transmitted light. Those 

 made by the red light were largest, and 

 those made by the violet, least; the in- 

 termediate colours having intermediate 

 breadths. 



From these experiments he inferred 

 that the rays which formed the fringes 

 in the red light passed by the body at a 

 greater distance than those which formed 

 the violet or any of the intermediate 

 colours. So that the action of the body 

 on the less refrangible rays at a given 

 distance was equal to its action on the 

 more refrangible rays at a less distance, 

 and thus occasioned the tints of the 

 fringes without changing the colour or 

 properties of any component part of the 

 solar light. 



Such was the state to which Newton's 

 experimental investigations had arrived, 

 when they were unfortunately inter- 

 rupted. In the queries, however, an- 

 nexed to this book, and which have 

 been already alluded to, he throws out 

 some suggestions for the consideration of 

 future inquirers. The following queries 

 relate to the subject with which we have 

 just been engaged. 



1 . Do not bodies act on light at a dis- 

 tance ; and by their action bend its rays ? 

 and is not this action strongest at the 

 least distance? 



2. Do not the rays which differ in re- 

 frangibility differ also in flexibility? and 

 are they not, by their different inflexions, 

 separated from one another, so as, after 

 separation, to make the colours in the 

 three fringes above described ? and after 

 what manner are they inflected to make 

 those fringes ? 





3. Are not the rays of light in passing 

 by the edges and sides of bodies bent 

 several times backwards and forwards 

 with a motion like an eel ? and do not 

 the three fringes of coloured light above- 

 mentioned arise from three such bend- 

 ings? 



4. Do not the rays of light which fall 

 upon bodies, and are reflected or re- 

 fracted, begin to bend before they arrive 

 at the bodies ; and are they not reflected, 

 refracted, and inflected by one and the 

 same principle, acting variously in va- 

 rious circumstances ? 



The theory hinted at in these queries 

 is sufficient to represent the principal phe- 

 nomena of inflexion ; but it must be con- 

 fessed that the undulatory theory affords 

 rather a more satisfactory generalization 

 of those complicated effects, and has been 

 more generally adopted in the investiga- 

 tions of modern philosophers. Neither the 

 material nor undulatory theory can be 

 said to be satisfactorily established ; but 

 it would seem that every phenomenon 

 which can be brought under the former, 

 can also, with equal facility, be explained 

 by the latter ; while there are some known 

 effects in strict accordance with the lat- 

 ter, which cannot, without great difficulty 

 and the introduction of gratuitous hy- 

 pothesis, be accounted for by the former. 

 For the most part, however, the lan- 

 guage of either may be translated into 

 the other. 



To discuss the question respecting the 

 experiments which have been just de- 

 scribed, and the inferences drawn from 

 them, it would be necessary to refer to 

 the original experiments and reasoning 

 of Grimaldi, and the later investigations 

 of Dr. Young and Fresnel. Such de- 

 tails are, however, foreign to the objects 

 of the present treatise. 



The remaining queries which termi- 

 nate this book relate to the probable 

 connexion and causes of heat and light, 

 the sense of vision, the cause of gra- 

 \itation, and other subjects in phy- 



