OPTICS. 



manner ; behind that hole in the second 

 board place a prism, with its refracting 

 angle downward, turn the first prism 

 slowly about its axis, and the light will 

 move up and down the second board ; let 

 the colours be transmitted successively, 

 and mark the places of the different co- 

 loured rays on the wall after their refrac- 

 tion by the second prism, the red will ap- 

 pear lowest, the violet highest, the rest 

 in the intermediate places in order. Here 

 then the light being very much simpli- 

 fied, and the incidences of all the rays 

 on ihe second prism exactly the same ; 

 the red was least refracted, the violet 

 most, &c. 



The permanency of these original co- 

 lours appears from hence, that they suf- 

 fer no manner of change by any number 

 of refractions, as is evident from the last 

 mentioned experiment ; nor yet by re- 

 flection ; for if any coloured body be 

 placed in simplified homogeneous light, it 

 will always appear of the same colour of 

 the light in which it is placed, whether 

 that differ from the colour of the body or 

 not ; e, g. if ultra marine and vermilion 

 be placed in a red light, both will appear 

 red; in a green light, green; in a blue 

 light, blue, &c. It is, however, to be al- 

 lowed, that a body appears brighter when 

 in a light of its own colour than in another; 

 and from this we see that the colours of 

 natural bodies arise from an aptitude in 

 them to reflect some rays more copious- 

 ly and strongly than others ; but lest this 

 phenomenon should produce a doubt of 

 the constancy of the primary colours, it 

 is proper to assign the reason of it, which 

 is this : that when placed in its own co- 

 loured light, the body reflects the rays of 

 the predominant colour more strongly 

 than any of those intermixed with it ; 

 therefore the proportion of the rays of 

 the predominant colour to those of the 

 others, in the reflected light, will be 

 greater than in the incident light; but 

 when the body is placed in a light of a 

 different colour from its own, for a similar 

 reason the contrary effect will follow, i. e. 

 the proportion of the predominant colour 

 to the others will be less in the reflected 

 than in the incident light, and therefore 

 as its splendour would be greater in the 

 former case, and would be less in the lat- 

 ter than if all the rays were equally re- 

 flected, the splendour of the predominant 

 colour will be much greater in the former 

 case than in the latter. 



As a solar ray was separated into seve- 

 ral others of different colours, so, on the 

 contrary, from these homogeneous ravs 



VOL. V. 



a ray of heterogeneous light may be com- 

 pounded, perfectly corresponding both in 

 appearance and properties with the solar 

 rays. 



The coloured rays (fig^. 16) diverging 

 from the prism are received by a double 

 convex lens, at the distance of twice its 

 focal length from the hole ; at the same 

 distance behind the lens, where they are 

 collected by its refraction, they are re- 

 ceived on a second prism, whose refract- 

 ing angle is equal to that of the former; 

 the divergence of the homogeneous rays 

 that would otherwise ensue, is counter- 

 acted by the second prism, and they are 

 made to proceed parallel to each other 

 from the place of their intersection, and 

 therefore are all compounded and mixed 

 together in the emergent ray A B, which 

 is exactly of the same appearance with 

 the solar rays, and by experiments made 

 on it similar to those usually made in 

 solar light, is found to possess the same 

 properties. 



Since then, 1. A solar ray may be re- 

 solved into several differently colon ted 

 rays ; 2. Since their colours are immutable 

 either by reflection or refraction, and 

 therefore probably not generated in those 

 operations ; and 3. Since from the mix- 

 ture of those coloured rays solar . light 

 may be formed, it seems an indisputable 

 conclusion that the differently coloured 

 rays do exist in solar light previously to 

 any separation that takes place in expe- 

 riments. 



White is compounded of all the pri- 

 mary colours, mixed in their due propor- 

 tions, for if a solar ray be separated by 

 the prism into its component parts, and 

 at a proper distance a lens be so placed 

 as to collect the diverging coloured rays 

 again into a focus, a paper placed per- 

 pendicularly to the rays in this point will 

 exhibit whiteness. 



The same conclusion may be drawn 

 from the experiment of mixing together 

 paints of the same colours as the parts of 

 the spectrum, and in the same proportion ; 

 the mixture will be white, though not of 

 a resplendent whiteness, because the 

 colours mixed are' less bright than the 

 primary ones ; this may likewise be prov- 

 ed, by fixing pieces of cloth of all the 

 different colours on the rim of a wheel, 

 and whirling it round with great velocity, 

 it will appear to be white. Though seven 

 different colours are distinguishable in 

 the prismatic spectrum, yet, upon exa- 

 mining the,, matter with more accuracy, 

 we shall see that there are, in fact, only 

 H 



