OF NEWTON'S OPTICS. 



31 



constructing a concave reflector of glass, 

 as being in some respects preferable to 

 metal, but does not seem to have car- 

 ried this into effect. Newton was fully 

 aware of the defects of reflecting tele- 

 scopes compared with refractors, owing 

 to the much greater loss of light in 

 reflection, and the greater aberration 

 proceeding from their spherical form. 

 These, however, he thought inconsider- 

 able when compared with those defects 

 of the refracting telescope, which pro- 

 ceeded from the unequal refrangibility 

 of light. 



CHAPTER V. 

 The Theory of Colours. 



(39.) THE colours exhibited by refracted 

 and reflected light were phenomena with 

 which philosophers had been familiar 

 before the time of Newton. These ef- 

 fects were generally ascribed to the ac- 

 tion of the reflecting or refracting body, 

 and to the edges of opaque bodies which 

 marked the limits of shadow, in impart- 

 ing to the light qualities which it did not 

 possess before encountering these bodies. 

 Thus it was thought, that in passing 

 through glass or other transparent sub- 

 stances formed into a prism, the solar 

 beam is endued with a virtue by the ac- 

 tion of the medium upon it, by which it 

 reddens, or otherwise colours any body 

 which it afterwards illuminates. In like 

 manner, it was supposed, that in passing 

 the edge of an opaque body a similar 

 effect might be produced. 



Before he proceeds to explain and 

 establish his theory of colours, Newton 

 shows that this hypothesis of his prede- 

 cessors is untenable and inconsistent 

 with facts. The coloured spectrum be- 

 ing produced by the prism in the usual 

 way, the lights of the several colours 

 may be successively intercepted by the 

 interposition of an opaque body, so that 

 any one of the colours may bound its 

 shadow. These colours will remain un- 

 altered by thus passing the edge of the 

 opaque obstacle ; and therefore he con- 

 cludes that the light in passing the body 

 receives no modification which affects 

 its colour. 



He further shows, that the same light, 

 refracted in the same manner, passing 

 the same opaque edges, will throw upon 

 the paper which it illuminates different 

 colours, according to the direction in 

 which the paper is placed with respect 

 to the rays. He argues, that if the co- 



lorific property were a virtue imparted 

 to the ray by the edges of the aperture 

 through which the light is admitted, or 

 by the refracting medium through which 

 it has passed, this could not happen, 

 inasmuch as the colouring quality would 

 then be independent of the position of 

 the paper. 



But perhaps the most conclusive ar- 

 gument against this theory is derived 

 from the experiment explained in (32). 

 It appears in that experiment that the 

 confines of shadow produce no effect 

 whatever ; for the colour of the whole 

 of the light emerging from the com- 

 pound prism is always the same, that 

 in the middle of the beam being in 

 nowise different from that at the bor- 

 ders. Neither can the colour proceed 

 in this case merely from the action 

 of the glass, because it changes from 

 white to yellow, orange, red, &c., that 

 action remaining the same. Besides 

 this, the refractions being equal, and 

 in contrary directions, would mutually 

 destroy each other's effects. It may 

 further be argued, that if the light owed 

 its colour to the action of the glass, it 

 would not have the colour before its 

 passage through the prism K I H ; yet 

 it was found, in that experiment, that 

 when all the colours in the spectrum P 

 were made to vanish, except the red, 

 the light producing that colour on the 

 screen P was found to produce the same 

 colour on a screen which received it be- 

 tween the compound prism and K I H, 

 before it was refracted by the latter. 

 Thus the light which reddens the screen 

 P would also redden it if unrefracted by 

 the prism K I H, and the same may be 

 said of the lights of other colours. 



From these and, indeed, all other ex- 

 periments which have been described, it 

 abundantly appears, that " all homo- 

 geneous light has its proper colour an- 

 swering to its degree of refrangibility, and 

 that this colour is unalterable, either by 

 refraction or by reflection." When pure 

 homogeneous light of any colour illu- 

 minates a body, whatever the natural 

 colour of that body may be, it will ap- 

 pear, when so illuminated, to have the 

 colour of that light only which shines 

 upon it. The apparent colour of the 

 body is that of the light which it re- 

 flects ; and it can reflect no light but 

 that which shines upon it. Thus if a 

 body whose natural colour is blue be 

 placed in a dark chamber, and illumi- 

 nated by the red light of the prismatic 

 spectrum, it will appear red ; and, on 



