Chap. 9.] Colours ofopake Bodies. <ijt 



ceives only green ones, which are the only rays it can 

 tranfmit. 



If the rays of the fun are made to fall very obliquely 

 upon the interior furface of a prifm, the violet-colour- 

 ed rays will be reflected, and the red, &c. will be 

 tranfmitted ; if the obliquity of incidence is augment- 

 ed, the blue will be alfo reflected, and the other tran- 

 mitted ; the reafon of which is, that the rays which 

 have the rnoft refrangibility are alfo thole which are the 

 eafieft reflected *. 



In whatever manner we examine the colour of a 

 fmgle prifmatic ray, we mall always find, that neither 

 refraction, reflection, nor any other means, can make 

 it forego its natural hue; but if we examine the artifi- 

 cial colouring of bodies by a microfcope, it will ap- 

 pear a rude heap of colours, unequally mixed. If we 

 mix a blue and yellow to make a common green, it 

 will appear moderately beautiful to the naked eye; but 

 when we regard it with microfcopic attention, it feems 

 a confufed mafs of yellow and blue parts, each particle 

 reflecting but one feparate colour. - 



To determine the caufe of the permanent colours of 

 opake bodies, a feries of experiments was inftituted 

 by Mr. Delaval, as noticed in the hiftorical part of this. 

 book. He prepared a great variety of coloured fluids, 

 which he put in phial bottles of a fquare form. The 

 backs of thefe phials he coated over with an opake 

 fubftance, leaving the front of the phial uncovered, and 

 the whole of the neck. On expofing them to the in- 

 cident light, he found, that from the parts of the phials 

 which v/ere covered at the back no light whatever was 

 reflecte \ but :t was perfectly black, while the light 

 tranfmitted through the uncoated parts of the phials 



* Briflbn, Traite-EIem. de Phyfiguc, tora. ii. page 361. 



was 



