210 Mr. Charles Tomlinson on 



disk may form an acute angle with the plane mirror — this 

 arrangement will separate the two images of the spectrum, 

 which is now to be observed. Suppose a green disk be 

 employed : the image of the spectrum, obtained from the 

 second surface of the green disk, will be entirely deficient 

 in red rays ; it will be, in fact, a spectrum deprived of red 

 and orange ; while the image obtained from the reflection 

 of the first surface will be entire. 



5. The first surface of coloured glass, considered hy itself, 

 is simply a reflecting surface; and objects presented to it 

 are returned unaltered in colour. This occurs when the 

 reflection from the first surface is obtained in such a 

 manner, that the second surface does not act ; but, in every 

 case, when white light falls upon the first surface, a portion 

 enters the glass and is absorbed, while another portion is 

 reflected unaltered from the first surface : but suppose the 

 coloured glass be arranged in a manner favourable to the 

 attainment of reflections from the two surfaces, and this is 

 eminently the case in the Perichromascope, and an object 

 illuminated by white light is presented to the coloured 

 glass ; of the white light so impinging on the first surface, 

 a principal portion is at once chromatically divided into 

 two complementary portions ; one portion, which is homo- 

 chromatic with the glass, enters readily and reflects the 

 imao-e of the same colour from the second surface ; another 

 portion, complementary in colour, does not enter the glass, 

 but is reflected by the first surface, mingled, however, with 

 a small portion of undecomposed light which did not enter 

 the glass : this gives the image from the first surface the 

 faint grayish appearance, which has been already noticed 

 by Mr. Cooper. — Records, vol. ii. p. 176. 



6. It is natural, then, to expect that if white light be 

 chromatically divided on entering the glass, that the colour 

 of the reflection from the second surface, combined with 

 the colour reflected from the first surface, will, in every 

 case, aff'ord all the colorific rays necessary to constitute 

 white light : and such, indeed, is the case ; the primary 

 colour and its complement always affording red, yellow, 

 and blue. These I assume to be the fundamental colours. 

 Were I to adopt Mr. Cooper's theory, which supposes them 

 to be red, green, and violet, I confess I should be unable 



