COLOURS OF THIN FILMS. 301 



in excellence. In illuminating with homogeneous light 

 the very regularly formed series of bands of which 

 Hooke had already made mention, and which originated 

 round the point of contact of two lenses pressed closely 

 together, he proved that for each species of simple 

 colour there exists, in thin films of every substance, a 

 series of thicknesses gradually increasing, at each of 

 which no light is reflected from the film. This result 

 was of capital importance; it included the key to all 

 these phenomena. 



Newton was less happy in the theoretical views which 

 these remarkable observations suggested to him. To 

 say, with him, that the luminous ray which is reflected 

 is "in a fit of easy reflexion," to say that the ray 

 which passes through the film entire, is " in a fit of easy 

 transmission," what is it but to announce, in obscure 

 terms, merely the same fact which the experiment with 

 the two lenses has already taught us ? * 



* In regard to the theory of the " fits," the author here seems to 

 represent Newton's view, as in fact mere tautology; while in other 

 places he is supposed to have indulged in a visionary theory on the 

 subject. Newton, however, expressly says, " what kind of action or 

 disposition this is; whether it consist in a circulating or vibrating 

 motion of the ray, or of the medium, or something else, I do not here 

 inquire." ( Optics, p. 255, ed. 1721.) 



The fact is, Newton in his optical researches expressed the same 

 avowed and systematic dislike to indulging in any gratuitous theories 

 as in his other inquiries, " Hypotheses non fingo " was his motto in 

 these as well as other researches. In adopting the idea of " fits of 

 easy reflexion and transmission," we are of opinion that he did not 

 violate that maxim, and that it was in fact the only legitimate first 

 expression of the conclusion which the facts warranted. At certain 

 points no light appeared; it was the legitimate inference, in the then 

 state of knowledge, that none was reflected. But light was clearly 

 under the same circumstances transmitted; at a distance a little 

 greater along the ray, an opposite effect was witnessed ; and so on. 



