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. 
