COLOURS OF THIN PLATES. 135 



it has been already stated, Newton framed the hypothesis 

 of the fits of easy reflexion and transmission already referred to. 

 Its application is obvious. The molecule of light is in a, fit of 

 easy transmission in its passage through the first surface ; this 

 is succeeded by a fit of easy reflexion, and so on alternately, 

 the spaces traversed during the continuance of the fits being 

 all equal. On arriving at the second surface, therefore, the 

 molecule will be in a fit of easy transmission, or easy reflex- 

 ion, according as the interval of the surfaces is an even or 

 an odd multiple of the length of the fit. Thus the alter- 

 nate succession of bright and dark rings, and the arithmetical 

 progression of the thicknesses at which they are exhibited, 

 are explained. 



To explain the second law, it is necessary to suppose that 

 the length of the fits varies ivith the colour of the light, being 

 greatest in red light, least in violet, and of intermediate mag- 

 nitude in light of intermediate refrangibility. Newton de- 

 termined the absolute lengths of these fits for the rays of 

 each simple colour, and found that they bore a remarkable 

 numerical relation to the lengths of the chords sounding the 

 octave. 



To account for the two remaining laws, Newton was con- 

 strained to make new suppositions, and to attribute pro- 

 perties to the fits which are inconsistent with every physical 

 account which has been given of them. Thus, to explain 

 the dilatation of the rings with the obliquity, he assumed 

 that the length of the fits augmented with the incidence, and 

 nearly in the ratio of the square of the secant of the angle of 

 incidence. This assumption is inconsistent with the physical 

 theory. If the fits are produced by the vibrations of 

 the ether which are propagated faster than the lumi- 

 nous molecules, and which alternately conspire with and 

 oppose their progressive motion, their lengths should 



