66 



the difference of the densities. This, it is thought, may be well illus- 

 trated, if not demonstrated, by the analogy of elastic bodies of dif- 

 ferent sizes. When an undulation is transmitted through a surface 

 terminating different media, it proceeds in such a direction that 

 the sines of the angles of incidence and refraction are in the constant 

 ratio of the velocity of propagation in the two media. When an 

 undulation falls on the surface of a rarer medium so obliquely that it 

 cannot be regularly refracted, it is totally reflected at an angle equal 

 to that of its incidence. And if equidistant undulations be supposed 

 to pass through a medium of which the parts are susceptible of per- 

 manent vibrations somewhat slower than the undulations, their ve- 

 locity will be somewhat lessened by this vibratory tendency; and the 

 more so in the same medium, the more frequent the undulations. If 

 we ascribe the sensation of colours to the different velocities of the 

 coloured beams or undulations, this last Proposition will afford a so- 

 lution to the phenomena of dispersion according to the new system. 



When two undulations from different origins coincide either per- 

 fectly or very nearly, in direction, their joint effect is a combination 

 of the motions belonging to each. This is the Eighth Proposition, 

 which, at first sight, appears so consistent with the most obvious 

 mechanical principles, as scarcely to need any illustration ; yet its 

 extensive utility in explaining the phenomena of colours renders it 

 perhaps the most important in the lecture. In a first corollary the 

 author treats of the colours of striated surfaces, where, after showing 

 in what manner these depend on the breadth of the undulations in 

 proportion to the distance and position of minute surfaces, it is shown 

 from original experiments in what manner this circumstance affords 

 a very strong confirmation of the theory. But a still more interesting 

 coincidence is shown in the second and third corollaries, which treat 

 of the colours of thin plates, and of thick plates. It is here explained 

 by what means the breadth and duration of the respective undulations 

 may be deduced from Newton's measures of the thicknesses reflect- 

 ing different colours ; and the law of variation of colour, in conse- 

 quence of the change of obliquity, which is very embarrassing on every 

 other supposition, and had never been reduced to any analogy, is re- 

 ferred to a simple and necessary consequence of the author's theory. 



The whole visible spectrum being estimated to be comprised within 

 the ratio of 3 to 5, the undulations of red, yellow and blue appear to 

 be related to each other in magnitude as the numbers 8, 7, and 6. 

 On these data a table is constructed, showing for each primitive 

 colour, and the intermediate ones between each *pair of them ; 1 . 

 The length of an undulation in parts of an inch in ah". 2. The num- 

 ber of undulations in an inch. And 3. The number of undulations 

 in a second. All these numbers agreeing accurately with the phae- 

 nomena, will probably be considered as a strong evidence in favour 

 of the theory. The appearances of colours in inflected light are like- 

 wise explained in a subsequent corollary. 



The last Proposition may be considered as the general result of the 

 whole investigation; in consequence of which, Dr, Young thinks him- 



