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 phaenomena of dispersion according to the new system.
When two undulations from diiferent origins coincide either per- fectly or very nearly, in direction, their joint etfect is a combination of the motions belonging to each. This is the Eighth Proposition, which, at first sight, appears so consistent with the most obviorm mechanical principles, as scarcely to need any illustration; yet its extensive utility in explaining the phzenomena 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 difi'erent 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 air. 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 phas- 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-
