THEORY OF LIGHT AND COLOURS. 165 



propofition, that when light is difperfed by refraaion, the 

 corpufcles of the refrading fubftance are in a date of aftual 

 alternate motion, and contribute to its tranfmiflion ; but it 

 mud be confefled, that we cannot at prefent form a very de- 

 cided and accurate conception of the forces concerned m 

 maintaining thefe corpufcular vibrations. 



PROPOSITION VIII. 

 When tzvo Undulations, from different Origins, coincide either Prop. VIII, 

 perfeaiy or very nearly in Direaion, their joint Effect is a JJj^^J^ 1 

 Combination of the Motions belonging to each, region will 



Since every particle of the medium is affected by each un- 

 dulation, wherever the directions coincide, the undulations 

 can proceed no otherwife than by uniting their motions, fo 

 that the joint motion may be the fum or difference of the fepa- 

 rate motions, accordingly as fimilar or diffimilar parts of the 

 undulations are coincident. 



I have, on a former occafion, infifted at large on the appli- 

 cation of this principle to harmonics; (Phil. Tranf. for 1800, 

 p. 130.) and it will appear to be of ftill more extenfive uti- 

 lity in explaining the phenomena of colours. The undulations Effe&s when 

 which are now to be compared are thofe of equal frequency, fal^cy. 

 When the two feries coincide exactly in point of time, it is 

 obvious that the united velocity of the particular motions muft 

 be greateft, and, in effect at leaft, double the feparate velo- 

 cities ; and alfo, that it muft be f mailed, and if the undula- 

 tions are of equal ftrength, totally deftroyed, when the time of 

 the greateft direct motion belonging to one undulation coin- 

 cides with that of the greateft retrograde motion of the other. 

 In intermediate ftates, the joint undulation will be of inter- 

 mediate ftrength ; but by what laws this intermediate ftrength 

 muft vary, cannot be determined without further data. It is 

 well known that a fimilar caufe produces in found, that effeel The beat in 

 which is called a beat ; two feries of undulations of nearly foun<1 - 

 equal magnitude co-operating and deftroying each other alter- 

 nately, as they coincide more or lefs perfectly in the times of 

 performing their refpective motions, 



Coroll ar y 1 . Of the Colours ofjlriated Surfaces, 



Boyle appears to have been the firft that obferved the co- Colours of ftri- 

 lours of fcratches on polilbed (urfaces. Newton has not no- atcd fj? 6 "* 



ticed explained, 



