8$ THEORY OF LIGHT AND COLOURS. 



tlier confirmed by calculations. (Lagrange, Mifc. Taur. Vol. 1. 

 p. 91 . Alfo, much more concifely, in my Syllabus of a courfe 

 of Lectures on Natural and Experimental Philofophy, about to 

 be publifhed. Article 289.) If the impulfe be fo great as ma- 

 terially to difturb the denfity of the medium, it will be no longer 

 homogeneous; but, as far as concerns ourfenfes, the quantity 

 of motion may be confidered as infinitely fmall. It is furprifing 

 that Euler, although aware of the matter of fact, mould ftill 

 have maintained, that the more frequent undulations are more 

 rapidly propagated, (Theor. muf. and Conject-. phys.) It is 

 poflible, that the acluai velocity of the particles of luminife- 

 rous ether may bear a much lefs proportion to the velocity of 

 the undulations than in found; for light may be excited by the 

 motion of a body moving at the rate of only one mile in the 

 time that light moves an hundred millions. 

 Law of the ve- Scholium 1 . It has been demonftrated, that in different me- 

 lociciei, ha aifFer- diums the velocity varies in the fubduplicate ratio of the force 

 directly, and of the denfity inverfely. (Mifc. Taur. Vol. I. 

 p. 91. Young's Syllabus. Art. 294.) 

 Undulations do Scholium 2. It is obvious, from the phenomena of elaftic 

 not mix. bodies and of founds, that the undulations may crofs each other 



without interruption. But there is no neceffity that the vari- 

 ous colours of white light mould intermix their undulations ; 

 for, fuppoling the vibrations of the retina to continue but a 

 five hundredth of a feeond after their excitement, a million 

 undulations of each of a million colours may arrive in diftinct 

 fucceffion within this interval of time, and produce the fame 

 fenfible effect, as if all the colours arrived precifely at the 

 fame inftant. 



PROPOSITION II. 



Prop. II. Na- 4n Undulation conceived to originate from the Vibration of a 

 tureof undula- fingle Particle, mujt expand through a homogeneous Medium 

 in afpherical Form, but icith different Quantities of Motion in 

 different Parts. 



For, fince every impulfe, considered as pofitive or negative, 

 is propagated with a conftant velocity, each part of the undu- 

 lation muft in equal times have paft through equal diftances 

 from the vibrating point. And, fuppofing the vibrating par- 

 ticle, in the courfe of its motion, to proceed forwards to a fmall 

 diitance in a given direction, the principal ftrength of the undu- 

 lation 



tion 



