624 



OK THE THEORY OF I-IGTIT AND COLOURS. 



been vejected without any good grounds, 

 and will probably very soon recover its po- 

 pularity. 



Let us suppose, that these vibrations are 

 less frequent than those of light; all bodies 

 therefore are liable to permanent vibrations 

 slower than those of light; and indeed almost 

 all are liable to luminous vibrations, either 

 when in astate of ignition, or in the circum- 

 stances of solar phosphori ; but much less 

 easily, and in a much less degree, than to 

 the vibrations of heat. It will follow from 

 these suppositions, that the more frequent lu- 

 minous undulations will be more retarded 

 than the less frequent ; and consequently, 

 that blue light will be more refrangible than 

 red, and radiant heat least of all ; a conse- 

 quence which coincides exactly with the 

 highly interesting experiments of Dr. Her- 

 schel. (Phil. Trans. 1800.284.) It may also 

 be easily conceived, that the actual exist- 

 ence of a state of slower vibration may tend 

 still more to retard the more frequent undu- 

 lations, and that the refractive power of so- 

 lid bodies may be sensibly increased by an 

 increase of temperature, as it actually appears 

 to have been in Euler's experiments. (Acad. 

 de Berlin. 1762. S28.) 



Scholium. If, notwithstanding these con- 

 «iderations, this proposition should appear to 

 be insufficiently demonstrated, they must be 

 allowed to be at least equally explanatory of 

 the phenomena with any thing that can be 

 advanced on the other side, from the doc- 

 trine of projectiles ; since a supposed accele- 

 rating force must act in some other propor- 

 tion than that of the bulk of the particles; 

 and, if we call this an elective attraction, 

 it is only veiling, under a chemical term, our 

 incapacity of assigning a mechanical cause. 



Mr. Short, when he found by observation 

 the equality of the velocity of light of all co- 

 lours, felt the objection so forcibly, that he 

 immediately drew an inference from it in 

 favour of the undulatory system. It is as- 

 sumed in the proposition, that when light is 

 dispersed by refraction, the corpuscles of the 

 refracting substance are in a state of actual 

 alternate motion, and contribute to its trans- 

 mission ; but it must be confessed, that we 

 cannot at present form a very decided and 

 accurate conception of the forces concerned 

 in maintaining these corpuscular vibrations. 

 The proposition is not advanced as adding 

 weight to the evidence in favour of the un- 

 dulatory system, but as explaining in some 

 degree a difficulty wjiich is common to all 

 systems; and there is still room for other il- 

 lustrations of the subject. The principal ar- 

 gument in confirmation of the system is 

 built on the next proposition, which appears 

 to be equally new and important. 



Proposition viii. JV/ien two Undula- 

 tions, from different Origins, coincide either 

 perfect/}/ or very nearh/ in Direction, their 

 joint effect is a Combination of the Motions 

 belonging to each. 



Since every particle of the medium is 

 affected by each undulation, wherever the 

 directions coincide, the undulations can pro- 

 ceed no otherwise, than by uniting their mo- 

 tions, so that the joint motion may be the 

 sum or difference of the separate motions, 

 accordingly as similar or dissimilar parts of 

 the undulations are coincident. 



I have, on a former occasion, insisted at 

 large on the application of this principle to 

 harmonics; (Fhil. Trans. 1800. 130.) and 

 it will appear to be of still more extensive 

 utility in explaining the phenomena of co- 



