33 
the Theory of Light and Colours . 
consequently, that blue light will be more refrangible than red, 
and radiant heat least of all ; a consequence which coincides 
exactly with the highly interesting experiments of Dr. Her- 
schel. (Phil. Trans, for 1800. p. 284.) It may also be easily 
conceived, that the actual existence of a state of slower vibra- 
tion may tend still more to retard the more frequent undulations, 
and that the refractive power of solid 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. 
p. 328.) 
Scholium. If, notwithstanding, this proposition should appear 
to be insufficiently demonstrated, it 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 doctrine of projec- 
tiles ; since a supposed accelerating force must act in some other 
proportion 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 velo- 
city of light of all colours, felt the objection so forcibly, that he 
immediately drew an inference from it in favour of the undula- 
tory system. It is assumed in the proposition, that when light 
is dispersed by refraction, the corpuscles of the refracting sub- 
stance are in a state of actual alternate motion, and contribute 
to its transmission ; 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. 
MDCCCII. 
F 
