45 
the Theory of Light and Colours . 
nature of the subject. It is clearly granted by Newton, that 
there are undulations, yet he denies that they constitute light ; 
but it is shown in the three first Corollaries of the last Proposi- 
tion, that all cases of the increase or diminution of light are 
referable to an increase or diminution of such undulations, and 
that all the affections to which the undulations would be liable, 
are distinctly visible in the phenomena of light ; it may there- 
fore be very logically inferred, that the undulations are light. 
A few detached remarks will serve to obviate some objections 
which may be raised against this theory. 
1. Newton has advanced the singular refraction of the Ice- 
land crystal, as an argument that the particles of light must be 
projected corpuscles ; since he thinks it probable that the dif- 
ferent sides of these particles must be differently attracted by 
the crystal, and since Huygens has confessed his inability to 
account in a satisfactory manner for all the phenomena. But, 
contrarily to what might have been expected from Newton's 
usual accuracy and candour, he has laid down a new law for 
the refraction, without giving a reason for rejecting that of 
Huygens, which Mr. Hauy has found to be more accurate than 
* 
Newton's ; and, without attempting to deduce from his own 
system any explanation of the more universal and striking effects 
of doubling spars, he has omitted to observe that Huygens's 
most elegant and ingenious theory perfectly accords with these 
general effects, in all particulars, and of course derives from 
them additional pretensions to truth : this he omits, in order to 
point out a difficulty, for which only a verbal solution can be 
found in his own theory, and which will probably long remain 
unexplained by any other. 
2. Mr. Michell has made some experiments, which appear' 
to show that the rays of light have an actual momentum, by 
