23 
the Theory of Light and Colours . 
Scholium 1. It has been demonstrated, that in different 
mediums the velocity varies in the subduplicate ratio of the 
force directly, and of the density inversely. (Misc.Taur. Vol. L 
p. 91. Young's Syllabus. Art. 294.) 
Scholium 2. It is obvious, from the phenomena of elastic 
bodies and of sounds, that the undulations may cross each other 
without interruption. But there is no necessity that the various 
colours of white light should intermix their undulations ; for, 
supposing the vibrations of the retina to continue but a five hun- 
dredth of a second after their excitement, a million undulations 
of each of a million colours may arrive in distinct succession 
within this interval of time, and produce the same sensible 
effect, as if all the colours arrived precisely at the same instant. 
PROPOSITION II. 
An Undulation conceived to originate from the Vibration of a 
single Particle, must expand through a homogeneous Medium 
in a spherical Form, but with different quantities of Motion in 
different Parts. 
For, since every impulse, considered as positive or negative, 
is propagated with a constant velocity, each part of the undu- 
lation must in equal times have past through equal distances 
from the vibrating point. And, supposing the vibrating particle, 
in the course of its motion, to proceed forwards to a small dis- 
tance in a given direction, the principal strength of the undula- 
tion will naturally be straight before it ; behind it, the motion 
will be equal, in a contrary direction ; and, at right angles to 
the line of vibration, the undulation will be evanescent. 
Now, in order that such an undulation may continue its pro- 
gress to any considerable distance, there must be in each part 
of it, a tendency to preserve its own motion in a right line from 
