g8 
Dr . Young’s Lecture on 
the motion of the denser, and destroying the reflection, (prop, 
iv.) while they themselves will be more strongly propelled 
than if they had been at rest ; and the transmitted light will be 
increased. So that the colours by reflection will be destroyed* 
and those by transmission rendered more vivid, when the double 
thicknesses, or intervals of retardation, are any multiples of the 
whole breadths of the undulations; and, at intermediate thick- 
nesses the effects will be reversed; according to the Newtoni an 
.observations. 
If the same proportions be found to hold good with respect 
to thin plates of a denser medium, which is indeed not impro- 
bable, it will be necessary to adopt the corrected demonstration 
of prop. iv. but, at any rate, if a thin plate be interposed be- 
tween a rarer and a denser medium, the colours by reflection 
and transmission may be expected to change places. 
From Newton’s measures of the thicknesses reflecting the 
different colours, the breadth and duration of their respective 
undulations may be very accurately determined ; although it is 
not improbable, that when the -glasses approach very near, the 
atmosphere of ether may produce some little irregularity. The 
whole visible spectrum appears to be comprised within the ratio 
of three to five, or a major sixth in music ; and the undulations 
of red, yellow, and blue, to be related in magnitude as the 
numbers 8, 7, and 6 ; so that the interval from red to blue 
is a fourth. The absolute frequency expressed in numbers is 
too great to be distinctly conceived, but it may be better ima- 
gined by a comparison with sound. If a chord sounding the 
tenor c, could be continually bisected 40 times, and should 
then vibrate, it would afford a yellow^green light : this being 
41 40 41 
denoted by c, the extreme red would be a, and the blue d. 
