the Theory of Light and Colours. 39 
The absolute length and frequency of each vibration is ex- 
pressed in the table ; supposing light to travel in minutes 
500,000,000000 feet. 
Colours. 
Length of an 
Undulation 
in parts of an 
Inch, in Air. 
Nufnber of 
Undulations 
in an Inch. 
Number of Undulations 
in a Second. 
Extreme 
.0000266 
3764,0 
463 millions of millions 
Red - 
.OOOO256 
39180 
482 
Intermediate 
.OOOO246 
40720 
501 
Orange 
.OOOO24O 
4l6lO 
512 
Intermediate 
.OOO0235 
4 2 5 lO 
523 
Yellow 
.0000227 
44 000 
542 
Intermediate 
.0000219 
45600 
561 (== 2 48 nearly) 
Green - 
.0000211 
474^0 
584 
Intermediate 
.0000203 
49320 
607 
Blue - 
.OOOOI96 
51110 
629 
Intermediate 
.OOOO189 
52910 
65 2 
Indigo 
.OOOOI85 
54070 
665 
Intermediate 
.OOOOlBl 
55240 
680 
Violet - 
.OOOOI74 
57490 
7°7 
Extreme - 
.OOOOI67 
59750 
735 
Scholium. It was not till I had satisfied myself respecting all 
these phenomena,' that I found in Hooke’s Micrographia, a pas- 
sage which might have led me earlier to a similar conclusion. 
<c It is most evident that the reflection from the under or fur- 
“ ther side of the body, is the principal cause of the production 
“ of these colours. — Let the ray fail obliquely on the thin 
“ plate, part therefore is reflected back by the first superficies, 
“ - — part refracted to the second surface, — whence it is reflected 
<c and refracted again. — So that, after two refractions and one 
