ON THE THEORY OF LIGHT AND COLOURS. 



527 



medium, which is indeed nol improbable, it 

 will be necessary to adopt the corrected de- 

 monstration of Prop. 4. but, at any rate, if 

 a thin plate be interposed between 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. The 

 whole visible spectrum appears to be com- 

 prised within the ratio of tiiree to five, which 

 is that of 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 con- 

 ceived, but it limy be better imagined 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 denoted by 



c, the extreme red would be a, and theblued. 

 The absolute length and frequency of each 

 vibratioii is expressed in the table : suppos- 

 ing light to travel in 8| minutes 500 000' 

 000 000 feet. 



Scholium. It was not till I had satisfied 

 myself respecting all these phenomena, that 

 I found, in Hooke's ^Iicrographia,a passage 

 which might have led me earlier to a similar 

 opinion. " It is most evident, that the re-, 

 flection from tlie under or furilier side of the 

 body, is llie principal cause of the produc- 

 tion of these colours. — Let the ray fall ob- 

 liquely on the thin plate, part thereof is re- 



flected back by the first superficies, — partr 

 refracted to the second surface, — whence it 

 is reflected and refracted again. — So that, 

 after two refractions and one reflection, there 

 is propagated a kind of fainter ray — ," and, 

 " by reaS|On of the time spent in passing and 

 repassing, — this fainter pulse comes behind- 

 the" former reflected " pulse ; so that hereby 

 (the surfaces being so near together that the 



