14 Dr. Young’s Experiments and Calculations 
focus, I have found them nearly such as Mr. Hewson has de- 
scribed them. 
V. REMARKS ON THE COLOURS OF NATURAL BODIES. 
Exper. 5. I have already adduced, in illustration of Newton’s 
comparison of the colours of natural bodies with those of thin 
plates. Dr. Wollaston’s observations on the blue light of the 
lower part of a candle, which appears, when viewed through 
a prism, to be divided into five portions. I have lately observed 
a similar instance, still more strongly marked, in the light 
transmitted by the blue glass sold by the opticians. This light is 
separated by the prism into seven distinct portions, nearly equal 
in magnitude, but somewhat broader, and less accurately defined, 
towards the violet end of the spectrum. The first two are red, 
the third is yellowish green, the fourth green, the fifth blue, the 
sixth bluish violet, and the seventh violet. This division agrees 
very nearly with that of the light reflected by a plate of air 
of an inch in thickness, corresponding to the 11th series of red 
and the 18th of violet. A similar plate of a metallic oxide, would 
perhaps be about ttuoo °f an thickness. But it must be 
confessed, that there are strong reasons for believing the colour- 
ing particles of natural bodies in general to be incomparably 
smaller than this ; and it is probable that the analogy, suggested 
by Newton, is somewhat less close than he imagined. The 
light reflected by a plate of air, at any thickness nearly corre- 
sponding to the 1 ith red, appears to the eye to be very nearly 
white; but, under favourable circumstances, the 11th red and 
the neighbouring colours may 'Still be distinguished. The light 
of some kinds of coloured glass is pure red ; that of others, red 
with a little green : some intercept all the light, except the extreme 
