gg6 Dr. Young's Account of some Cases 
broadest. We have only to suppose each particle of tallow to 
be, at its first evaporation, of such dimensions as to produce the 
same effect as the thin plate of air at this point, where it is 
about of an inch in thickness, and to reflect, or perhaps 
rather to transmit, the mixed light produced by the incipient 
combustion around it, and we shall have a light completely re- 
sembling that which Dr. Wollaston has observed. There 
appears to be also a fine line of strong yellow light, separate 
from the general spectrum, principally derived from the most 
superficial combustion at the margin of the flame, and increas- 
ing in quantity as the flame ascends. Similar circumstances might 
undoubtedly be found in other cases of the production or modi- 
fication of light ; and experiments upon this subject might tend 
greatly to establish the Newtonian opinion, that the colours of 
all natural bodies are similar in their origin to those of thin plates ; 
an opinion which appears to do the highest honour to the sa- 
gacity of its author, and indeed to form a very considerable 
step in our advances towards an acquaintance with the intimate 
constitution and arrangement of material substances. 
I have lately had an opportunity of confirming my former 
observations on the dispersive powers of the eye. I find that, 
at the respective distances of 10 and 15 inches, the extreme red 
and extreme violet rays are similarly refracted, the difference 
being expressed by a focal length of 30 inches. Now the interval 
between red and yellow is about one-fourth of the whole spec- 
trum; consequently, a focal length of 120 inches expresses a 
power equivalent to the dispersion,, of the red and yellow, and 
this differs but little from 132, which was the result of the 
observation already described. I do not know that these expe- 
riments are more accurate than the former one; but I have 
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