310 DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 
The mass of white light is finely seen in the impressions taken upon tin, but 
never appears upon isinglass. 
The preceding experiments do not afford any precise data for determining 
the influences of refractive power. The realgar and the isinglass give fewer 
periods of colour so as to indicate that, cseteris paribus, a diminution of refrac- 
tive power, produces a diminution in the number and orders of colours, or 
causes the minima to be developed at a less incidence. This indication, how- 
ever, is opposed by the fact, that as the isinglass dries and consequently in- 
creases in refractive power, the periods diminish in number, and the minima 
are produced at less incidences. The modification of the tints by a change 
of refractive power is here masked by the influence of other causes, namely, an 
inferiority in the sharpness of the impression to that of the original surface, 
and a rounding of the narrow spaces n subsequently produced by induration. 
In the specimen of isinglass, therefore, already mentioned, which gave the 
first limit of pink and blue at nearly the same angle as the steel, it is probable 
that it would have developed the same limit at a greater inclination had the 
impression been as sharp as the original. 
In this uncertainty I conceived that the influence of a variable refractive 
power would be best obtained by placing different fluids on the surface of the 
grooved steel ; and upon using alcohol and oil of cassia my expectations were 
fulfilled. 
The following were the results 
Number of 
grooves in 
an inch. 
Maximum tint without a fluid. 
312 No colour 
Maximum tint, with water, alcohol, and oil of cassia. 
. Water. Tinge of yellow. 
. Alcohol. Tinge of yellow. 
. Oil of cassia. Faint reddish yellow. 
. Water. Tinge of red. 
500 Citron yellow of first order 2. Alcohol. Diluted pink. 
. Oil of cassia. A bluer pink. 
. Water. Faint pink of second order. 
. Alcohol. Ditto more pink. 
. Oil of cassia. Bluish pink of second order. 
. Water. Pinkish red, second order. 
. Alcohol. Brilliant pink, ditto. 
Oil of cassia. Greenish blue, third order. 
625 Reddish yellow of second order 
1000 Yellowish green of second order 
1250 Bluish green faint 
{ 
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S. 
1. Water. Yellow of second order. 
2. Alcohol. Yellower. 
3. Oil of cassia. Yellowish pink. 
