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. ^ u„.:u/ uhr y 



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 , 



500 Citron yellow of first order 



625 Reddish yellow of second order 



1000 Yellowish green of second order 



1250 Bluish green faint , 



(1 



{i 



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. 



. 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. 



Water. Yellow of second order. 



Alcohol. Yellower. 



Oil of cassia. Yellowish pink. 



