20* 



EXPERIMENTS OX DOUBLE VISION. 



Method of 

 snaking the ex- 

 periment. 



Results of the 

 combined per- 

 ception of dif- 

 ferent colours 

 by refraction. 



glare ; and those which, reflecting colours proper to them- 

 selves, vitiate the results : such as candles, lamps, &c. , 

 which diffuse too abundant light in yellow rays. The best 

 method is to take a piece of white paper, about eight inches 

 in diameter; place it on a black or brown ground at the 

 end of a moderately light room opposite the window, and, 

 standing two or three yards from it, with the back to the 

 window, look at it with both eyes, each having its proper 

 prism before it. In this manner I obtained the following 

 results. 



Red and yellow produced orange. 



Red and orange 

 Red and blue 

 Red and violet 

 Red and green 

 Red and indigo 

 Orange and yellow 

 Orange and blue 

 Orange and green 

 Orange and violet 

 Yellow and blue 

 Yellow and green 

 Yellow and violet 

 Blue and green 

 Blue and violet 



aurora. 



violet. 



a pleasing rose colour. 



a muddy red. 



an indeterminate colour. 



light orange. 



muddy green. 



light green. 



muddy rose colour. 



muddy green. 



light green. 



harsh red. 



sea green. 



a deep violet. 



The colouring In order to enable those who wish to repeat these cxperu 



matters that raen ts to execute them with greater facility, and render the 

 were employed. ;_ . . * 



results uniform, 1 shall add here an account of the colour- 

 ing substances, that were employed. The red was a decoc- 

 tion of brazil brightened by an acid. I likewise employed 

 for this colour red wine, and a decoction of cochineal. The 

 yellow was prepared from quercitron bark, the decoction 

 of which must be weak, well filtered, and brightened : the 

 orange, from French berries, or turmeric: the blue was 

 aqua ca>lestis, a solution of copper in ammonia: the green, 

 an infusion of mallow flowers changed by potash : the violet 

 was prepared from litmus and violets. These experiments 

 are easy to execute; they only require a little practice, and 

 the habit of distinguishing different tints of colour. 



After 



