EXPERIMENTS ON DOUBLE VISION. 203 



not only because all colouring substances have neither the 



same solubility nor the same transparency, but because Many not the 



they hare not all the same tint by reflected and refracted same b J re f flec " 

 3 % ■ 3 lion and refrac- 



light; as Spallanzani observed with respect to the globules tion. 



of the blood, which appear red or yellow in the microscope, Blood. 



according to the manner in which they are acted upon by 



the light. It has long been known, as may be seen in 



Newton's Optics, that the lignum ncphriticum exhibits a Nephritic 



phenomenon of this kind. The infusions of violets and li- wo ° d > 



r violets, htums. 



turns, which have a pure blue tint by reflected light, have 



a decided violet by refraction. But it is particularly diffi- 

 cult to obtain the desired tints among the yellows. Those Yellows. 

 that are the purest yellow by reflected light have a decided 

 orange by refracted. They can only be divested of this red 

 hue, that alters them, by filtering them a great many times, 

 after diluting them with a considerable quantity of water. 

 This effect, which depends apparently on the opacity of the 

 colouring particles, and the force with which they repel the 

 most easily reflected rays, and admit only those that are the 

 least, seems to me well adapted to explain most facts of this 

 kind. In reality it is the least refrangible colour, the red, 

 that generally produces those differences observed in colour- een eraii "Th* 

 ed mediums by reflected and refracted light ; and these dif- cause. 

 ferences are diminished by weakening the tinctures, and di- 

 luting them with water. 



Two prisms of glass being filled with different coloured Glass prisms, 



liquors, and applied one to each eye, if we direct both eyes ^ lled Wltn co " 



H r . . •', loured liquors 



at once to the same object, we receive a double impression, applied before 



the perception corresponding to which is simple, and that tlie e y ei 

 of the colour resulting from a mechanical mixture of ana- 

 logous colouring substances. Thus a yellow prism, and a 

 red prism, applied one to the right eye, the other to the 

 left, produce the sensation of orange, as a mixture of ver- 

 milion and yellow ochre would do. But not to enter into 

 tedious details, I shall give a tabular view of the results of 

 the numerous experiments I made on this subject. 



As the results of these experiments cannot be exact, un- Cautions. 



less the colours be distinct and pure, in reflecting the light 



toward the eye we ought to exclude two kinds of bodies, 



those that are too bright, and disturb the sight by their 



P 2 glare : 



