30 RELATION OF NON-LUMINOUS RAYS TO THE EYE. 



second light prevented the occurrence of the phenomenon. The blue colour 

 was considered by Dr. Tourtual to be complementary of the reddish-yellow 

 colour of the lamp ; and he was led by the circumstance to undertake some 

 additional experiments on the subject. On placing a strip of black silk, 

 about three lines broad, on a piece of orange-coloured cloth, and on 

 directing the axes of both eyes towards it at a moderate distance, and in 

 clear daylight, the distinction between the two colours was clearly seen. 

 But on closing the left eye, and gradually approximating the object 

 towards the right eye, whose direction and point of adaptation remained 

 unaltered, a bright margin appeared around the black silk, and the silk 

 itself assumed a dark blue colour. When, in a similar experiment, the 

 black silk was placed on a purple ground it assumed a green colour, on a 

 violet ground a yellow colour, and so on; the colours assumed being 

 always complementary of that on which the silk was placed. A similar 

 phenomenon was observed when, instead of the black silk, a strip of white 

 paper of the same breadth was employed. The change of colour may be 

 observed also, when, instead of altering the position of the object, the eye 

 be directed to a point about an inch on one side of the black stripe ; or 

 when the one or both eyes are made to accommodate themselves to 

 distant vision. The size of the pupil has no direct influence on this 

 phenomenon ; it appears, in the opinion of Tourtual, to arise from 

 indistinctness of vision alone. 



An interesting fact in relation to complementary colours, has also been 

 noticed by M. Briicke.* He found that the transmitted portion of the 

 rays of light falling on any given part of the fibrous tapetum of animals, 

 possessed a colour exactly complementary of that reflected by the same 

 part of the tapetum. The various colours reflected by the tapetum (not 

 including the red, which is due to the blood in the vessels of the retina 

 and choroid) are yellow, merging into orange, yellow itself, green, blue, 

 and blue merging into violet ; those transmitted are all complementary 

 of these, viz., violet merging into blue, violet itself, red, orange, and 

 orange merging into yellow.f 



Belation of non-luminous rays to the eye. Several very ingenious 

 experiments have been performed by Ernst Briicke, J to determine 

 whether the chemical and calorific rays of light are transmitted through 

 the transparent media of the eye as well as the luminous ones. To 

 ascertain this with regard to the chemical rays, those namely which are 

 situated outside the violet, he took advantage of the property which these 

 rays possess of changing the colour obtained from guaiacum wood first to 

 a bright green, and then to a deep greenish-blue tint. Having coloured a 

 small porcelain plate with tincture of guaiacum and dried it in the dark, 



* Miiller's Archiv. 1844, p. 449. 



t For an account of the Simple and Complementary colours, see Miiller's Physiology, 

 p. 1103. Miiller's Archiv. 1845, p. 262. 



