of Colour -disease. 



95 



It has further been established as a law, that each colour- 

 blind person, in a complementary pair of colours that he sees 

 equal in this his characteristic colour-equation, has always a 

 feebler sensitiveness for the blue or red side than for the green 

 and yellow. Experience seems, finally, to show that all colour- 

 mistaken persons are red- seers, with the exception of violet-seers 

 under the santonine stimulation. 



For investigating colour-mis- 

 taken persons I use my colour- 

 measurer*, as now constructed 

 by M. Oertling according to my 

 direction, for practical use even 

 at the bedside. In a polarizing- 

 apparatus (two Nicols with di- 

 vided circles) there are fixed 

 an achromatic doubly-refracting 

 spar prism and a rock-crystal 

 plate cut perpendicularly half a 

 centimetre thick. The image of 

 the diaphragm is seen through it 

 always double, the two near each 

 other and of complementary co- 

 lours. The colour depends on the 

 position of the upper Nicol (0); 

 the difference in the intensity 

 of the two images on that of 

 the lower one (U). 



The colour-stamp of plane 

 Daltonists gives, in the form of 

 an equation, the readings on 

 both divided circles, at which 

 both images appear to them 

 quite identical. 



Such colour-stamps are, for in- 

 stance, for 



Messrs. 







A. 



+ 22 =U - 



- 57 



B. 



+ 12 =U 



53 



F. 



+ 15 =U 



60 



G. 



+ 13 =U 



60 



H. 



+ 12 =U 



50 



J. 



+ 17 =U 



48 



N. 



+ 16 =U 



52 



* Compare woodcut. I am quite ready to have the small correction of 

 the instrument determined by a Daltonist of my acquaintance, so that 

 the instruments may be comparable. 



