THE EYE AND THE PHYSIOLOGY OF VISION. 569 



an. entirely different basis. Both theories, however, start 

 from facts and observations which are afforded by instances 

 of color blindness, and so before giving these theories it 

 may be desirable to describe somewhat in detail the various 

 kinds of color blindness observed. Two distinct kinds of 

 color blindness are found called respectively monochro- 

 matic, and dichromatic colorblindness. Of the dichromatic 

 blindness two varieties occur called red blindness and green 

 blindness. More correctly, however, the terms monochro- 

 matics and dichromatics are applied to the persons them- 

 selves showing these defects. 



COLOE BLINDNESS. 



Monochromatics . As the name indicates, these are per- 

 sons who see but one color. This color seems to be a bluish 

 white. Such monochromatics see the world, about as we 

 do, as far as distinctness is concerned. They can recognize 

 objects readily. The world appears to them about as it 

 appears to us in a photograph or an etching. It will be 

 remembered that in photographs and etchings but shades 

 of white and dark, as a rule, are used. Yet so true may a 

 photograph be, using only these two colors, that we fail to 

 notice frequently the absence of the spectral tints. It is 

 even possible for such monochromatics to live for some 

 time without really suspecting their defect, attributing their 

 lack of color perception frequently to stupidity in that di- 

 rection rather than a real optical disability. Such mono- 

 chromatics show when they are looking at objects a pecu- 

 liar habit of squinting the eye, as if they were focussing 

 with effort. They seem to have some difficulty to turn 

 their point of acutest vision on the object in question. 

 Strange as it may seem, such monochromatics may some- 

 times successfully recognize colors, but when they do so it 

 is for the same reason that a photographer may recognize 

 colors on his photograph, having learned by experience 

 what the appearance of red or yellow in the photograph will 

 be, and so these monochromatics having red pointed out 



