THE PHYSIOLOGY OF COLORS. 685 



ing the tint that is suited to them, and present very different aspects 

 according to the mode in which they are lighted. A red rihbon, for 

 example, placed successively in the different colors of the spectrum, 

 appears black, except in the red region ; it therefore returns by reflec- 

 tion an almost homogeneous light. A rose ribbon appears very un- 

 equally luminous in different parts of the spectrum. The light which 

 it reflects is, therefore, complex. 



We may ask, then, What would be the condition of nature if the 

 light that shines upon us were absolutely homogeneous ? Some bodies 

 would absorb it completely, and would appear dark like black velvet ; 

 others would reflect it more or less actively, and would have a corre- 

 sponding degree of brilliancy. As there would be no criterion for com- 

 parison, the eye would have only the sensations of white, black, and 

 the intermediate rays. 



Pascal said that nothing better enabled him to comprehend the 

 properties of the air than what took place where there was none. So, 

 nothing better enables us to comprehend the properties of colors than 

 the appearance of the world under an illumination of homogeneous 

 light. The volatilization of a salt of soda in the flame of a Bunsen 

 burner almost perfectly fulfills this condition. With such a light, cloths 

 dyed in the richest colors show only white, black, and gray, and the 

 art of painting has no place. 



The estimation of color being connected with the impression pro- 

 duced upon the retina, it is readily to be seen that the human eye will 

 not always equally well perform that function. The different points of 

 the retina are not alike ready to appreciate colors. To distinguish the 

 details of au object, it is necessary to direct the look toward it, or, in 

 other words, to produce an image upon the central region of the 

 retina, where the acuteness of physiological perception is much the 

 greatest. The same is the case for colors. When we keep the look 

 in a determined direction, and put a colored body in the visual field 

 in such a way that its image is produced laterally, we remark that the 

 notion of color is more and more weakened as we remove from central 

 vision, and disappears at the limits of the field. But the most impor- 

 tant fact is, that in the different views the colors are not distinguished 

 from one another with equal facility, and that we sometimes come to 

 the point of confounding colors which really seem to be most discord- 

 ant, as green and red. The discovery of this particular form of in- 

 firmity is due to Dalton, who was very strongly affected by it, and 

 who carefully analyzed the errors of his judgment. This fault, which 

 remained unperceived for so long a time, is in reality quite frequent. 

 About ten persons in a hundred make mistakes in the comparison of 

 colors marked enough to be detected by an attentive examination. 

 Generally the imperfection is not accompanied with grave inconven- 

 iences, and is corrected unconsciously by the operation of habit, the 

 recollection of objects, and the judgments of others. But the annoy- 



