THE PHYSIOLOGY OF COLORS. 687 



could perceive the whole ultra-violet spectrum as it is revealed by- 

 photography. Having had occasion to study the light emitted by 

 metallic vapors, I have ascertained that, with a prism of Iceland spar, 

 an ordinary sight can distinguish an ultra-violet spectrum three or 

 four times as extended as the luminous spectrum ; one of my co-labor- 

 ers saw much farther still, and pointed out in advance all the rays 

 which it was possible for me to photograph. If instead of regarding 

 the refraction of these rays, which varies with the nature of the sub- 

 stances, we define them by their wave-lengths or by the duration of 

 their undulations, we may say that the ordinary luminous spectrum 

 comprises the interval of an octave, and that it is possible to perceive 

 a second higher or more acute octave. 



Sir William Thomson has expressed surprise that Nature has for- 

 gotten to give us a special sense for perceiving the magnetic phenom- 

 ena amid which we are living. In the case of light, we are in the 

 presence of rays that are not luminous in sunlight, or at least are 

 not seen by us, which are energetically absorbed by most transparent 

 media and especially by the humors of the eye, for which we give our- 

 selves no concern whatever in current life, and which nevertheless act 

 upon the retina. Does it not seem as if we possessed in this respect a 

 superfluous sensibility, and as if there were a lack of harmony between 

 the structure of the organ and the wants to which it should respond ? 

 A question has been raised on this subject that presents a very great 

 interest in the philosophical point of view, as to whether man is sus- 

 ceptible of an organic development, and if it is possible to detect a 

 trace of any progress that may have been accomplished in the vision 

 of colors, and consequently in the structure of the eye. An eminent 

 Englishman has not disdained to engage himself with this question. 

 Mr. Gladstone has summed up all the expressions used by Homer to 

 designate the color of objects, from which it appears that the great 

 poet was accustomed to apply the terms in a very uncertain manner, 

 and confounded green with yellow and blue with black. Before con- 

 cluding, from this curious observation, that the sense of color was but 

 little developed in Homer's age, we should, perhaps, remark that the 

 interval that separates us from him is but a short time in the history 

 of mankind ; that the Greeks afterward made much use of colors in 

 their pictures and in the painted statuettes of which we possess nu- 

 merous specimens ; that the frescoes of Pompeii exhibit the most vari- 

 ous colors ; and that a careful examination of modern authors might 

 lead us to draw the same conclusions with respect to their time as 

 would be drawn from the Homeric writings. Is it not singular that 

 in the middle of the seventeenth century, when Lesueur was using 

 blue extensively in painting, that emphatically naturalistic poet, La 

 Fontaine, did not once employ the term blue to designate any colored 

 object or the color of the sky ? 



Even if mankind were capable of a rapid progress toward perfec- 



