Page:Popular Science Monthly Volume 27.djvu/707

Rh 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-laborers 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 substances, 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 forgotten to give us a special sense for perceiving the magnetic phenomena 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 ourselves 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 susceptible 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 concluding, 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 numerous specimens; that the frescoes of Pompeii exhibit the most various 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