the brain. One of these sets of nerve-fibers is supposed to respond most readily to red, the other to green, the third to violet, or to a blue which verges closely upon violet. When all these nerve-fibers are absolutely at rest, we see nothing. Improperly speaking, we might say that we then experience the sensation of black, for absolute black really produces no sensation, but is rather the result of the absence of all sensation. On the contrary, when all the nerve-fibers are excited simultaneously and to an equal degree, we experience the sensation of white, provided that the amount of excitation is tolerably great. If the excitation is only feeble, we see what we call gray—gray being simply white of a low degree of luminosity. All other color-sensations are produced by the excitation of groups of nerves variously combined. Thus, whenever the fibers which respond to red and those which respond to green are excited simultaneously, we experience the sensation of yellow; when the two groups which respond respectively to green and to violet are simultaneously excited, we experience the sensation of blue, and so on through the whole scale of colors. Again, when all the nerve-fibers are excited at once, but to an unusual degree, we perceive the result of the mixture of one predominating color with the others. If we suppose the nerves responding to red to be the most violently excited, we shall experience the sensation of red mixed with white, or, in other words, of light red.
It will readily be seen that this hypothesis explains the curious condition of color-blind persons very satisfactorily. In the case of total color-blindness, we need only to assume that the nerve-fibers are in an abnormal condition, so that each set, instead of responding to only one sensation, responds equally to all. The result must necessarily be a total absence of color in the impressions received through the eye. In the case of a red-blind person, the nerves which ought to respond to red may either be paralyzed or they may be wanting altogether, and all other defects in color-vision may be explained upon the same principle.
To a limited extent the inability to tell the difference between certain colors, which is due to partial color-blindness, may be overcome by the use of variously colored glasses; but, after all, no artificial palliative will compensate for the want of a naturally perfect eye.
By H. N. DRAPER, F.C.S.
SO much has already been written by way of contribution to our knowledge of the different species of the eucalyptus-tree, that, interesting as the subject is, it may well be considered to have received already a fair share of attention. There is one aspect of it,