94 THE POPULAR SCIEXCE MONTHLY, 



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 ab- 

 solutely at rest, we see nothing. Improj^erly speaking, we might say 

 that we then experience the sensation of black, for absolute black real- 

 ly 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-sensa- 

 tions 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 uneaual deofree, 

 we perceive the result of the mixture of one predominating color with 

 the others. If we suppose the nerves resjDonding 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 exj^lains the curious con- 

 dition 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 miist 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 jmnciple. 



To a limited extent the inability to tell the difference between cer- 

 tain colors, which is due to j^artial 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. 



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THE EUCALYPTUS I^ THE EOMAK CAMPAG^A. 



By H. N. DEAPEK, 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 re- 

 ceived already a fair share of attention. There is one aspect of it, 



