864 



SCIENCE. 



[N. S. Vol. XVII. No. 439. 



official, naval and military life are features 

 which have always constituted a charm and 

 attractiveness to the many American biolo- 

 gists who have already experienced them. 



Another consideration very important to my 

 mind is the educational value to young biol- 

 ogists — ^prospective investigators — to be ob- 

 tained from such an establishment.- The 

 broadened conception of the possibilities of 

 the animal and plant world which even a short 

 experience within the tropics affords is very 

 desirable. To wander amid the beauty and 

 luxuriance of life on a coral reef, or pass 

 amongst the intricacies and remarkable adap- 

 tations within a tropical forest, gives an in- 

 spiration not to be experienced in temperate 

 regions. For this purpose a station having 

 the gTeatest variety of both land and marine 

 conditions is obviously most desirable. I 

 conceive that many professors will think it 

 worth while to take or send their most promis- 

 ing students, the idea of a general acquaint- 

 ance with a tropical fauna and flora predomi- 

 nating over that of discovering material for 

 research. For many years such has been the 

 custom of Professor Brooks with regard to his 

 students, and the conditions found in Jamaica 

 have most nearly approached the ideal. 



j. e. duerden. 



Uni^'eesity of Noeth Carolina. 



8H0BTEB ARTICLES. 

 THE PHYSICAL BASIS OF COLOR. 



At the present time no one, I thinkj ques- 

 tions the validity of the wave-theory of light. 

 We may hold various ,views as to the nature, 

 or even the existence, of that omnipresent 

 medium, the ether; and the physicist, though 

 unable to get along without it, is continually 

 changing his conceptions of its manner of 

 action; but the broad general principles upon 

 which the theory is built remain unshaken. 



The backbone of the theory is periodicity. 

 Innumerable measurements of extreme accu- 

 racy have been made whilst experimenting in 

 the various domaius of optics, all of which 

 agree in the conclusion that light, in its very 

 essential nature, is periodic; and the simplest 

 image one can form in his mind of such a 

 phenomenon is a wave-motion, while the 



simplest method of representing it mathe- 

 matically is by the circular functions of the 

 sine and cosine. 



The three quantities which determine a 

 wave-motion are its amplitude, its wave- 

 length (and, therefore, its frequency or 

 period), and the form or contour of the waves. 

 The mechanical measure of the intensity is 

 proportional to the square of the first of these, 

 while the sensation of color is in some way 

 indissolubly connected with the second — ^pos- 

 sibly, also, with the third, though I do not 

 know of any direct evidence on the question. 



It has been usual to assert that color is 

 purely a function of the wave-length, just as 

 pitch is a function of wave-length in acous- 

 tics. Light of one wave-length would excite 

 one color, light of another wave-length, a dif- 

 ferent color, etc. In an article on ' Color 

 Saturation,' which appeared in the American 

 Journal of Psychology (Vol. VII., No. 3, 

 April, 1896), my friend and colleague. Pro- 

 fessor A. Kirschmann, expresses dissent from 

 the view generally accepted (by physicists, at 

 any rate), and it is the question raised by him 

 that I wish to briefly consider. 



Dr. Kirschmann remarks : ' It is claimed 

 that light of one certain wave-length causes 

 the impression of red, another that of green, 

 etc. ; but this is mere hypothesis, for nohody 

 has ever seen light of one wave-length.' Per- 

 haps it would be fairer to state the proposi- 

 tion as I have done in the preceding para- 

 graph. The physicist surely does not claim 

 that he has ever worked with light of abso- 

 lutely a single wave-length, though we shall 

 see how near he has been able to approach to 

 it. If a writer on the wave-theory should in- 

 dulge in such superficial dogmatic statements, 

 he must not be taken too literally, and the 

 true value of the theory must not be judged 

 therefrom. 



Dr. Kirschmann supposes a ' pure ' spec- 

 trum, a meter in length, to be produced on a 

 screen, and discusses the nature of a narrow 

 band of this image 1/100 of a millimeter in 

 width. Taking the number of vibrations per 

 second, corresponding to extreme red and ex- 

 treme violet, to be 413 million million and 

 790 million million, respectively, we see that 



