480 



SGIENGE. 



[N. S. Vol. III. No. 65. 



hands too full to stop and examine all these 

 competing theories. To test them is the privi- 

 lege of the psychologists. Pending the estab- 

 lishment of some one of these new theories by an 

 exhibition of approximate unanimity among 

 the psychologists, the rest of us will be apt to 

 content ourselves as best we may with the the- 

 ory of vision that has thus far seemed no more 

 objectionable than its successors, and which is 

 fortified by the authority of the greatest Ger- 

 man physicist of the nineteenth century. 



We are fully aware of certain facts in the 

 history of science that may quite naturally 

 be cited in this connection. The great au- 

 thority of Newton caused more than a century 

 of delay in the acceptance of the undulatory 

 theory of light. The modification of this the- 

 ory by Maxwell received but a small share of 

 the credit it deserved until Hertz published the 

 experimental evidence upon which light was 

 shown to be very probably an electro-magnetic 

 phenomenon. As soon as any new theory of 

 visual perception is established upon evidence 

 comparable with that brought out by Hertz, if 

 it conflicts with the Helmholtz theory of vision, 

 this will become of only historic interest, like 

 the emission theory of light. Its fate, how- 

 ever, has not yet been sealed. 



In this connection it may be permissible to 

 express my hearty accordance with the views 

 set forth by Mrs. Franklin in a recent contribu- 

 tion to Tlie Nation regarding the desirability of 

 greater precision in the use of the word 'light.' 

 The meaning of a word is determined by cus- 

 tom rather than argument. But custom may 

 be gradually modified if those who have occa- 

 sion most frequently to use a special word or 

 form of expression will agree among themselves 

 to guard against ambiguity. No careful physi- 

 cist at present includes the ultra-violet or infra- 

 red ether vibrations among light vibrations. 

 The distinction between luminous and non- 

 luminous energy waves is generally accepted 

 and applied. But we need to habituate our- 

 selves to the use of the term ' light-sensations, ' 

 rather than ' light,' when reference is made to 

 what is carried to the brain by the optic nerve, 

 whether the origin of the sensation is found in 

 luminous, electi-ic or mechanical energy. The 

 American sense of linguistic esthetics may be 



depended upon to prevent the adoption of such 

 cumbrous unhyphened compound words as are 

 tolerated by our German friends. But the 

 scientific demand is for clearness combined with 

 accuracy, for an application of the doctrine of 

 conservation of energy in the giving and takings 

 of ideas. Whatever differences may exist be- 

 tween the physicist and psychologist regarding 

 the explanation of light-sensation, they can 

 certainly clasp hands and agree not to deceive 

 each other by unnecessary vagueness in the use 

 of language. W. Le Conte Stevens. 



THE PHILADELPHIA BEICK CLAYS, ET AL. 



I HAD not thought there was occasion for re- 

 sponding to the article of Prof G. Frederick 

 Wright (Science, No. 59, p. 242), until inquiry 

 concerning the truth of the matters touched 

 upon began to be made by correspondents. I 

 shall not now take space to state the case fully, 

 but only to say that the term 'Columbia,' as 

 used by Prof. Wright, and indeed as it has 

 been generally used in the past, is a somewhat 

 ambiguous one. It has been made to cover for- 

 mations, chiefly extra-glacial, widely separated 

 in time, ranging indeed from the beginning of 

 the glaciitl period nearly to the present. The 

 Jamesburg formation of New Jersey falls within 

 the limits of the Columbia, according to this 

 usage, but the term Jamesburg has never 

 been extended to the extra-morainic drift dis- 

 cussed somewhat fully in the New Jersey geo- 

 logical report of 1893. Most of the Jamesburg 

 deposits of New Jersey are, I take it, relatively 

 young, as indicated by Prof. Wright's citation 

 from my report. But if I interpret rightly, 

 there are remnants of a much older division of 

 the ' Columbia ' formation, not referred to ex- 

 plicitly in the report from which Prof Wright 

 quotes. These remnants are in scattered 

 patches, and are quantitatively unimportant ; 

 but they are, as I believe, very significant. If 

 present interpretations be right, there was 

 very extensive erosion after the deposition of 

 the formation of which these patches are rem- 

 nants, this erosion antedating the deposition of 

 the great body of material which passes under 

 the name of ' Columbia. ' Just where in the 

 complex ' Columbia ' the ' Philadelphia brick 

 clays ' belong, is a question I have nowhere 



