August 23, 1907] 



SCIENCE 



255 



variations in displacement, its distribution of 

 the throw over several near-lying and gen- 

 erally parallel planes, and, finally, the general 

 persistence with which the zone of dislocation 

 adheres to a definite course. 



The object of this reply is to make clear 

 that with the exception of the minor differ- 

 ences above referred to, the theses which Pro- 

 fessor Davis has defended in his review, are 

 just those which I have myself set up in 

 the report reviewed, as well as in some other 

 papers upon structural geology. 



Wm. H. Hobbs 



Uni^'ersitt of Michigan, 

 July 22, 1907 



RAILWAY SIGNALS 



To THE Editor of Science : By some inad- 

 vertence Dr. J. W. Baird, of the University 

 of Illinois, in criticizing a recent article of 

 mine on " Railway Signals," in the Century 

 Magazine, has attributed to me the belief that 

 the human retina at night is color-blind; and 

 he wonders how, according to my doctrine, an 

 engineer ever distinguishes his color signals 

 at night. As a matter of fact, I distinctly 

 state, in the very article he criticizes, that at 

 night the eye is not color-blind: "Colors are 

 readily seen at night if they are intense 

 enough.'" The passage of mine which he 

 quoted speaks explicitly of faint lights; for 

 the signal-lights, bright enough in themselves, 

 often become faint by distance, fog, smoke or 

 storm. And of faint lights it is demonstrably 

 true, as I said, and as every careful student 

 of the subject knows, that the eye " no longer 

 detects their proper colors." 



2. As to the relative sensitiveness of the 

 outlying portions of the retina for color and 

 for form, it should be said that at a certain 

 angular distance from the fovea a red danger- 

 light can appear " white " — a common sign of 

 safety. But in my own ease I can easily dis- 

 tinguish correctly a horizontal from a vertical 

 line, still farther off to the side. And even 

 when, with greater angular distance, the direc- 

 tion becomes obscure, I find no tendency in a 

 line-signal to appear to be its very opposite, 



' " Railway Disasters at Night," The Century 

 Magazine, May, 1907, p. 120, col. 2. 



as in the case of certain color-signals. So far 

 as the practical problem of signaling is con- 

 cerned, therefore, it seems probable that indi- 

 rect vision would be less likely to cause dis- 

 astrous misperception of a line-signal than of 

 color; and that Dr. Baird's contention here is 

 not stichhaltig . 



3. The fact that some illuminated sema- 

 phores have failed ■ would hardly seem to 

 justify the judgment that what I recommend 

 is " antiquated " and a failure. As I shall 

 attempt to show elsewhere, there is an essential 

 difference between the long line of lights which 

 I propose for signaling, and the devices that 

 have failed. 



4. Dr. Baird charges me with promulgating 

 the " erroneous conception " that there are 

 individuals weak in their color sense but by 

 no means color-blind; and declares that 

 " several thousand cases of ' color-weakness,' " 

 examined by Nagel, of Berlin, turned out in 

 every instance to be color-blind. This is cer- 

 tainly astonishing. For ISTagel himself, in the 

 very latest issues of his journal,^ affirms that 

 he has found many cases of markedly weak 

 color-sense that were not color-blind at all. 

 He finds the color-weak to be usually " anom- 

 alous trichromates " ; but quite recently he has 

 examined carefully a person who showed in 

 a pronounced way the characteristic marks of 

 color- weakness (Earbenschwache), and yet was 

 not even " anomal." Except for the color- 

 weakness, his color-system was the normal 

 " three-color " system. The " popular " and 

 " erroneous " conception that there are color- 

 weak persons who are not color-blind, seems 

 thus destined to continue. 



It is the more striljing that these misrepre- 

 ' ZeitsoTirift fiir Sinnesphysiologie, Vol. 41, pp. 

 250 f.; Vol. 42, pp. 65 ff. Could Dr. Baird's " sev- 

 eral thousand cases of ' color-wealcness,' " all 

 proved by Nagel to be color-blind, have perhaps 

 been drawn from the following passage in Nagel? 

 ■ — " Among many thousand persons lohose color- 

 sense I have investigated, I have found not a 

 single instance of markedly weak color-sense that 

 did not on closer examination turn out to he an 

 anomalous trichromatic color-sense." (Ibid., Vol. 

 41, p. 251). It is perhaps needless to add that 

 " dichromatic " would have been used by Nagel 

 had he meant (even partially) color-blind. 



