December 



1895] 



NATURE 



125 



there is much more to criticise in so crude a present- 

 ation cf the case. Is not brown just as truly a " native'' 

 or " fundamental " sensation as green or blue ? Without 

 ;in education it would be impossible to pronounce it to 

 l)c a mixture of any other two of the '' native " sensations. 

 The writer would go further, and include amongst 

 native sensations that of purple, the pure full purple so 

 rarely occurring in nature. To him, indeed, violet 

 suggests a mixture of blue and purple, while crimson 

 suggests a mixture of the sensations of purple and red. 

 Further, it is not strictly true of the six supposed native 

 sensations connoted by the six names given above, that 

 they are in each pair complementary to each other. The 

 true complementary to yellow is violet, not blue ; the 

 true complementary to blue is orange, not yellow. The 

 true complementary to red is not green, but blue»-green 

 or peacock ; and the true complementary to green 

 (full green) is not red, but a crimson tending toward 

 purple. 



In other parts of the work, Hering's theory is com- 

 pared with Young's theory ; and a really decisive point 

 in favour of the latter is made on p. 136, where it is 

 shown, from the experiments made on the gradual 

 extinction of luminosity down to the tints that persist 

 last when all others have become invisible, that the 

 finality of disappearance in the case of persons of mono- 

 chromatic vision is inexplicable on Hering's physiological 

 assumptions. 



Many other points in the work show acute observation. 

 The remark that no colour-match can be accepted unless 

 we know the portion of the retina used in the operation 

 is an instance, for the match will be different if the por- 

 tion used is the macula lutea instead of a larger area of 

 retinal surface ; while, again, the colour-vision at 10^ 

 obliquity differs even in the normal eye from that of 

 the forea centralis. The instructions how to pick out 

 from the spectrum tints those which excite sensations 

 that are primary, on p. 93, though not too clearly 

 expressed, are readily understood ; and of great value 

 to future workers. The experiment of producing the 

 neutral or a-chromatic sensation of light by means of a 

 glow-lamp under a shade of blotting-paper, is both novel 

 and neat. 



Unfortunately the author occasionally uses words and 

 phrases in a special or technical sense of his own devising, 

 and which he does not define. It is left to the reader to 

 discover, if he can, what the meaning is. For example, 

 on p. 112, the words "the extinction" are used not to 

 mean the disappearance of the light, but to mean, 

 apparently, the percentage degree to which the light has 

 to be reduced in order to be practically imperceptible. 

 Unless one finds out by some sort of intuition that the 

 word is being used in this unusual sense, one is puzzled 

 to read three pages further on that " if we multiply the 

 extinction by the luminosity, we shall get what we want.' 

 Indeed, the word "luminosity," used so often throughout 

 the work, needs to be better defined ; and it ought to be 

 used, when defined, only in one sense. There are several 

 passages in which the word occurs, where it is not evident 

 whether the term "luminosity" refers to the number of 

 candles per unit area (the sense in which one would speak 

 of the intrinsic or specific luminosity of the crater of the 

 electric arc, or of the flame of a lamp), or whether it 

 NO. 1363. VOL. 53] 



refers to the illumination of a surface in terms of the 

 illumination due to a standard light at a standard dis- 

 tance, or, lastly, whether it refers to the apparent 

 luminosity as viewed by a person of possibly abnorma 

 vision. Yet at the bottom of p. 1 1 5 we are told that the 

 author has taken " the luminosity of the yellow light near 

 D [in a particular experiment] as one amyl acetate lamp." 

 Taken literally and grammatically, luminosity here means 

 simply " one lamp " of a kind that is known to be equal 

 to about 087 of a standard candle. The plaintive remark 

 on p. 181, that the word "brightness" is misleading to 

 some people who are uneducated, is a curious com- 

 mentary on the use of a word that will mislead some who 

 are not uneducated. In a certain "criterion by the 

 luminosity method" (p. 182), red is placed beside white, 

 and the subject is asked to say which he considers the 

 darker. This is to avoid asking him (lest it should be 

 misleading) which he considers the brighter. Does the 

 word luminosity as here used mean anything more than 

 brightness ? Again, on p. 1 10 we read : " The spectrum 

 was of such a brilliance that the intensity of the square 

 patch ... of the orange light (D) was exactly that of an 

 amyl-acetate lamp, placed at one-foot distance from the 

 receiving screen. Knowing this, the actual luminosity of 

 all other rays of the spectrum can be derived from the 

 curve of luminosity." In this passage, has not the word 

 " intensity " precisely the same meaning as " luminosity " .'' 

 And does not the word "brilliance" again mean the same 

 thing ? Would not the passage be exactly as clear if the 

 word " brightness " had been used instead of the three 

 different words in the places where they occur ? 



The most valuable part of the work is that which re- 

 lates to colour-blindness, where, for the first time, there 

 are given charts showing the degree of apparent 

 luminosity (the qualifying adjective is the reviewer's) in 

 different parts of the spectrum to persons having one 

 or other variety of colour-blindness. One case, men- 

 tioned on p. 85, is of exceptional interest. This is the 

 case of a person who, though he sees each of the three 

 fundamental colours quite correctly, red as red, green as 

 green, violet as violet, is relatively less sensitive to green 

 than other persons of normal vision. " He is defective 

 in the green sensation, although it is present to a large 

 extent." Here is a person who certainly would be quite 

 capable of doing duty as an engine-driver or as a sea- 

 man, perfectly able to distinguish green lights from red,, 

 but who yet would be condemned, if tested by the much 

 over-rated method of Holmgren, as being colour-blind. 

 Indeed, the least satisfactory part of Captain Abney's- 

 book is that wherein, in the face of this very case in 

 which Holmgren's wool tests would unjustly condemn a 

 man, he describes and praises the Holmgren method of 

 testing colour-vision. There is not one word of warning 

 as to its uselessness in such cases, or as to its mis- 

 employment by practitioners devoid of any training in 

 the optical laboratory. Captain Abney says that he " is 

 glad to say" that the Holmgren system has been adopted 

 by the Board of Trade and by most of the railway 

 companies in the United Kingdom. The pity is, that 

 no one explains to those who have to use it how easily 

 the coloured wool test may be (and actually is) misused, 

 and how in certain cases, like that cited, its indications, 

 may inflict a serious injustice. S. P. T. 



