Oct. 30, 1879] 



NATURE 



637 



coast of Europe and Asia was perfectly practicable to 

 that river. Some of the scientific results of these ex- 

 peditions were published in Nature at the time, and it is 

 well known that so far as the immediate object was con- 

 cerned the expeditions were completely successful. Full 

 details will be found in Mr. Leslie's volume. From what 

 we have said it will be seen that comparatively young as 

 Prof. Nordenskjold is, he has done an amount of work 

 rarely accomplished even in a long lifetime. Appended 

 to Mr. Leslie's volume is a long bibliography of the 

 published results of these expeditions of Nordenskjold, 

 and from this it is evident that they have borne rich 

 fruit in nearly every department of science. 



HERING'S THEORY OF THE VISION OF 



LIGHT AND COLOURS^ 



II. 



BEFORE propounding his theory, the author thinks it 

 necessary to devote one memoir- — the fourth —to an 

 essay, the object of which is to define clearly the nature of 

 the sensations of black and white and their mixture gray. 

 He remarks that it is a habit to treat visual sensations 

 rather according to their physical origin than by their own 

 nature ; and this peculiarly influences the ideas enter- 

 tained about the sensations of black and white. We know 

 that physically, white light is a combination of rays of 

 all wave-lengths, and we have no physical notion of black 

 except a negative one, namely, as an absence of light of 

 any kind. Hence, transferring our physics to our phy'sio- 

 logy, we consider that our sensation of white is a positive 

 one, but that our idea of black arises simply from the 

 absence of all sensation ; or, to use a metaphor drawn 

 from painting, black is our canvas, or background, on 

 which all our sensation-pictures are drawn in white or 

 colours ; as a result of this, all our reasoning is confined 

 to the pictures, while the background receives no 

 attention. 



The author, as one of the main points of his theory, 

 strongly objects to this view. He denies that the natural 

 unimpressed state of the visual sensation corresponds 

 with black, appealing to every-day experience in support 

 of the opinion. Any one who carefully examines his 

 impressions after being for some time in a perfectly dark 

 room, will observe a dark field, it is true ; but if he tries, 

 in imagination, to compare this with his sensation of a 

 piece of the blackest velvet, he will be obliged to admit 

 that the field is nothing approaching the latter in dark- 

 ness ; it is, in fact, only dark gray. Or as an easier and 

 simpler test, let him compare the black after-image of a 

 white disk with the general field given by his closed and 

 darkened eyes, and he will observe a similar contrast. 



The author's view is that the impression of black, like 

 that of white, can only be derived from external sources ; 

 and that consequently black is a perfectly independent 

 visual sensation, which should be studied physiologically 

 like those of white, or red, or blue. On physiological 

 grounds it is no more reasonable to consider black as the 

 absence of white, than white as the absence of black, or 

 blue as the absence of yellow, or to consider a sphere as 

 the absence of solids of every other form. 



On this principle he proceeds to discuss the sensation 

 of gray. He objects to the usual mode of defining dif- 

 ferent shades of gray as merely different intensities of 

 light. He considers any sensation of gray as a combina- 

 tion of the two independent sensations, black and white, 

 in certain proportions ; he calls this accordingly a black- 

 luhite sensation, and he proposes to express it in a mathe- 

 matical form. The full and perfect extreme sensations 

 are practically unknown, and therefore no positive quanti- 

 tative expressions can be used for them. But it is quite 

 permissible to gire an algebraical idea of the difference 

 between intermediate gradations, and this may be done in 



< Contiuued from p. 613. 



the form of a ratio, or fraction, of which the two com- 

 ponents express the assumed amounts of white and black 

 respectively that are combined in the sensation. For 

 example, there must be a practical gray (though we cannot 

 identify its exact shade) which is intermediate between 

 white and black, resulting from an equal force of each 

 sensation. Here therefore if W = the force of the white 



sensation, and B = that of the black one, —- = - or = i. 



B I 



For a lighter gray in which there is twice as much white 

 as black, -^=-=2. Andfor a darker gray, in which there 



is twice as much black as white, 



W 

 B 



On this principle 

 the pure white sensation would be expressed by — =00, 



and the pure black sensation by - = o. 



It is possible, still retaining the principle, to give a 

 more convenient expression for the brightness or lightness 

 {Heili^keit) of any black-white sensation ; thus, the 

 degree of brightness may be expressed by the ratio which 

 the white element bears to the whole sensation, or 



— Thus the brightness of the medium gray will 



be = 



W+B" 

 I 



= o'5 ; and that of the mixture of two 



I -+- I 2 



white to one black will be = — ? — = - = o'66 ; and that 



2+1 3 



ofthe mixture of two black to onewhite = — ; — =- = 0*33. 



1+2 3 



The brightness of pure white will be = tjT";; ~ '' ^^^ 



that of pure black = 



1 + 



= o. This mode of definition 

 o+i 



corresponds to the usual practical idea of the intensity of 

 white in gray, but it differs from it by acknowledging the 

 independent black element in the composition. 



In the fifth memoir we at length get a statement of the 

 fundamental features, of the author's theory, so far as 

 the black-white sensation is concerned. 



He begins by objecting to the treatment of white as a 

 mixture of complementary colours, as blue and yellow, or 

 red and green, or of all colours together, an idea which 

 has arisen solely from physical considerations. No one, 

 he says, can pretend that the least trace of any other 

 colour can be distinguished in a pure white sensation ; 

 all that can be said is that the sensation of white is pro- 

 duced by a mixture of light of different wave-lengths. 

 But the sensation is a perfectly independent one, like black, 

 or red, and must be so considered in an investigation into 

 the rationale of the visual perceptions. 



Since the physiologist considers all sensations as called 

 into existence by physical processes of the nervous system 

 (for otherwise every physiological investigation would be 

 objectless), he must assume so-called psycho-physical 

 processes or movements which correspond to the sensa- 

 tions of black, of white, and of all shades between them. 

 In what part ofthe nervous system these psycho-physical 

 processes are situated it is impossible to say ; suffice it 

 that, somewhere in the nervous apparatus of the eye and 

 the parts of the brain standing in functional relation 

 therewith, a substance must be sought, with the changes 

 or motion of which the sensation is bound up; this 

 substance may be called the "visual substance ' (Seh- 

 sitbstanz). 



The action of this substance may be studied in two 

 ways : either il priori, by considering the physical influ- 

 ences brought to bear upon it, or, i\ posteriori, by con- 

 sidering the sensations.'resulting from its changes. The 

 former mode has hitherto been of little profit, for 



