Nov. 6, 1879] 



NATURE 



i7 



character and name to the mixed sensation. If any one 

 fundamental sensation is so strong in predominance over 

 all the others, that the latter are not distinguishable, it 

 approaches the idea of purity, which, however, in reality- 

 can never exist. 



When the visual organ has been for a long time 

 protected from any external stimulus, it assumes, more 

 or less perfectly, that neutral condition in which the 

 assimilation and dissimilation, as well as the D-excita- 

 bility and the A-excitability are equal for all the three 

 visual substances. 



In this state, in order that a mixed light should produce 

 a colourless impression, it is necessary that this light 

 should have an approximately equal assimilating and 

 dissimilating moment, by which is meant the product of 

 the stimulus and the excitability. Such mixed light may 

 be called objectively colourless light. 



But the same light will appear, say, greenish, if the red- 

 green substance is no longer in the neutral condition, but 

 has its green excitability greater than that of the red. 

 For in this case the A and D-moments will not be equal, 

 and a small difference will exist to the advantage of the 

 green. 



Now when a part of the previously neutrally tuned visual 

 organ has been stimulated by coloured light, the condition 

 of this part will become so altered that the excitability 

 for the perceived colour diminishes and becomes less 

 than the excitability for the opposite colour. Under 

 these circumstances any mixed light which, in the neutral 

 condition appeared colourless, will now appear coloured 

 with the opposite colour. And if a part of the visual 

 organ has been affected, say, by the action of green light, 

 on looking at a blue or yellow surface, the blue or yellow 

 will appear tinged with red. 



The phenomena of simultaneous colour-contrast and of 

 colour-induction are explained in the same manner as for 

 the corresponding phenomena in black and white. 



It has been seen that by the white illumination of any 

 part of the visual organ, the other parts of it, and particu- 

 larly those adjoining, are, by the indirect action of the 

 stimulus, darkened ; or the sensation of black is intensi- 

 fied ; and in a similar way, under the action of coloured 

 light on any part, the sensation of the opposite colour is 

 strengthened in the adjoining parts. 



As a consequence of this the relations of excitability 

 are altered ; for, according to this theory the sensation of 

 any colour implies also a change of those relations to the 

 disadvantage of this colour and to the advantage of its 

 opposite. If, then, light be allowed to fall on the whole 

 retina, which, under a neutral condition, would be colour- 

 less, it now appears coloured; the colour on the pre- 

 viously excited part will be the opposite colour (successive 

 contrast), while that around it will be the same colour as 

 that previously observed (successive induction). In fact, 

 all the phenomena explained for white and black may be, 

 in this way, transferred to the colour-sensation. 



The author makes some further remarks on the Young- 

 Helmholtz theory. He admits that the attempt of Young 

 to reduce the great variety of colour-sensations to a small 

 number of physiological variables was a most important 

 step in advance, but he considers that if, as before ex- 

 plained, every psychical result must correspond to some 

 physiological process, the number of fundamental colour- 

 sensations ought, as has been often urged, to be increased 

 to four, and that separate physiological sensations ought 

 to be allotted to white and black. But he considers the 

 great defect of the theory to lie in its only acknowledging 

 one kind of excitability, excitation, and fatigue, namely, 

 that which he denotes by D, and that it ignores entirely 

 the antagonistic relations of certain rays to the visual 

 organ ; hence it regards the production of white out of 

 "complementary colours" as a result of their mutual 

 combinations, and not of their mutual extinction. 



He also remarks on the difficulties of explanation of 

 many phenomena, on this theory, and in particular on the 

 inconsistencies it causes in the attempted explanation of 

 colour-blindness, as shown in the most modern literature 

 on the subject. 



In conclusion he gives some remarks on the chief 

 points of his theory, which it may be instructive to repeat 

 here. 



The theory, although immediately dictated by a free 

 and unbiased analysis of the visual sensations, is essen- 

 tially based on certain fundamental principles, taken from 

 the acknowledged phenomena of organic and psychical 

 life, and it is by these principles that the author's views 

 are brought into connection with the doctrines of physio- 

 logy generally. 



With respect to the doctrine of light and colour, the 

 first thing to mention is the natural system of visual sen- 

 sations, founded on their internal similarity ; and further, 

 the grouping of the six fundamental sensations in three 

 pairs of opposite colours. 



Next comes, as of fundamental importance, the appre- 

 ciation of the visual sensations as the psychical correlatives 

 of the nutritive processes, or changes of matter in the 

 visual substance, which leads to the separation of the 

 D and A sensations, and further, to the principle that 

 every D sensation implies a decrease, every A sensation 

 an increase of the visual substance. Corresponding to the 

 three pairs of simple or fundamental sensations are 

 assumed three kinds of D and A processes in the visual 

 substance and three kinds of specific D and A excita- 

 bility. The colourlessness of mixed light formed out of 

 "complementary" rays, is explained by their antagonistic 

 relations. 



Further, here, for the first time the proof is methodi- 

 cally and comprehensively produced that the separate 

 parts of the nervous visual substance are in internal 

 functional changingrelationship(Jl'Vt'/:^/for/V//«;/j'), which 

 is to be regarded as reciprocally connected with the 



change of matter; for when _ is greater on a stimulated 



part, it is less in the surrounding parts, and vicevcrsd ; so 

 that after the stimulus the excitabilities of both parts 

 change in opposite directions. 



These propositions and their consequences afford the 

 means of explaining the various phenomena mentioned, 

 but it often happens that several explanations are possible 

 for one and the same phenomenon, and that the decision 

 between them must be reserved for more detailed inquiry. 

 What we immediately appreciate in a visual sensation is 

 the ratio of the corresponding D and A processes to each 

 other, for this determines the quality of the sensation. A 

 change of sensation gives only an indication of the change 

 of this ratio, and not of the changes of its two com- 

 ponents. Then it is that we have so often the choice 

 between an increase of assimilation and a stoppage or 

 decrease of dissimilation, and vice vcrsii. But the theory 

 itself gives means of determining these, by further and 

 more detailed and intricate investigations ; and the 

 author promises future communications by which the 

 details will further be supplied, without as he hopes, any 

 material alteration of the principles he has laid down. 



William Pole 



THE "PARASOL" ANTS OF TEXAS: HOW 

 THE Y CUT AND CA RR Y LEA VES : ORIGIN 

 OF CASTES BY EVOLUTION 



IN Mr. McCook's recent investigation (Nature, vol. xx. 

 p. 583, andProc. Acad. Nat. Sci. Phil., 1879, p. 35) 

 he stated that he observed carefully both the mode of 

 cutting and the svstem of carrying cut portions of oak- 

 leaves at Camp Wright, and at a vegetable garden near 

 Austin, Texas. 



To investigate successfully he found it best to thrust 



