Oct. 30, 1879] 



NATURE 



639 



lus by light a reaction is set up not only by the parts 

 directly stimulated, but by the surrounding parts; the 

 former through increased dissimilation, the latter through 

 increased assimilation, which, however, is most powerful 

 close to the lighted part, and diminishes fast with the 

 distance from it. 



This explains why, in a lighted room, the parts in shade 

 appear black, much darker than our sensation with closed 

 eyes, although the D-stimulus is equally active in both 

 cases ; and not only do the so-called dark parts really 

 reflect some light, but a portion of dispersed hght by 

 objective irradiation enters the eye, which latter is 

 strongest in the immediate neighbourhood of the bright 

 object. But the increase of the assimilation prevents the 

 perception of this light, and thus the ground is darkened, 

 and the boundaries of the bright object are more sharply 

 defined and brought out. 



It is a result of this theory, that when two neighbouring 

 parts of the retina are both stimulated by light at the 

 same time, each reacts on the other by increased assimila- 

 tion, the effect of which is to reduce the brightness of 

 both. Hence, a small white surface is brighter than a 

 large one of the same objective material. This may be 

 easily seen by putting a large sheet of white paper, and a 

 small strip of the same, both on a black ground. Or hold 

 against the sky a large sheet of black paper, near the 

 edge of which a small hole has been pierced ; the point 

 of light thus produced will be far more intense than that 

 perceived round the edge of the paper. This is also the 

 explanation of the great apparent brilliancy of the stars, 

 the objective illumination of which is so very weak. 



Explanation of Simultaneous and Successive Light In- 

 duction. Following up the process above described for 

 simultaneous contrast, suppose the white object on the 

 black ground to be further steadfastly observed for a 

 longer time. The increase of assimilation in the parts 

 immediately surrounding the white will cause an increase 

 of the excitable substance, and will thus bring about an 

 increase of excitability there. Hence the constantly 

 working inner stimulus and the weak dispersed light of 

 the black ground will acquire more dissiniilating effect, 

 while the assimilation gradually becomes weaker. Hence 

 will follow an increase of apparent brightness, on the 

 parts previously darkened by contrast ; this is simul- 

 taneous light induction. At the same time that the 

 ground brightens in this way, the part of the visual organ 

 impressed by the white surface suffers, by the prolonged 

 dissimilation, a diminution of the excitable substance, 

 whereby the excitability diminishes and the apparent 

 brightness consequently diminishes also. If the contrast 

 be made with only a slight difference in shade, and if the 

 observation be carried on long enough, a phase will 

 ultimately set in, in which by the gradual brightening of 

 the ground and darkening of the whiter surface, they will 

 both acquire the same appearance, and the distinction 

 between them will disappear. This maybe easily proved 

 by experiment, but to prevent the confusing effect of the 

 outlines, it is better shown by making, on white paper, 

 dark patterns with shaded edges, when the effect will be 

 soon apparent. 



As the illumination of the light surface decreases it 

 loses its power to favour the assimilation in the neigh- 

 bouring dark parts, while the dissimilation under the 

 influence of the inner D-stimulus not only goes on, but 

 finds a greatly increased excitability to work upon ; i.e., 



according to the theory, the proportion — [—\ becomes 



greater, which means that the sensation increases in 

 brightness. Hence, after sufficiently long steadfast 

 observation, the bright-light space appears, and when the 

 eyes are shut the sensation remains, as before described, 

 giving successive light induction. 



The explanation of Successive Contrast is given in four 

 illustrations: — r. Observe fixedly a white stripe left 



between two large black surfaces ; it will be seen that the 

 original brightness gradually diminishes, and if the black 

 surfaces be suddenly removed so as to leave an entirely 

 white ground, the stripe will appear upon it as an after- 

 image of a dark gray. This is, acccording to the theory, 

 the result of the sudden bright illumination of the neigh- 

 bouring parts ; before this took place the dissimilation 

 was powerful on the stripe, and (as before explained) 

 excited an assimilation on the neighbouring black parts ; 

 this increased the D-excitability, and when the black 

 surfaces were removed the white suddenly began to act 

 with great power, setting up an assimilating action on the 

 stripe where the excitability had been just before dimi- 

 nished, and so resulted the darkening effect on the latter. 



2. Observe a small black stripe between two large 

 sheets of white paper ; it will at first appear very dark, but 

 will gradually become lighter, and if the white sheets be 

 suddenly removed, it will appear light on the black 

 ground. This is the ordinary simultaneous and succes- 

 sive light induction already explained. 



3. Lay a narrow white stripe on a black ground, observe 

 it for a time, and then suddenly remove it ; the place 

 where it was will then appear blacker, and its neighbour- 

 hood lighter, than before. While observing the white 

 stripe, the excitability upon it was diminished, and that 

 around it increased, on the principles already explained : 

 and on its removal the neighbouring parts were more im- 

 pressed, and the place of the stripe less so, by the inner 

 stimulus and the faint light of the general black ground. 



4. Observe a black stripe on a white ground, and then 

 suddenly remove it, leaving the whole field white. The 

 place of the stripe will appear a brighter white surrounded 

 by a darker space of a gray tinge. This is simply the 

 converse of No. 3. The excitability was raised on the 

 stripe and lowered around it ; and when the whole field 

 became active as a stimulus, the sensation was more 

 powerful in the former place than the latter. 



Lastly, the author devotes a chapter to the considera- 

 tion of Ihe fatigue of the visual organ. He says this may 

 arise from two causes. When a light-stimulus is received, 

 both a D-action and an A-action are set up, as previously 

 explained : the D-action will naturally fatigue the organ 

 by the dissimilation of the visual substance, but a similar 

 result may follow also from the A-action if the assimilation 

 goes on at a greater rate than fresh matter can be 

 provided by the blood-supply. After looking at any very 

 bright object, as the sun, and then covering the eyes, the 

 after-image is not at first negative, but positive, and this 

 bright impression may last for a long time, although, if 

 the vision be thrown on white paper, the image will 

 appear darker. 



The explanation is as follows : — While looking at the 

 bright object there is set up not only a strong dissimila- 

 tion, but also a very considerable though less strong, 

 assimilation. By the first the D-excitability will be 

 greatly diminished, and by the latter the material in store 

 will be quickly used up. Hence in the darkened eye, the 

 sensation caused by the inner D-stimulus will be opposed 

 only by a very weak A-action, so giving an after-impres- 

 sion of light, but of no great power. As the blood affords 

 fresh matter, the etiuilibrium will be restored, and the 

 appearance will die away. 



In making the experiments hereinbefore mentioned on 

 subjective vision, many different phases set in ; these, 

 though generally attributed to chance, are really due, in a 

 large measure to the complicated influences of fatigue, 

 caused as above described, and to their interference with 

 the regular course of the light-induction and other 

 processes hereinbefore described. 



The author states, in conclusion, that he is far from 

 believing that the theory he has developed is perfect, or 

 incapable of correction, but he considers it comes nearer 

 the truth than any other. WILLIAM POLE 



(To be continued.) , 



