Oct. 23, 1879] 



NATURE 



61 



of the retina illuminated cither not at all or very faintly, 

 but whose immediate neighbourhood was brightly illumi- 

 nated, gave, in the negatire after-image, a much stronger 

 brightness than one, the neighbourhood of which was not 

 so illuminated. The phenomena of contrast show, on the 

 other hand, that the light-perception of a weakly illumin- 

 ated part of the retina is diminished when its surrounding 

 part is brightly illuminated. Thus the lowering of the 

 perception of brightness (or excitement) during the obser- 

 vation of an object, corresponds to the raising of this 

 perception in the after-image, so that the successive 

 light-induction appears as the opposite or reversal 

 (Gegensatz) of the simultaneous contrast-action. And 

 the latter may be denoted as a simultaneous negative 

 light-induction. 



It is, therefore, easy to imagine an internal causal 

 connection between both phenomena, and to consider the 

 lowering of the light-perception (excitability) which takes 

 place during the contrast, as the cause of the raising of 

 the perception which takes place afterwards. 



The third memoir treats of "Simultaneous Light-Induc- 

 tion" and "Successive Contrast." To illustrate the 

 former, look fixedly at any point in a line dividing a 

 black space from a white one ; after looking some 

 time you will see that the originally deep black begins 

 to lighten into gray ; the brightening is the greatest 

 close to the division line, and diminishes farther away, 

 but after sufficiently long observation it will extend more 

 or less over the whole black field. A contrary effect 

 takes place, but less prominently, in the other half, the 

 white darkening in a similar way. 



This appearance is usually explained in the following 

 half physiological, half psychological way. From the 

 increasing fatigue of the retina, the white sensation 

 becomes gradually weaker, and, as a consequence of this, 

 the contrast action between the white and the black 

 becomes weaker, and the latter appears lighter, through 

 the same false judgment invoked to explain contrasts 

 generally. 



To show the insufficiency of this explanation the 

 following experiment is proposed. Put a strip of black 

 velvet on a white ground, well lighted by a gas flame ; 

 after observing it steadfastly for a time gradually diminish 

 the light, and it will be seen that the black strip becomes 

 brighter, and the white ground darker, until at length the 

 former appears actually lighter than the latter. If the 

 explanation is applied to this it must be assumed that the 

 fatigue of the retina has become so great that the 

 moderated white produces actually a less powerful im- 

 pression than the black stripe, which gives out light of 

 verj' small intensity, or in other words, that the power of 

 the retina is so crippled that a moderate light is no longer 

 able to produce an impression equal to the inner self- 

 illumination of the unfatigued organ. This can be also 

 shown by another trial which is described. 



A further experiment enables a direct comparison to be 

 made. In the middle of a broad white stripe laid on a 

 black ground, cut a longitudinal opening ; this will show 

 an internal black stripe, surrounded by a white surface 

 which is again surrounded by a black ground : then fix the 

 eyes on the middle of the figure, and, after observing some 

 time, gradually reduce the.light. It will soon be found that 

 the black internal stripe appears lighter than the external 

 black (ground, although both are objectively equally dark, 

 and the parts of the retina brought into action are, as it 

 is said, equally free from fatigue. 



The changes consequent on the reduction of the illu- 

 mination show that the simultaneous light-induction 

 passes over into the successive, the latter being, indeed, 

 only a more clearly manifested continuation of the former. 

 The connection of the three phenomena hitherto treated 

 of may be illustrated simply as follows : — When you first 

 observe a line of division between dark and light, the 



dark part, especially near the division, appears- darker 

 than it would do if the white were absent ; this is simul- 

 taneous contrast. Continue the fixed observation a longer 

 time, and the darkening gradually changes to a contrary 

 effect of brightening, which also is most powerful near the 

 division; this is simultaneous light-induction. Lastly, 

 close and darken the eyes, and this brightened space con- 

 tinues a long time visible in the after-image. This is 

 successive light-induction. 



The last of the subjective phenomena treated of by the 

 author he calls " Successive Light Contrast." It differs 

 from the three last mentioned in that, while they all refer 

 to the comparison of one visual space with another close 

 adjoining, this refers to a comparison of the impressions 

 derived from the same space at two successive times. 

 The simplest example is as follows : — Put a strip of white 

 paper on a black ground, and observe it steadfastly for a 

 time ; then let it be suddenly removed, keeping the eyes 

 fixed on the same spot. A negative after-image will 

 result, of a changeable character, but showing generally 

 a space corresponding to the paper, much darker than the 

 already dark ground. This is very striking ; an inex- 

 perienced observer will be loth to believe that he can see 

 anything darker than the blackest velvet ; but there can 

 be no mistake about the impression conveyed. The usual 

 explanation of this is that the part of the retina stimu- 

 lated by the white paper is more fatigued than the rest, 

 and therefore, after it is removed, is less sensitive to the 

 faint light given off by the black ground, making it thus 

 appear still fainter. The author deems this explanation 

 insufficient, as it is inconsistent with many of the changing 

 phases observed in the after-image ; and he describes 

 several other experiments to support this view. He 

 points out that the "successive light-induction," before 

 described, may play an important part in these pheno- 

 mena, but he does not attempt here to give any complete 

 explanation of them. 



It is pointed out at the close of this part of the subject 

 that many of the experiments may be made with colours, 

 instead of with simple light and shade, it being under- 

 stood that black and white hold the same relation to each 

 other, for this purpose, as blue and yellow, or red and 

 green. William Pole 



(To be continued.') 



ALFRED HENRY GARROD 



T T has seldom been our lot to have to record the pre- 

 ••• mature close of a career so full of promise as that 

 which ended with the death of Alfred Henry Garrod, at his 

 father's house in Harley Street, on Friday last, October 

 17. The son of an eminent physician. Dr. Alfred Baring 

 Garrod, F.R.S., he was born in London on May 18, 1846, 

 received a medical education at King's College, London, 

 and in 1868 entered St. John's College, Cambridge. He 

 graduated (B.A.) in 1871, taking the highest place in the 

 natural science tripos. In due course he took his M.A. 

 degree, and was elected a Fellow of his college in 1873. 

 His earliest scientific predilections were chiefly for 

 mathematics and physics, and the knowledge of these sub- 

 jects which he acquired when a student was of great value 

 to him in his biological researches. The mechanics of 

 physiology was the subject to which he first turned his 

 attention as a scientific investigator, and, while still an 

 undergraduate, he communicated a paper on the cause 

 of the diastole of the ventricles of the heart to the 

 Journal of Anatomy (voL iii., 1869). About the same 

 time he sent to the Royal Society the results of an in- 

 teresting series of experiments made upon himself with a 

 view of ascertaining the causes of the minor fluctuations 

 in the temperature of the human body while at rest, 

 from which he concluded that these fluctuations mainly 

 result from alterations in the amount of blood exposed at 

 the surface to the influence of absorbing and conducting 



