6l2 



NATURE 



\Oct. 23, 1879 



culty. He objects to this, alleging that these phenomena 

 ought to be capable of purely physiological explanations, 

 and he considers that, when properly investigated, they 

 afford the best means of determining the true character of 

 light-vision. 



Acting on this idea, he devotes his first three papers to 

 an examination of these subjective phenomena. He 

 describes a series of experiments of great simplicity, illus- 

 trating the various points of importance, and he founds 

 upon them, as he goes on, a chain of reasoning which 

 leads up eventually to the statement of his doctrine- 

 It will be necessary briefly to notice this introductory 

 matter. 



The first paper treats of what the author calls " Succes- 

 sive Light-Induction," which he illustrates in several 

 different ways. 



If you look steadfastly for, say,fifteen seconds to a minute, 

 at the centre of a small white circular disk, laid on a large 

 black ground (white paper on dull black velvet is best), then, 

 closing and covering the eyes, you will soon perceive a 

 negative after-image [Nachbild) of what you have seen. 

 The disk itself will appear dark, generally much darker 

 than the general visual area, and will be sharply defined, 

 but it will be surrounded by a peculiar "light-space" 

 (Lichtliof), brightest close to the disk, and becoming 

 gradually darker as it recedes, until it fades into the 

 general dark area around. The appearance is that of a 

 halo, or more exactly that of the sun during a total 

 eclipse, where the intensely black circle is seen surrounded 

 by the corona. 



This phenomenon is usually explained as follows : — 

 There is a certain internal light-stimulus which acts when 

 the eyes are closed ; but the part of the retina which has 

 been exposed to the bright light from the disk has become 

 fatigued thereby and is less sensitive to the internal light- 

 stimulus than the parts around ; so that the fatigued circle 

 appears darker than elsewhere. This explanation, which 

 is purely physiological, answers very well so far as the 

 disk is concerned, but it gives no account of the .sur- 

 rounding light-space, for which the psychical dejcs ex 

 inachind is called in,^, it being asserted that the halo is 

 only a mental delusion caused by the contrast between 

 the dark circle and the less dark space around. The 

 author objects altogether to the sufficiency of this sugges- 

 tion, and has devised an experiment to show that the 

 appearance of the light- space is a real and not an 

 imaginary one. 



He proves .that when the light-spaces of two neighbour- 

 ing dark negative after-images are superposed, they cause 

 an increase of brightness. To show this, cut out two 

 squares of white paper and place them side by side on 

 the black ground, leaving a small interval between them. 

 Observe then, as before, fixing the eyes steadfastly on a 

 point in the black interval. In the after-image it will be 

 noticed that the narrow intervening strip is much brighter 

 than the space around the other three sides of each 

 square, showing the effect of the superposition in that 

 place of the two light-spaces together. 



For a further study, examine the negative after-image 

 of a 'narrow strip of black paper laid on a white 

 ground; and direct attention first to the absence of 

 any partial darkening of the general ground close to the 

 strip (which ought to result if it were merely the impres- 

 sion of contrast), and, secondly, to the intensity of the 

 brightness of the image of the strip itself. Lastly, to 

 prove that this brightness is far greater than can be due 

 to the self-working inner light-excitation of the retina, an 

 ingenious experiment is shown (easy to carry out, but 

 somewhat lengthy to describe), by which an objective test 

 of the strength of the illumination can be made. The 

 negative light after-image of a black stripe is found, by 

 direct comparison, to be much brighter than a certain 

 amount of objective light thrown on a fresh and unused 



part of the field of vision, proving, therefore, that the 

 former must be due to something more than the mere 

 unfatigued inner fight of the retina. 



In concluding this paper the author gives some general 

 conclusions from these experiments. From an analogy 

 with certain effects of colour, he names the light produced 

 in the after-image, around a dark area, induced light, and 

 the general effect successive light-induction. This takes 

 place on any part of the retina where there has, in the 

 object originally looked at, been a boundary between light 

 and dark, the light part inducing an impression of light 

 in the neighbouring part, so that when the eyes are shut 

 the latter appears bright. The induced light is naturally 

 most powerful in the immediate neighbourhood of the 

 boundary, and diminishes as the distance from this in- 

 creases, until it fades into the general aspect of the visual 

 field. The author draws two important conclusions from 

 this part of the investigation: (i) that the activity of one 

 part of the retina corresponds with that of others, the 

 usual idea of the independence of each part being untrue; 

 (2) that the so-called independent or inner light of the 

 retina is capable, within certain physiological limits, of 

 being considerably increased in intensity. The pheno- 

 mena of light-induction are subsequently examined more 

 fully from a more general point of view ; in the meantime 

 it is considered as sufficiently shown that the ordinaiy 

 fatigue theory is not sufficient to explain the appearances 

 observed. 



Thesecondmemoiris on" Simultaneous Light-Contrast." 

 The author begins with the simplest example :^If a stri p 

 of gray paper is placed alternately against a black and a 

 white background, in the former case it will appear much 

 lighter than in the latter. The psychical theory explains 

 this as a false mental judgment, assuming that the abso- 

 lute sensation conveyed by the gray paper to the eye is 

 the same in both cases. The author controverts this, 

 insisting that the impression is, as a matter of reality and 

 not of imagination, different in the two cases, which he 

 proves by the following experiment : — Make a ground half 

 white and half black, and lay two narrow strips of the same 

 gray paper "parallel to each other and a short distance 

 apart, one on each half of the ground. The one on the 

 white ground will look so much darker than the other, 

 that a new-comer would scarcely believe they were both 

 the same shade. Now fix the eyes for half a minute or 

 a minute intently on a point midway between the strips; 

 then close and darken the eyes and observe the after- 

 image. The difference in luminosity of the two strips 

 will be even greater than the apparent difference of shade 

 in the original. And as this after-image cannot recognise 

 any matter of judgment, but only faithfully records the 

 actual state of the retina, it proves that the difference in 

 shade observed was real, and not imaginary; and the 

 inapplicability of the false-judgment-by-contrast explana- 

 tion is manifest by the fact that the difference between 

 the two strips will often remain after the background, 

 forming the contrast, has disappeared. 



The author then goes on to give his explanation oi 

 simultaneous contrast, namely, that the light-perception 

 of any part of the retina depends not only on the illumi- 

 nation of that part, but also on the illumination of other 

 parts adjoining. He adds that as in modern physiology 

 it is customary to explain the varying strength of reaction 

 consequent on a uniform stimulus, as \Vi.xymg'excitability 

 (Erregbarkeit), it may be said that here the effect of 

 contrast is due to the fact that the excitability, and 

 consequently the excitement, of one part of the retina is a 

 function of the contemporary illumination of the whole 

 retina, or at least of the parts immediately around. 



The next step is to point out the connection that exists 

 between the effect of contrast treated of here, and the 

 light-induction described under the former head. The 

 phenomena of successive light-induction showed that a part 



