BINOCULAR VISION. 



1123 



object with well-marked contours, this will tend to prevail in the 

 struggle between the two fields, and will not disappear periodically, 

 although the background may continue to do so. If each field contain 

 an object with contours, and these coincide, there will be marked rivalry, 

 especially if the objects differ greatly in form. At times only one of 

 the objects will be seen, while at other times parts of the two objects 

 will be seen together. Kivalry may occur when one eye is closed ; if a 

 sheet of white paper is observed close to the eyes while one eye is 

 closed, a periodical darkening of the paper occurs, which is due to the 

 influence of the intrinsic light of the closed eye (Purkinje, Hering). 

 In 1864, Hering l pointed out that the behaviour of black in retinal 

 rivalry was evidence in favour of black being a positive sensation. 



The explanation given of retinal rivalry has been chiefly psycho- 

 logical. Helmholtz referred the phenomenon to the influence of attention, 

 this being applied at one moment to the field of one eye, and at the 

 next to the field of the other. The variations may take place with 

 considerable regularity, and the periods of variation show some 

 correspondence with those of the variations of after-images. We are at 

 present wholly in the dark as to the physiology of this phenomenon. 



Brightness of binocular field. The apparent brightness of a given 

 illumination is not the same for one and for two eyes ; according to old 

 observations, a light seen with two eyes appears about one-tenth brighter 

 than when seen with one eye alone. When the illumination of the two 

 eyes is different, the apparent brightness of the combined field varies with 

 the intensity, and with the difference of illumination of the two eyes. 

 Fechner 2 found that if a dark glass were held before one eye, the apparent 

 brightness of the field increased when this eye was closed or screened, and 

 diminished again on reopening the eye. In this experiment the bright- 

 ness of the sensation increases when the amount of light entering the 

 eyes is diminished, and decreases when it is augmented, and it is con- 

 sequently known as " Fechner's paradoxical experiment." After a time 

 the brightened field may be observed to darken again owing to rivalry. 

 If the glass used lets through much light, this effect does not occur, and 

 the field brightens and darkens with increase and decrease of illumination. 

 For each individual, Fechner found an indifferent point, corresponding 

 to a glass with which the brightness of the combined field remained the 

 same whether the eye was closed or open. Such a glass was found on 

 the average to be one which transmitted two-thirds of the light. 



The greatest amount of darkening occurred with a glass which let 

 through about ^V of the light, and on either side of this point glasses 

 could be found with which the combined field was equally bright ; thus a 

 glass transmitting |- of the light gave a combined field of the same 

 brightness as one transmitting -^-, and such glasses were said by 

 Fechner to be of conjugate intensity. It has been supposed that the 

 paradoxical experiment is due to the pupil ; closing the eye behind the 

 dark glass would lead to a consensual widening of the pupil of the other 

 eye, and might in this way brighten the field. Fechner found, however, 

 that the experiment succeeded with a small artificial pupil, and the 

 phenomenon also occurs as well with atropinised pupils as in the normal 

 condition. No satisfactory physiological explanation has been given. 

 It has been supposed that the brightness of the combined field is the 



1 " Beitr. z. Physiologie," Leipzig, 1864, Heft 5, S. 309. 

 -Abhandl.d. k. sacks. Gesellsch. d. Wissensch., 1861, Bd. v. S. 416. 



