354 THE SPECIAL SENSES. 



so to speak, simultaneously with two different sensations. Under 

 such circumstances what is known as the struggle of the visual 

 fields ensues. If the image on one eye consists of vertical lines 

 and on the other of horizontal lines we see only one field at a time, 

 first one then the other, or the field is broken, vertical lines in part 

 and horizontal lines in part ; there is no genuine fusion into a con- 

 tinuous, constant picture. The struggle of the two fields is better 

 illustrated when different colors are thrown on the two retinas. 

 When red and yellow are superposed on one retina we obtain a com- 

 pound sensation of orange; if they are thrown one on one retina, 

 one on the other, no such fusion takes place. We see the field 

 alternately red or yellow or a mixture of part red and part yellow, 

 or at times one color, as it were, through the other. If, however, 

 one field is white and the other black a peculiar sensation of glitter 

 is obtained, quite unlike the uniform gray that would result if the 

 two fields were superposed on one retina. 



Judgments of Solidity. Our vision gives us knowledge not 

 only of the surface area of objects, but also of their depth or solidity, 

 that is, from our visual sensations we obtain conceptions of the 

 three dimensions of space. The visual sensations upon which this 

 conception is built are of several different kinds, partly monocular, 

 that is, such as are perceived by one eye alone, partly binocular. 

 If we close one eye and look at a bit of landscape or a solid object 

 we are conscious of the perspective, of the right relations of fore- 

 ground and background, and those individuals who have the 

 misfortune to lose one eye are still capable, under most circum- 

 stances, of correct visual judgments concerning three dimen- 

 sional space. Nevertheless it is true that with binocular vision 

 our judgments of perspective are more perfect, and that under 

 certain circumstances data are obtained from vision with two eyes 

 which give us an idea of solidity far more real than can be obtained 

 with one eye alone. This difference is shown especially in the 

 combination of stereoscopic pictures, and in ordinary vision when 

 the light is dim, as in twilight, or in exact judgments of perspective 

 in the case of objects close at hand. If, for example, we close one 

 eye and attempt to thread a needle, light a pipe, or make any similar 

 co-ordinated movement that depends upon an exact judgment of 

 the distance of the object away from us, it will be found that the 

 resulting movement is far less perfectly performed than when two 

 eyes are used. The sensation elements upon which our judgments 

 of depth or perspective are founded may be classified as follows :* 



The Monocular Elements, That is, those that are experienced 

 in vision with one eye. (a) Aerial perspective. The air is not en- 

 tirely transparent, and, therefore, in viewing landscapes the more 

 distant objects are less distinctly seen, as is illustrated, for instance, 

 * See Le Conte, "Sight," vol. 31 of "The International Scientific Series," 1881 



