344 THE SPECIAL SENSES. 



sons suffering from " squint" (strabismus). In this condition the 

 individual, for one reason or another, is unable to adjust the contrac- 

 tions of his eye muscles so as to unite his lines of sight upon the 

 object looked at. The image of the object falls upon non-corre- 

 sponding points and should give double vision, diplopia. This 

 would undoubtedly be the case if the condition came on suddenly ; 

 just as double vision results when we dislocate one eyeball by 

 pressing slightly upon it. But in cases of long standing one of the 

 images, that from the abnormal eye, is usually suppressed. The 

 act of suppression seems to be a case of a stronger stimulus prevail- 

 ing over a weaker one in consciousness, just as a painful sensation 

 from stimulation of one part of the skin may be suppressed by a 

 stronger pain from some other region. 



Struggle of the Visual Fields. When the images of two dis- 

 similar objects are thrown, one on each retina, the mind is presented, 

 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 



