180 EXPERIMENTAL PHYSIOLOGY. 



in visual judgments because they are so largely subjective in 



nature. The student is expected to make observations for himself 



along the lines indicated below. He should also use the various 



charts which have been prepared to assist in this part of the work. 



Experiment 75. Some of our accuracy depends on the use of 



both eyes together. Try to touch with the tip of your pencil a 



black dot in the middle of a sheet of white paper which you 



hold obliquely in front of you. Now close one eye and see 



whether or not you can perform the movement as well as before. 



MONOCULAR ELEMENTS IN JUDGMENT OF DISTANCE. One of 



these is the HAZINESS or clearness of the air between the object 



and the observer. Objects at a distance are hazier, all other things 



being equal, than those near by, so that we usually interpret 



hazy objects as being distant. That is why objects "loom large" 



in a fog. Since the things look hazy we conclude that they are 



farther away than they really are. The size of the images which 



they cast on the retina is not changed, however, and this together 



with our idea of their distance is the only clue which we have as 



to the size of the objects. If they were really as far away as we 



imagine them to be, to throw images of this size on the retina they 



would have to be larger than they really are, and we form our 



judgments accordingly. 



A second and more important factor in monocular judgment is 

 that known as MATHEMATICAL PERSPECTIVE. It is familiar that 

 lines which are really parallel to one another appear as they recede 

 into the distance to come closer and closer together. The rails of 

 a level stretch of track, for instance, appear to one standing be- 

 tween them to form a V, with its point beyond the horizon. We 

 are so familiar with this effect of distance that when we see lines 

 which we believe to be parallel, or nearly so, appearing much 

 closer together at the farther than at the nearer end of an object, 

 we judge the object to have a good deal of depth. 



THE MUSCLE SENSE of the ciliary muscles gives important 

 information as to the depth of objects which are close at hand, 

 because of the difference in the extent of accommodation necessary 

 to bring into focus first their nearest and then their farthest point. 

 The greater depth they have, the more difference there is in the 

 effort of accommodation in the two cases. 



