SENSE ORGANS. 



153 



produce. Hence the perspective is far clearer when 

 the picture is looked at with one eye only. 



JUDGMENTS OF SIZE AND DISTANCE. 



The eye perceives at once direction up and down and 

 right and left, and therefore outline form and surface 

 contents, for this is a combination of directions. Thus 

 two dimensions of space — viz., angular diameter in all 

 directions — are given immediately. But this does not give 

 size, unless distance, or the third dimension, is also 

 known. Now, this third dimension is not given in sense, 

 but is a judgment. The direct gifts of sight are light, its 

 intensity, its color, and its direction, and therefore also out- 

 line form. But size, distance, and solid form are judg- 

 ments based on these gifts. Moreover, size and distance 

 are closely correlated, so that a mistake in one will cause 

 a corresponding mistake in the other. 



Distance. — We judge of distance by the various 

 forms of perspective already explained. Being a judg- 

 ment, we are liable to error. We often say " our senses 

 deceive us." Not so. We make false judgments on true 

 reports of the senses. 



Size. — The size of an object is judged by its angular 

 diameter, or size of its retinal image, multiplied by its 



Fig. 94. 



estimated distance. For example, in Fig. 94 the reti- 

 nal image a may be made by A or A' or A", and the ap- 

 parent size of the spatial correspondent will vary ac- 



