THE SENSES. 731 



By a mental operation of this kind, we obtain a correct idea of the size 

 of individual objects, as well as of the extent of the field of vision. 

 To illustrate this, it will be well to refer to fig. 433. 



The angle #, included between the decussating central rays of two 

 cones of light issuing from different points of an object, is called the 



Fig. 433. 



optical angle angulus opticus sen visorius. This angle becomes larger, 

 the greater the distance between the points A and B ; and since the angles 

 x and y are equal, the distance between the points a and b in the image 

 on the retina increases as the angle becomes larger. Objects at different 

 distances from the eye, but having the same optical angle x for exam- 

 ple, the objects, c, ^, and 0, must also throw images of equal size upon 

 the retina; and, if they occupy the same angle of the field of vision, 

 their image must occupy the same spot in the retina. 



Nevertheless, these images appear to the mind to be of very unequal 

 size when the ideas of distance and proximity come into play; for, from 

 the image a b, the mind forms the conception of a visual space extend- 

 ing to e, d) or c, and of an object of the size which that represented by 

 the image on the retina appears to have when viewed close to the eye, or 

 under the most usual circumstances. 



Estimation of Size. Our estimate of the size of various objects is 

 based partly on the visual angle under which they are seen, but much 

 more on the estimate we form of their distance. Thus a lofty mountain 

 many miles off may be seen under the same visual angle as a small hill 

 near at hand, but we infer that the former is much the larger object 

 because we know it is much further off than the hill. Our estimate of 

 distance is often erroneous, and consequently the estimate of size also. 

 Thus persons seen walking on the top of a small hill againts a clear 

 twilight sky appear unusually large, because we over-estimate their dis- 

 tance, and for similar reasons most objects in a fog appear immensely 

 magnified. The same mental process gives rise to the idea of depth in 

 the field of vision; this idea being fixed in our mind principally by the 

 circumstance that, as we ourselves move forward, different images in 

 succession become depicted on our retina, so that we seem to pass 

 between these images, which to the mind is the same thing as passing 

 between the objects themselves, 



