656 THE SENSE OF SIGHT. 



it becomes difficult to say what belongs to mere sensation, 

 and what to the influence of the mind. 



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 understand this, it will be 

 necessary to refer again to fig. 178, p. 652. 



The angle #, included between the decussating central 

 rays of two cones of light issuing from different points of 

 an object, is called the optical angle angulus opticus seu 

 visorius. This angle becomes larger, the greater the dis- 

 tance between the points A and B ; and since the angles x 

 and y are equal, the distance between the points a and I 

 in the image on the retina increases as the angle x becomes 

 larger. Objects at different distances from the eye, but 

 having the same optical angle, x for example, the objects, 

 c, d, and e, 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 extending 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. A land- 

 scape depicted on the retina, as a J, and viewed under the 

 angle x, is therefore conceived by the mind to have an 

 extent of two miles perhaps, if we know that its extent is 

 such, or if we infer it to be so from the number of known 

 objects seen at the same time. And in the same way that 

 the images of several different objects, viewed under the 

 same angle, thus appear to the mind to have a different 

 size in the field of vision, so the whole field of vision, which 

 has always the same absolute size, is interpreted by the 

 mind as of extremely various extent : and, for this reason 



