446 ELEMENTARY PHYSIOLOGY less 



covered by the image of a large object is larger than that 

 covered bij a sirudl object; while that covered by an object 

 when near is larger than that covered by the same object 

 when distant; and, other conditions being alike, a near 

 object is more brilliant than a distant one. Furthermore, 

 the shadoius of objects differ according to the forms of their 

 surfaces, as determined by touch. 



Conversely, if these visual sensations can be produced, 

 they inevitably suggest a belief in the existence of objects 

 competent to produce the corresponding tactile sensations. 



What is called perspective, whether solid or aerial in 

 drawing, or painting, depends on the application of these 

 principles. It is a kind of visual ventriloquism — the 

 painter putting upon his canvas all the conditions requisite 

 for the production of images on the retina, having the size, 

 relative form, and intensity of colour of those which would 

 actually be produced by the objects themselves in nature. 

 And the success of his picture, as an imitation, depends 

 upon the closeness of the resemblance between the images 

 it produces on the retina, and those which would be pro- 

 duced by the objects represented. 



To most per.sons the image of a pin, at three or four 

 inches from the eye, ap[)ears blurred and indistinct — 

 the eye not being cai)able of adjustment to so short a 

 focus. If a small hole be made in a piece of card, the 

 circumferential raj's which cause the blur are cut off, and 

 the image becomes distinct. But at the same time it is 

 magnified, or looks bigger, because the image of the pin, 

 in spite of the loss of the circimiferential rays, occupies a 

 much larger extent of the retina when close than when 

 distant. All convex glasses produce the same effect — 

 while concave lenses diminish the apparent size of an 

 object, because they diminish the size of its image on the 

 retina. 



Objects, as is well known, appear larger when seen in a 

 fog. In this case the actual size of the image on the 

 retina is the same as if there were no fog. But the indis- 



