SENSE OF VISION. 125 



perpendicular to that point of the retina on which the image is 

 formed, and that as all these perpendiculars meet in the centre of 

 the eye, the line of direction is identical with the prolonged radius 

 of the sphere, is not optically correct. Some of these lines cross 

 each other at a point behind, and some before the lens, and they 

 thus fall on the retina at such different angles, that no general law can 

 be laid down respecting them. The notion of erectness, which we 

 form by the combined use of our eyes and hands, is really the 

 result o'i education and experience. 



Single vision ivith two eyes, is explained by the fact that the rays 

 of light proceeding from a luminous object fall upon parts of the 

 retina which are accustomed to act together. It is not necessary that 

 these points should be equi-distant from the optic nerve, as is evi- 

 dent in strabismus. Double vision almost always follows the ope- 

 ration for its cure, till the parts become accustomed to act together. 

 The vanishing of images that fall on particular parts of the re- 

 tina, as shown in Marriotte's experiments, is due to the fact that the 

 rays fall directly upon the centre of the optic nerve, at the point 

 where the middle artery of the retina passes in ; and as this has 

 no special sensibility, no impression, of course, is conveyed to the 

 brain. 



By the association of visual and tactual sensations, we obtain know- 

 ledge of various properties of bodies, such as smoothness and rough- 

 ness, form, and to a certain extent, of distance. In the latter both 

 eyes seem to be required. The distinctness of outline also assists 

 us in the estimate of distances, haziness of outline being generally 

 associated with distance. If we know the real size of an object, we 

 often judge of its distance by its apparent size. 



Impressions made upon the retina continue for some time after the 

 removal of the stimulus. It is this which causes the appearance of 

 a circle of light when an ignited point is rapidly moved in a circle. 

 It also accounts for the optical delusion in the toy called thauma- 

 trope. 



The phenomena of accidental colours is thus explained. When 

 the eye is steadily fixed for a length of time upon one particular 

 colour, as for instance, upon a red wafer, the retina becomes fa- 

 tigued and loses its impressibility to that colour ; if now the eye be 

 turned to a white ground, a spot having a different colour will be 

 seen ; this will be made up of all the colours of the solar spectrum, 

 minus the red, and the resulting one colour is called the comple- 

 mentary or accidental colour. In this case it will be bluish-green. 

 The laws of harmonious colouring in paiiiting, are founded upon the 

 combination of complementary colours. 



The subjective phenomena of vision are familiar to all. Examples 

 are seen in the flashes of light before the eyes in congestion of the 



11* 



