284 



SENSE OF SIGHT. 



apparent magnitude of objects; 2. The vivacity of their colours; 3. 

 The distinction of their smaller parts; 4. The necessary conformation 

 of the eyes for seeing distinctly at different distances; 5. The direction 

 of their axes; and 6. The interposition of objects. Dr. Brown 1 re- 

 duces them to three 1. The difference of the affections of the optic 

 nerve ; 2. The different affections of the muscles, employed in varying 

 the refracting power of each eye, according to the distance of objects, 

 and in producing that particular inclination of the axes of the two eyes 

 which directs them both equally on a particular object; and 3. The 

 previous knowledge of the distance of other objects, " which form, with 

 that we are considering, a part of one compound perception." Lastly, 

 Dr. Arnott 2 enumerates four modes by which this is effected 1. The 

 space and place, occupied by objects in the field of view, measured by 

 what is termed the visual angle. 2. The intensity of light, shade, and 

 colour. 3. The divergence of the rays of light and 4. The converg- 

 ence of the axes of the eyes. This enumeration may be adopted with 

 some slight modifications. The circumstances, in our opinion, to be 

 considered, are: 



1. The visual angle, or that formed by two lines, which shave the 



extremities of an object and cross at 

 F ' i s- 127. the centre of the crystalline ; so that 



the visual angle, subtended by the 

 object, as A B, Fig. 127, is exactly 

 equal to that subtended by its image 

 a b on the retina. It is obvious, from 

 this figure, that if all objects were 

 equidistant from the eye, and of the 

 same magnitude, they would subtend 



Visual Angle. the same angle; and if not of the 



same magnitude, the difference would 



be accurately indicated by the difference of the visual angles subtended 

 by them. The two arrows, however, which are of different sizes, sub- 

 tend the same visual angle, and are alike represented on the retina by 

 the image a b. It is clear, then, that the visual angle does not give 

 us a correct idea of the relative magnitudes of bodies, unless we are 

 acquainted with their respective distances from the eye; and, con- 

 versely, we cannot judge accurately of their distances, without being 

 aware of their magnitudes. A man on horseback, when near us, sub- 

 tends a certain visual angle; but, as he recedes from us, the angle be- 

 comes less and less; yet we always judge accurately of his size, because 

 aware of it by experience; but if objects are at a great distance, so as 

 not to admit of their being compared with nearer, by simple vision, we 

 are in a constant state of illusion, irresistibly believing, that they are 

 much smaller than they really are. This is the case with the heavenly 

 bodies. The head of a pin held close to the eye subtends as large a 

 visual angle as the planet Jupiter, which is one thousand two hundred 

 and eighty-one times bigger than this earth, and is eighty-six thousand 

 miles in diameter. In like manner, a five-cent piece, held at some 



1 Lectures on the Philosophy of the Human Mind, vol. i., Boston, 1826. 

 8 Elements of Physics, new Amer. edit., p. 383, Philad., 1841. 



