APPRECIATION OF MOTION, ETC., OF BODIES. 289 



sary, that figures, placed on lofty columns, should be of colossal mag- 

 nitude. 



It is owing partly to the intervention of bodies, that the sun and 

 moon appear to us of greater dimensions, when rising or setting, although 

 the visual angle, subtended by them, may be the same. "The sun and 

 moon," says Dr. Arnott, 1 "in appearance from this earth are nearly 

 of the same size, viz. : each occupying in the field of view about the 

 half of a degree, or as much as is occupied by a circle of a foot in 

 diameter, when held one hundred and twenty-five feet from the eye 

 which circle, therefore, at that distance, and at any time, would just 

 hide either of them. Now, when a man sees the rising moon apparently 

 filling up the end of a street, which he knows to be one hundred feet 

 wide, he very naturally believes, that the moon then subtends a greater 

 angle than usual, until the reflection occurs to him, that he is using, as 

 a measure, a street known, indeed, to be one hundred feet wide, but of 

 which the part concerned, owing to its distance, occupies in his eye a 

 very small space. The width of the street near him may occupy sixty 

 degrees in his field of view, and he might see from between the houses 

 many broad constellations instead of the moon only ; but the width of 

 the street afar off may not occupy, in the same field of view, the twen- 

 tieth part of a degree, and the moon, which always occupies half a 

 degree, will there appear comparatively large. The kind of illusion, 

 now spoken of, is yet more remarkable, when the moon is seen rising 

 near still larger known objects for instance, beyond a town or a hill 

 which then appears within a luminous circle." 



Such are the chief methods by which we form our judgment of the 

 distance and magnitude of bodies; 1st, by the visual angle 2dly, by 

 the intensity of light, shade, and colour 3dly, by the convergence of 

 the axes of the eyes and 4thly, by the interposition of known objects. 



The eye also enables us to appreciate the motion of bodies. This it 

 does by the movement of their images upon the retina; by the variation 

 in the size of the image ; and by the altered direction of the light in 

 reaching the eye. If a body be projected with great force and rapidity, 

 we are incapable of perceiving it; as in the case of a shot fired from a 

 gun, especially when near us. But if it be projected from a distance, 

 as the field of view is very extensive, it is more easy to perceive it. 

 The bombs, sent from an enemy's encampment, in the darkness of night, 

 can be seen far in the air for some time before they fall; and afford 

 objects for interesting speculation regarding their probable destination. 



To form an accurate estimation of the motion of a body, we must be 

 ourselves still. When sailing on a river, the objects, that are stationary 

 on the banks, appear to be moving; whilst the boat, which is in motion, 

 seems to be at rest. Bodies, that are moving in a straight line to or 

 from us, scarcely appear to be in motion. In such cases, the only mode 

 we have of detecting their motion is by the gradual increase in their 

 size and illumination when they approach us; and the converse, when 

 they are receding from us. If at a distance, and the visual angle be- 

 tween the extreme points of observation be very small, the motion of an 



1 Op. citat. 

 VOL. I. 19 



