ORGANS OF PERCKl'TIOX. 1<T7 



is obvious, that the sources of error are numerous, and 

 that the information thus gained is of little value, unless 

 corrected by the experience of the sense of touch. 



As we judge of the magnitude of objects, by the angle 

 which they form with the eye, so, in like manner, do we 

 employ the same means to determine their Distance, when 



stricted to the organs of vision alone. But as bodies of 

 the same dimensions, when placed at different distances, 

 will form with the eye different angles, and as bodies of 

 unequal dimensions may be so arranged, as to appear un- 

 der the same angle, our notion of distance acquired by un- 

 assisted vision, must likewise be very imperfect. It is true, 

 that in judging of the distance of objects, their degree of 

 illumination is likewise attended to. But as objects are not 

 always luminous in proportion to their proximity, it may 

 often happen that this character, instead of contributing to 

 an accurate result, shall rather generate error. 



In judging of the condition of the Surface of any object, 

 in order to ascertain its inequalities by means of the eye, we 

 attend to the degree of illumination of the different parts, 

 and form our opinions from the manner in which the light 

 and shade are distributed. But as the eye has not the 

 power of perceiving the difference between a dark ground 

 and a shaded place, nor between an illumined and a white 

 spot, it is obvious, that, without the assistance of the sense 

 of touch, it could not be determined whether the surface 

 was rough or smooth, pitted or even. 



The eye assists us in determining the Motions of bodies.; 

 but, like its intimations with regard to size and distance, it 

 frequently misleads us. Thus, when an object is moving 

 in a straight line from us, it appears as if at rest. When 

 sailing on a river in a boat, the objects which are stationary 

 on the banks seem to be in motion, while the boat which is 

 in motion appears to be at rest. In like manner, the sun 



