Ixxx 



INTRODUCTION TO OPTICS. 



road which separates the two rows forms a smaller visual angle, in pro- 

 portion as it is more distant from us ; therefore the width of the road 

 seems gradually to diminish as well as the size of the trees, till at length 

 the road apparently terminates in a point, at which the trees seem to 

 meet. 



In sculpture we copy Nature as she really exists ; in painting we 

 represent her as she appears to us that is to say, we do not copy the 

 objects, but the image they form on the retina of the eye. 



If an object, with an ordinary degree of illumination, does not sub- 

 tend an angle of more than half a minute of a degree, it is invisible. 

 There are, consequently, two cases in which objects may be invisible, 

 either if they are too small, or so distant as to form an angle less than one 

 second of a degree. The fixed stars subtend much smaller angles, and 

 yet are visible ; but they are bodies luminous in themselves, and possess 

 much more than an ordinary degree of illumination. In like manner, if 

 the velocity of a body be so small that the arc which it describes in an 

 hour does not subtend an angle of more than twenty degrees, its motion is 

 imperceptible : consequently a very rapid motion may then be imper- 

 ceptible, provided the distance of the moving body be sufficiently great ; 

 for the greater its distance, the smaller will be the angle under which its 

 motion will appear to the eye. It is for this reason that the motion of 

 the celestial bodies is invisible, notwithstanding their immense velocity ; 

 for the greatest apparent motion of any celestial body does not exceed 

 fifteen degrees in an hour, being that seemingly produced in a body at 

 the equator by the revolution of the earth. The greatest of the real 

 motions is that of the moon, and even that does not exceed about thirteen 

 degrees in a day. The real velocity depends altogether on the space 

 comprehended in each degree ; and this space depends on the distance 

 of the object and the obliquity of its path. Now we cannot judge of the 

 velocity of a body in motion unless we know its distance ; for, supposing 

 two men to set off at the same moment from A and B (Jig. 9), to walk 

 each to the end of their respective lines C and D, if they perform their 



walk in the same space of time, they 

 must have proceeded at a very different 

 rate ; and yet to an eye situated at E, 

 they will appear to have moved with 

 equal velocity, because they will both 

 have gone through an equal number of 

 degrees, though over a very unequal 

 length of ground. Sight cannot be 

 implicitly relied on : it deceives us, 

 both in regard to the size and the dis- 

 tance of objects indeed our senses 

 would be very liable to lead us into 

 error, if experience did not set us right. 

 Nothing more convincingly shows how 

 requisite experience is to correct the 

 errors of sight, than the case of a young 

 man who was blind from his infancy, 

 and who recovered his sight at the 

 age of fourteen, by the operation of 

 couching. At first he had no idea 

 either of the size or distance of ob- 

 jects, but imagined that every thing he saw touched his eyes j and it was 



Fig. 9. 



A 



