THE EYE. 



be sufficiently slow to give the necessary time for the production 

 of a perception of it. 



57. The time thus necessary to obtain the perception of a 

 visible object varies with the degree of illumination, the colour, 

 and the apparent magnitude of the object. The more intense the 

 illumination the more vivid the colour, and the greater the appar- 

 ent magnitude the less will be the time necessary to produce a 

 perception of the object. 



58. In applying this principle to the phenomena of vision, it 

 must be carefully remembered that the question is affected, not by 

 the real but by the apparent motion of the object, that is to say, 

 not by the velocity with which the object really moves through 

 space, but by the angle which the line drawn from the eye to 

 the object describes per second. Now this angle is affected by two 

 conditions, which it is important to attend to : 1. the direction of 

 the motion of the object compared with the line of vision; and 

 2. by the velocity of the motion compared with the distance of 

 the object. If the object were to move exactly in the direction of 

 the line of vision, it would appear to the eye to be absolutely 

 stationary, since the line drawn to it would have no angular 

 motion ; and if it were to move in a direction forming an obliq 

 angle to the line of vision, its apparent motion might be indefini' 

 slow, however great its real velocity might be. 



For example, let it be supposed that the eye being at E, fig. 7, 

 an object o moves in the direction of o o', so as to move from o to 



j 

 alar 

 ique 



* 



0' in one second. Taking E as a centre, and E o as a radius, let a 

 circular arc o o" be described. The apparent motion of the object 

 will then be same as if, instead of moving from o to o' in one 

 second, it moved from o to o" in one second. 



The more nearly, therefore, at right angles to the line of vision 

 the direction of the motion is, the greater will be the apparent 

 motion produced by any real motion of an object. 



59. A motion which is visible at one distance may be invisible 

 at another, inasmuch as the angular velocity will be increased as 

 the distance is diminished. 



Thus if an object at a distance of 57| feet from the eye move 



at the rate of a foot per second, it will appear to move at the rate 



of one degree per second, inasmuch as a line one foot long at 5 



72 



