172 THE SENSES. 



What instrument is devised to take advantage of this ? 



The stereoscope. In this instrument two photographs are taken 

 by cameras so placed as to represent the position of the eyes in 

 vision, and the two views of one object are then superimposed by 

 the use of prisms. 



What determines the clearness of vision ? 



The space between the cones in the point of clearest vision, the 

 macula lutea. It has been calculated that an object must sub- 

 tend an arc of at least 60 to 70 seconds in the field of vision to be 

 clearly seen. Such an object makes an image of about i^wir* of 

 an inch in the retina ; and this is about the distance between the 

 cones at the macula lutea. Similarly, two points to be clearly 

 distinguished must be separated sufficiently to allow this amount 

 of separation in the retinal image. 



What are after-images? 



It has already been noted that vision lasts longer than the 

 stimulus which excites it. Under some conditions it may last 

 a perceptibly long time : it is then known as an after-image. 

 If one looks at an intense light, the sun, the sense of light remains 

 for some time in the eye. Similarly, if one looks intently at a 

 white spot on a black background, and then turns to a white 

 surface, one has the image of a gray spot. The first of these 

 conditions cited is known as a positive after-image, and the latter 

 as a negative. In the first case the phenomenon results in a con- 

 tinuation of the same sensation, and in the latter a new perception 

 results. 



What peculiarity do the after-images of colored objects present? 

 They appear to have the complementary color of the original 

 object; thus, green excites a reddish after-image; orange, blue; 

 and so on. 



How are after-images explained? 



They may be explained as a result of exhaustion. The part of the 

 retina on which the image has fallen becomes tired, and when the 

 eye is turned upon a white ground, the white light coming to the 

 retina does not produce as much sensation in the tired portion. 

 The colored negative after-images may be similarly explained. 



* Variously estimated at T^TJS to 7?^^ f an inch- 



