MOVEMENTS OF THE EYEBALL. 129 



amplification, which is generally in proportion to their bright- 

 ness. An illustration of this is afforded by the simple ex- 

 periment of looking at two circles, one black on a white 

 ground, and the other white on a black ground. Although 

 the actual dimensions of the two circles are identical, the irra- 

 diation of rays from the white circle makes this appear the 

 larger. In a circle with one half black and the other white, 

 the white portion will appear larger for the same reason. 

 This deception increases sensibly when we look steadily at 

 the object. These phenomena are due to what has been 

 called by physiologists irradiation ; and their explanation is 

 very simple. It is probable that luminous impressions are 

 never confined absolutely to those parts of the retina upon 

 which the rays of light directly impinge, but that the sensi- 

 tive elements immediately contiguous are always more or less 

 involved. In looking at powerfully illuminated objects, the 

 irradiation is considerable, as compared with objects which 

 send fewer luminous rays to the eye. 



In experiments analogous to those just described, made 

 with strongly colored objects, it has been observed that the 

 border of irradiation takes a color complementary to that of 

 the object itself. This is particularly well marked when the 

 objects are steadily looked at for some time. Illustrations of 

 this point also are very simple. If we looked fixedly at a 

 red spot or figure on a white ground, we soon see surrounding 

 the red object a faint areola of a pale green ; or, if the image 

 be yellow, the areola will appear pale blue. These appear- 

 ances have been called accidental areolse. 



Movements of the Eyeball. 



The eyeball nearly fills the cavity of the orbit, resting, by 

 its posterior portion, upon a bed of adipose tissue, which is 

 never absent, even in extreme emaciation. Outside of the 

 sclerotic, is a fibrous membrane, the tunica vaginalis oculi, or 

 capsule of Tenon, which is useful in maintaining the equilib- 

 rium of the globe. This fibrous membrane surrounds the 



