THE BLIND SPOT 427 



the page, held at ten or twelve inches distance from 

 the eye. 



The black dot will be seen quite plainly, as well as the 

 cross. Now, move the book slowly towards the eye, which 

 must be kept steadily fixed upon the cross ; at a certain 

 point the dot will disappear, but, as the book is brought 

 still closer, it will come into view again. It results from 

 optical principles that, in the first position of the book, 

 the image of the dot falls between that of the cross 

 (which throughout lies upon the yellow spot), and the 

 entrance of the optic nerve : whUe, in the second position, 

 it falls on the point of entrance of the optic nerve itself ; 

 and, in the thii'd, it falls on the other side of that point. 

 The three positions of the dot and cross, and of the 

 resulting images of each on the retina, are shown in the 

 accompanying figure, 141. 



So long as the image of the spot rests upon the entrance 

 of the optic nerve, it is not perceived, and hence this 

 region of the retina is called the blind spot. The experi- 

 ment proves that the vibrations of the ether are not able 

 to excite the fibres of the optic nerve itself. 



10. The Duration of a Luminous Impression.— The 

 impression made by light upon the retina not only remains 

 during the whole period of the direct action of the light, 

 but has a certain duration of its o^vn, however short the 

 time during which the light itself lasts. A flash of light- 

 ning is practically instantaneous, but the sensation of 

 light produced by that flash endures for an appreciable 

 period. It is found, in fact, that a luminous impression 

 lasts for about one-eighth of a second ; whence it follows, 



