792 SPECIAL SENSES. 



We have a knowledge of the presence and an indefinite idea of the general form of large 

 objects situated outside of the area of distinct vision ; but, when we wish to note such 

 objects exactly, the eyeball is turned by muscular effort, so as to bring them at or very 

 near the axis of the globe. This tact, with what we know of the mechanism of refraction 

 by the cornea and lens, makes it evident that the area of the retina upon which images 

 are formed with perfect distinctness is quite restricted. A moment's reflection is sufficient 

 to convince any one that, in order to see any object distinctly, we must look at it, or 

 bring the axis of the eye to bear upon it directly. Let us see, now, how far this fact is 

 capable of positive demonstration. 



If we examine the bottom of the eye with the ophthalmoscope, we can see the yellow 

 spot with the fovea centralis, apparently free from blood-vessels, and composed, as we 

 know, chiefly of those elements of the retina which are sensitive to light. If, at the 

 same time, we examine an image for which the eye is perfectly adjusted, it will be seen 

 that this image is perfect only at the fovea centralis ; and, if the object be removed from 

 the axis of vision, we see a confused image upon the retina removed from the fovea, at 

 the same time that the subject is conscious of indistinct vision. In the words of Helm- 

 holtz, " It is only in the immediate vicinity of the ocular axis that the retinal image pos- 

 sesses entire distinctness ; beyond this, the contours are less defined. It is in part for 

 this reason that in general we see distinctly in the field of vision, only the point that we 

 fix. All the others are seen vaguely. This lack of distinctness in indirect vision, in 

 addition, depends also upon diminished sensibility of the retina : at a slight distance from 

 the fixed point, the distinctness of vision has diminished much more than the objective 

 distinctness of retinal images." 



At the point of penetration of the optic nerve, the retina is insensible to luminous 

 impressions ; at least, its sensibility is here so obtuse as to be entirely inadequate for 

 the purposes of vision. This point is called the punctum caecum ; and its want of 

 sensibility was demonstrated many years ago (1668) by Mariotte. The classical ex- 

 periment by which this important fact was positively ascertained, which is gener- 

 ally known to physiologists as Mariotte's experiment, is so curious that we quote it 

 verbatim : 



"I fasten'd on an obscure Wall about the hight of my Eye, a small round paper, to 

 serve me for a fixed point of Vision ; and I fastened such an other on the side thereof 

 towards my right hand, at the distance of about 2. foot ; but somewhat lower than the 

 first, to the end that it might strike the Optick Nerve of my Right Eye, whilst I kept my 

 Left shut. Then I plac'd myself over against the First paper, and drew back by little 

 and little, keeping my Right Eye fixt and very steddy upon the same ; and being about 

 10. foot distant, the second paper totally disappear'd." 



In this experiment, the rays of light from the paper which has disappeared from view 

 are received upon the punctnm ccecum, at the point of entrance of the optic nerve. If 

 the observer withdraw himself still farther, the second circle will reappear, as the rays 

 are removed from the punctum cascum. With the ophthalmoscope, the point of penetra- 

 tion of the optic nerve may be readily seen in the living eye. If the image of a flame 

 be directed upon this point, the sensation of light is either not perceived or it is very 

 faint and indefinite, and it is then probably due to diffusion to other portions of the 

 retina. 



The relative sensibility of different portions of the retina has been accurately meas- 

 ured by Volkmann and has been found to be in an inverse ratio equal to about the square 

 of the distance from the axis of most perfect vision. This observer calculated the dis- 

 tance between the sensitive elements of the retina at which he supposed that two par- 

 allel lines would appear as one. In the axis of vision, the distance was 0-00029", and, at 

 a deviation inward of 8, it was 0'03186", a diminution of acuteness of more than a hun- 

 dred times. 



