FORMATION OF IMAGES IN THE EYE. 699 



If a ray of light fall upon the border of the cornea, at a right angle to the 

 axis of the eye, it is refracted by its surface and will pass through the pupil 

 to the opposite border of the retina. Above and below, the circle of vision 

 is cut off by the overhanging arch of the orbit and the malar prominence ; 

 but externally the field is free. With the two eyes, therefore, the lateral 

 field of vision must be equal to at least one hundred and eighty degrees. 

 It is easy to demonstrate, however, by the ophthalmoscope, as well as by 

 taking cognizance of the impressions made by objects far removed from the 

 axis of distinct vision, that images formed upon the lateral and peripheral 

 portions of the retina are confused and imperfect. One has 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 it is desired 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 fact, with what is 

 known 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, it is necessary 

 to bring the axis of the eye to bear upon it directly. 



In examining the bottom of the eye with the ophthalmoscope, the yellow 

 spot, with the fovea centralis, can be seen, free from large blood-vessels, and 

 composed chiefly of those elements of the retina which are sensitive to light. 

 If at the same time, an image for which the eye is perfectly adjusted be ob- 

 served, 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, there is 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 Helmholtz, " It is only in 

 the immediate vicinity of the ocular axis that the retinal image possesses 

 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 dis- 

 tinctness in indirect vision, in addition, depends also upon diminished sensi- 

 bility of the retina : at a slight distance from the fixed point, the distinct- 

 ness 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 ; or at least, its sensibility is here so obtuse as to be 

 entirely inadequate for the purposes of vision. This point is called the punc- 

 tum caecum ; and its want of sensibility was demonstrated many years ago 

 (1668) by Mariotte. The classical experiment by which this important fact 

 was ascertained is generally known as Mariotte's experiment. The following 

 account is quoted 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 



