86 SPECIAL SENSES. 



experiment in a dark chamber, Yolkmann has observed such 

 an image in the human subject. 1 The most satisfactory ob- 

 servations, however, have been made with the ophthalmo- 

 scope. With this instrument, the retina and the images 

 formed upon it may be seen during life with perfect distinct- 

 ness. 2 



All parts of the retina, except the point of entrance of 

 the optic nerve, are sensible to light ; and the arrangement 

 of the cornea and pupil is such, that the field of vision is, at 

 the least estimate, equal to the half of a sphere. 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 border of the retina upon the oppo- 

 site side. 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, there- 

 fore, the lateral field of vision must be equal to at least one 

 hundred and eighty degrees. It is easy to demonstrate, how- 

 ever, 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 im- 

 perfect. We have a knowledge of the presence and an in- 

 definite 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 fact, 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 suf- 

 ficient 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 



1 VOLKMANN, Sehen, in WAGNER, Handworterbuch der Physiologic, Braun- 

 schweig, 1846, Bd. iii., Erste Abtheilung, S. 287. 



2 HELMHOLTZ, Optique physlologique, Paris, 1867, p. 87. 



