THE SENSE OF SIGHT. 181 



crystalline lens assumes <a greater curvature, a curvature so much the stronger in 

 proportion as the object is nearer, the magnitude of the curvature being in every 

 case such as to bring the image which falls too far behind tin; retina just into 

 the plane of that membrane. The correctness of this statement is ingeniously 

 shown by means which wc must be content very briefly to indicate. The sur- 

 faces of the transparent structures of the eye, the cornea as well as the lens, do 

 not permit the passage of all the light which foils upon them ; a part of it is 

 thrown back or reflected; hence if a luminous object is before them, they form a 

 reflex image of it, an image which, from physical laws not necessary to be here 

 dwelt upon, is an exceedingly small one. Further, this little image traced on 

 the outer convex surfaces of the cornea and lens stands upright like the object, 

 and seems to lie behind the reflecting surface, while that of the concave surface 

 behind the lens is inverted and occupies a position before it. The little im^ge 

 of the cornea is easily perceived ; the lustrous appearance of the eye consists 

 indeed in the light thus reflected. If we hold a candle flame near our head and 

 observe the eye of another person, we shall readily perceive on its cornea the 

 small sharply deflned image of the flame. The images cf both surfaces of the 

 leus are seen with more difficulty because they are fainter and are also somewhat 

 eclipsed by the brighter image of the cornea; besides that only a small portion 

 of the reflecting surface is accessible to view through the pupil. Now, it is well 

 known that the reflex image of a curved surface changes in magnitude and position 

 according as the form of the surfoce is changed, as its curvature is either increased 

 or diminished. On this fact rests the experimental proof of the change of form of 

 the crystalline lens in the act of looking at near and at distant objects. iJy means 

 of a peculiar telescope adapted to the most exact measurement, the change in 

 position and size of the reflex image traced on the front surface of the lens may 

 be accurately ascertained if the observed eye be alternately fixed on a far and 

 near object at a measured distance without any change of its own position ; from 

 these measurements again has been calculated the magnitude of the variation of 

 curvature which the surface in question undergoes in the action of accommoda- 

 tion. That the front surface of the lens becomes more strongly curved when 

 viewing a near object, may be seen even without an instrument, if the eye be 

 observed in profile; the pupil will then be seen to project further beyond the 

 edge of the cornea ; it must be understood, however, that the eye which is ob- 

 served shall not in the least change its place in transferring its contemplation 

 from the remote to the near object. 



After having so far considered the eye as aa optical instrument, a camera oh- 

 scura, explained the origin of small inverted images of outward objects on its 

 retina, and shown the means by which well defined images of objects at all dis- 

 tances are brought into the plane of this sensitive membrane, let us turn our 

 attention to the proper physiological part of the problem, the explanation of sight 

 itself. The existence of the image on the rerina is not vision ; the soul sits not 

 in the retina, nor receives into itself this image as such Avithout further elabora- 

 tion ; there is no second interior eye through which it observes the small pictures 

 of the retina, as with our visual organ we observe the images of the camera obscura. 

 Between the image and the pf^rception of it there still lies a whole series of pro- 

 cesses through which the image operates indirectly on the sensorium, so as to ex- 

 cite sensations of light and color and lead t- a recognition of the local relations 

 of external objects. The rays of light, united for the formation of the image, act 

 first on certain appaiatus of the retina ; this apparatus acts in turn on the fibres 

 of the optic nerve, in which are thus set in motion the mysterious couriers known 

 as '' nervous currents ;" these currents flow to the brain, and take effect there on 

 other apparatus, which stands in the m^re immediate service of the soul, and 

 from whose activity the latter derives sensations and ideas. If we would explain 

 this succession of incidents, we must begin with the first member of the series. 

 What, then, is light ? What is a ray of light which, through air or glass, pro- 



