THE EYE AS AN OPTICAL INSTRUMENT. 521 



The Camera Obscura, as used by photographers, is an in- 

 strument which serves to illustrate the formation of images 

 by converging systems of lenses It consists of a box blackened 

 inside and Having on its front face a tube containing the 

 lenses; the posterior wall is made of ground glass. If the 

 front of the instrument be directed on exterior objects, in- 

 verted and diminished images of them will be formed on the 

 ground glass; those images only are well defined, at any one 

 time, which are at such a distance in front of the instrument 

 that the conjugate foci of points on them fall exactly on the 

 glass behind the lens: objects nearer or iarther off give con- 

 fused and indistinct images; but by altering the distance be- 

 tween the lenses and the ground glass, in common language 

 " focussing the instrument," either can be made distinct. For 

 near objects the lenses must be farther from the surface on 

 which the image is to be received, and for distant nearer. 

 The reason of this may readily be seen from Fig. 148. If the 

 system of lenses brings the parallel rays a c and b d, proceed- 

 ing from an infinitely distant object, to a focus at x 9 then the 

 diverging rays f c and f d, proceeding from a nearer point, 

 will be harder to bend round, so to speak, and will not meet 

 until a point y, farther behind the system than x is. The 

 more divergent the rays, or what amounts to the same thing, 

 the nearer the point they proceed from, the farther behind 

 the refracting system will y be. 



ment. 



FIG. 148. Diagram illustrating the need of " focussing " m an optical 



int-. 



The refracting media of the eye form a convergent optical 

 system, made up of cornea, aqueous. humor, lens, and vitreous 

 humo.r. These four media are reduced to three practically, 

 by the tact tha^ the indices of refraction of the cornea and 

 aqueous -humor are the same, so that they act together as one 

 converging lens. The surfaces at which refraction occurs 

 are (1) that between the air and the cornea, (2) that between 

 the aqueous humor and the front of the lens, (3) that between 

 the vitreous humor and the back of the lens. The refractive 



