180 ON THE SENSES 



effect results from habitual occupation witli remote objects ; hence we find stu- 

 dents, engravers, &c , almost always short-sighted; huntsmen, on the other 

 hand, far-sighted ; in old age usually every eye loses more or less its capacity 

 of seeing objects which are close at hand. We will now consider in what ac- 

 commodation consists, and the mechanism by which it is. produced. 



We have seen in the camera obscura two means of adjustment working to the 

 same end, the forward and backward movement of either the glass lens or the 

 black glass tablet. For the employment of these means in the eye it is neces- 

 sary either that the screen of the retina which intercepts the image should be 

 movable in the direction of lhe crystalline lens, or that the lens should be capa- 

 ble of movement forwards and backwards as i-egards the retina; forwards in the 

 case of near- sight, that the image which would otherwise fall too far behind the 

 retina might be brought forward to the plane of the latter ; backwards in the 

 case of far-sight, that the image now formed in front of the retina might be 

 thrown back upon its plane. In the eye the displacement of the retina alone is. 

 impossible; it cannot leave the choroid which lies behind it, nor can an appara- 

 tus for such a purpose be easily imagined. Mediately, indeed, through a change 

 of form of the whole globe of the eye, the distance of the posterior layer of the 

 retina, which more directly serves for distinct vision, might be changed as regards 

 the lens — that is to say, might be increased by such a lateral compression of the 

 eye that its diameter from before backwards would be augmented at the expense 

 of the transverse diameter, or be diminished by a contraction acting from front 

 to rear. This somewhat rude mechanism of accommodation had, in fact, many 

 advocates in earlier times, who asserted that the compression in question was 

 effected through the small muscles which surround the eye and execute the 

 movements of the ball. This view, nevertheless, must be regarded as wholly 

 untenable, and was indeed contradicted by many insurmountable objections even 

 before the true mechanism of accommodation had been discovered. The second 

 of the above-named means, the movement of the crystalline lens, was accepted 

 by others, though no direct proof of it could be adduced nor an answer given 

 to the question how the fluids before and behind the lens could yield to the dis- 

 placement — without a possibility, in short, of pointing out any mechanism by which 

 this displacement could be accomplished. At present we know with certainty 

 that in the act of accommodation the lens does not change its place. It follows, 

 therefore, that tlie adjustment of the eye must take place in a different manner 

 from that of the camera obscura. And in fact a third means remains through 

 Avhich the shifting of the image delineated by a lens may be brought about, a 

 means not indeed applicable to our artificial inflexible glass lenses, but capable 

 of adaptation to the elastic and organized lens of the eye. This means is a 

 change in the form of the lens. The physical facts on which this kind of ac- 

 commodation rests are briefly the following : a lens refracts rays of light more 

 strongly the greater the curvature of its surfaces; if we have therefore a 

 slightly curved and a greatly curved lens formed of the same glass, and an ob- 

 ject at a definite and constant distance, the first lens refracts the rays proceed- 

 ing from this object less than the last lens ; the consequence is that the more 

 strongly refracted rays unite sooner than the rays more weakly refracted ; hence 

 the point of union, that is, the image of the object, lies in the case of the strongly 

 curved lens nearer behind it than in the case of the weakly curved lens. Thus, 

 if we conceive an object so near the eye that its image with a given distance be- 

 tween the crystalline lens and retina falls behind the plane of the latter, this 

 image will be made to move forward so soon as the surfaces of the lens assume 

 a greater curvature, since the convergence of the rays proceeding from the lens 

 into the vitreous body will be greater. Now this actually takes place in the eye. 

 The eye at rest is adjusted to objects at a certain distance — that is to say, main- 

 tains habitually such a curvature that the images of objects at some degree of 

 lemoteness fall upon the retina; would we observe objects moi-e closely, the 



