474 ON PHTSIOLOGICAL LIMITS OF MICEOSCOPIC VISION, 



presents surfaces of relatively large area which are much brighter 

 than, the other parts, corresponding portions of the retinal 

 percipient surface become fatigued, and vision is disturbed by aft&r 

 images^ whilst those parts which have relatively too low a tone 

 fade from sight. The act of seeing thus exercised upon images of 

 very minute objects in the midst of large bright fields is greatly 

 burdened in the attempt to discern fine structure. On the other 

 hand the more an object is needlessly magnified the less clearly 

 will its outlines be defined because the delineating pencils of light are 

 scattered and the intermediate details lose needful light and shade. 

 Whether therefore the brightness of image be too much with low 

 amplification, or too little with high amplification, the eye will not 

 perceive what the microscope with proper management can never- 

 theless perfectly delineate. 



Although we have barely glanced at a few of the principal defects 



inherent in the anatomical constitution of the eye, the limits of this 



paper have been so far exceeded that it is impossible to enter upon 



any discussion of the imperfections of the dioptric media of the eye 



considered as an optical apparatus. It has long been known that 



spherical and chromatic aberrations do exist in the most healthy 



organ; and it has been the province of the physiologist to account 



for the general perfection of vision, in spite of the very numerous 



structural and functional defects of the eye. It has, for instance, 



been shown how accommodation for focal distance is provided for by 



muscular movement operating upon the position of the lens, and how 



imperfect form and colour are counteracted, and contrast of light 



and darkness, appreciated by the simple act of changing the axial 



direction of the eye. Eut it is necessary to go much further than 



this in any attempt to set up a theory of vision. In the first place, 



the sensation of sight has to be explained, or at least the furthest 



boundary of physical impression has to be traced, and the means 



discovered by which the excitation of the separate nerve-fibrils is 



produced. And there seems little reason to doubt that anatomical 



investigation and physical science will eventually accomplish these 



ends. A theory of vision, so far as it relates to the complete and 



