OS PHYSIOLOGICAL LIMITS OF MlCfiOSCOPIC VISION. 463 



tinguishing capacity of the retina, where the conversion of physical 

 impression into special sensation takes place ; and that the capacity 

 of the retina for transferring its sensations (or, perhaps, still only 

 modified physical impressions) to the brain, stands in exact pro- 

 portion to the subtlety of impression which it has received through 

 the separate elements of the retinal layer of rods and cones. Further, 

 we should expect to find that the dioptric performance of the 

 structures of the eye in front of the retina was just as accurately 

 adapted to the capacity of the retina for receiving separate impres- 

 sions, yet not beyond it. But, as there appears no a priori 

 reason why the sense of sight should exceed in potential subtlety 

 the limits of any possible act of seeing, or the capacity of the 

 apparatus on which the sense depends, so there seems no ground 

 to attribute the limit of visibility to any antagonistic '' physical 

 character of light." If we suppose a dioptric apparatus to be 

 perfect in its capacity of transferring isolated pencils of light, giving 

 separate impressions from each illuminated point in an object, the 

 limit of vision for that object will be measured by the visual magni- 

 tude of interspace between the illuminated points, provided that 

 interspace does not exceed or fall short of the measure of the 

 percipient element on which each single impression falls. There- 

 fore the finer the percipient element and their interspaces, the finer 

 may be the delineation of detail belonging to an object which detail 

 shall yet be distinctly seen. But microscope objects generally (and 

 some in particular, e.^., closely ruled lines) which cannot be dis- 

 tinguished by the naked eye because their retinal images (16 times 

 smaller) can have no appreciable dimensions, may be rendered 

 visible with the help of such amplification as will spread their 

 detail, e.^., lines and interspaces, so as to fall singly upon the per- 

 cipient retinal elements. And, so far, magnifying power is necessary 

 for vision of minute objects, in order that the images of their 

 structural details may correspond with the dimensions of the per- 

 cipient elements. But each object so magnified should receive its 

 due illumination in order that the impression of each detail on the 

 retina may fall with sufficient intensity, as well as on the right 



