ON PHYSIOLOGICAL LIMITS OP MICROSCOPIC TlSIOlf. 465 



capacity of the eye to perceive distinctly the objective images of 

 minute structural details under conditions of illumination which 

 are not suited to the distinguishing power of the retina. The 

 constitution of the eye is, in short, the master condition which fixes- 

 the limit of resolving or defining powers. 



Let us now consider the possible performance of the retina as 

 measured by the actual dimensions and position of the percipient 

 elements. The cones of the bacillary layer of the retina at it& 

 most sensitive spot (the macula luteaj have a diameter at their base 

 (where the focal points of the image fall) of about -g- oVtr inch, and 

 the distance between their centres is about q^q-^ inch. The rods 

 which are not found at the yellow spot, but which everywhere else 

 are crowded closely together, have a diameter of about i-o^-^-q- inch, 

 and the distance between their centres is nearly the same — say^ 

 \2h-5 iiich. 



The dimensions of the retinal image of an object subtending 

 a visual angle of 60 seconds is yyVo i^^ch. 



To show the relation of these figures to each other the following^ 

 extract from Donders will be useful : — 



*' The first exact appreciation of the physiological question, we 

 find in Hook's essay on Distinct and Indistinct Yision (1738). 

 He investigates the angular distance required to observe two fixed' 

 stars separately, and he found that among one hundred persons 

 scarcely one is in a position to distinguish the two stars when the 

 apparent distance is less than 60 seconds. Subsequently similar 

 investigations were carried on by Mayer, and in our own time by 

 Volkmann, Harting, Weber, Bergmann, and Helmholtz, for the 

 most part with parallel lines or gauge net. It is evident that, for 

 two minute points of light to be seen separately, the centres of their 

 images must lie further apart from one another than the hreadth of a 

 percipient element of the retina (about one and a half times). If the 

 centres fall at both sides precisely on the boundaries of the same 

 element, this element alone will then receive as much light as the 

 two adjoining elements between which it is situate ; while, in order 

 to see two separate points, a less illuminated space must remain 



