ON PHYSIOLOGICAL LIMITS OF MICSOSCOPIC TISION. 475 



accurate apprehension of the form and colour of external objects, 

 may then be possible. Meanwhile the facts which have been 

 ascertained from the study of the dioptrics of the eye, cannot be 

 disregarded in our estimate of microscope vision. For the capacity 

 of the eye as an optical instrument has a direct relation to the 

 performance of the microscope ; whilst as a percipient or sensory 

 organ it dominates over the whole subject of light and its properties. 

 The limit of vision, whether by the unassisted eye or when assisted 

 by optical apparatus, remains fixed by the distinguishing power of 

 the retina. And under ordinary conditions of illumination, the 

 images formed by the eye in its normally healthy state are equal in 

 delicacy and accuracy to the limits which the dimensions of the 

 retinal cones and rods set to the delicacy and accuracy of sensation. 

 To these limits the microscope sets, properly speaking, no opposi- 

 tion. The function of this instrument is to bring minute objects 

 within the powers of recognition which the eye possesses. And as 

 a magnifying instrument it can do more than this; bat the failure 

 lies in the conditions of illumination on the one hand, and in the 

 limits of visual perception on the other. 



