458 ON PHYSIOLOGICAL LIMITS OF MICROSCOPIC VISION. 



microscope for separating lines so near together that they may he 

 obscured, or their numher falsijled, hy interference fringes.''^ 



The '^physiological part of the question " has, nevertheless, been 

 always considered by astronomers, mathematicians, and physiologists 

 to be an essential part of the theory of vision, whether by the 

 naked eye, or with the aid of optical instruments. All that relates 

 to the distinguishing power of the eye, all that we know of 

 magnifying power, all our outward experiences of light, colour, 

 form, size, distance, direction, proportion, and perspective are 

 gained through the sense of sight. And it is impossible to under- 

 stand the physical characters of light without considering how far 

 they are dependent upon and related to the action of the instrument 

 through which we become acquainted with them. I confess myself 

 unable to comprehend to what "important character of light," 

 allusion is made by Mr. Sorby, as *' limiting the power of the 

 microscope for separating lines," &c. If it is merely intended to 

 convey the idea that the effects of diffraction tend to limit the 

 power of the microscope before the fall advantages of magnifying 

 power are obtained, the question certainly merits full consideration, 

 and indeed has been already fully discussed in the papers of 

 Helmholtz and Abbe. 



But if it be thereby meant that the eye can distinguish the three- 

 millionth of an inch, or the -jo oVo^» ^^ ^^^^ tToVtttT' ^^ ^^ ^^^^ 

 with the microscope, because when, unarmed, the eye ''distinctly 

 appreciates " a visual angle of six seconds, and that consequently 

 •when aided by high magnifying power the "constitution of the eye'* 

 renders it capable of a defining power which is only limited by the 

 intervention of certain " physical characters of light," such an 

 inference can, as it seems to me, only be arrived at by ignoring 

 entirely the study of the dioptrics of the eye. Por in the eye, as an 

 optical apparatus, the distinction between acuteness of vision and 

 clearness of vision covers the same ground as the discussion of 

 ''resolving" and "defining" powers of an artificial lens-system. 

 Spherical aberration and chromatic dispersion are alike concerned in 

 "the image-forming process of eye and microscope. But the 



