462 ON PHYSIOLOGICAL LIMITS OF MICROSCOPIC VISION. 



opening through which a strong light is viewed, the effect being the 

 same whether produced by a pin hole in a card or by a minute optical 

 aperture of very highly magnifying lenses, combined or not with 

 deep oculars. The diffraction then which limits the resolution of 

 objects in the microscope is a physiological effect attendant upon 

 conditions which do not occur in the ordinary use of the eye, but which, 

 whenever they do occur, are the source of obstruction not to the 

 performance of the microscope, but to the function of vision. For 

 it is evident that a phenomenon which is as readily produced by a 

 pin hole in a card, or by various other physical means, as by looking 

 through a microscope, cannot be attributed to faulty ''definition" 

 or ''colour dispersion" of this instrument, bat that it appertains 

 to the general conditions of vision exercised under circumstances 

 unfavourable to the dioptric performance of the eye as an optical 

 instrument. Whatever, therefore, may be the limit of its per- 

 formance, it must be sought in the retina or in the dioptric media of the 

 eye. Supposing the microscope picture to be well delineated by a lens 

 of as perfect construction as can be made, the delineating pencils of 

 light which pass out of the microscope in a concentrated bundle of 

 intensely bright rays would, if thrown upon a sensitive chjemical 

 preparation, form a well resolved image. But if thrown upon the 

 front of the eye, this bundle of image-forming rays has to undergo 

 a fresh series of refractions and reflections, and the assumption that 

 nothing is thereby changed is not consonant with our present 

 experience. We are not justified in assuming that the eye as an 

 optical instrument does, or does not, deteriorate the performance of 

 the microscope (separately considered), or that, on the other hand, 

 the eye, armed with the microscope, would be capable of greatly 

 superior performance but for restricting conditions placed thereon 

 by certain physical qualities of light. For we must not forget that 

 light itself is but a sensation of the eye, and the physical qualities 

 of light but arbitrary expressions of particular effects which ether- 

 undulations produce on the organ of sight. And it certainly appears 

 most conformable with physiological fact and law, to believe that 

 the sense of sight is neither more nor less subtle than the dis- 



