ON PHYSIOLOGICAL LIMITS 



MICROSCOPIC VISION. 



461 



from visual angles of minute obj 

 normal vision with the naked eye t, 

 dispersion of the light passing tbr 

 indistinctness of retinal percepti 

 delineation of image and insufficii 

 But if little value can be placed 

 apparent, magnitudes of the visual 

 is to be thought of the assumption i 

 observing an object with the naked 

 of vision, can be made the basis 

 obtained by looking through a mic-. 

 without supposing the calculatioii 

 altered conditions of vision, and 

 illumination ? Even the magnifyi 

 due to the microscope, (say Uioo\^ i 

 the instrument, the retinal image I 

 the image presented to the eye a 

 ** distance of clear vision " (10 inch 

 that acuteness of vision does not ( 

 further than is necessary to separd 

 sufficiently to be capable of giviii,- 

 distance is clearly governed by th 

 retina. So again distinctness of i 

 dioptric function of the eye equal. 

 Lastly, in regard to diffraction, wl 

 exert a potent influence on the mii 

 it must be borne in mind that thi- 

 any faulty quality of a lens-system 

 of the optical arrangement necess 

 before the eye, because such a; 

 ** angular amplification," whereb\ 

 which the image is delineated, is ; 

 centrated form, and reduced secti. 

 Professors Helmholtz and Abbe ha 

 that diffraction in the microscopt 



-3. For we see that even in 



retinal images are falsified by 



igh the eye structures, and by 



, consequent upon defective 



. contrast of light and shadow. 



I calculations of actual, from 



agles of minute objects, what 



alt a visual angle, obtained by 



re and under normal conditions 



t' calculation of a magnitude 



cope magnifying 1000 times, 



)o be affected by the widely 



y a wholly different mode of 



;• power taken for granted as 



I compound effect of the eye and 



Lijg about 16 times less than 



I supposed to be seen at the 



). And we have already seen 



jend upon magnifying power 



two objects from each other 



;''parate impressions, and this 



distinguisMncf capacity of the 



ige is dependent on accurate 



with that of the microscope. 



;!i indeed has been proved to 



-cope image as seen hij the eye, 



iffraction is not produced by 



j'dt is the simple consequence 



y' to bring an enlarged image 



ngement necessarily implies 



he whole mass of light, by 



sented to the eye in a con- 



l area, in front of the pupil. 



insisted on the circumstance 



; caused by the narrowness of 



