ON" PHTSrOLOOICAL LIMITS OF MICROSCOPIC VISION. 459 



dioptric function of the eye ia further impeded by circumstances 

 which do not occur, or if they occur can be better remedied in the 

 achromatic lens-combination of the optician. For instance, animal 

 tissues and fluids are inferior in translucency, homogeneity, re- 

 fracting, and dispersing power to the materials of which an 

 objective is made ; opaqae particles fixed or floating in the eye- 

 structures occasion shadows ; diffraction spectra of the crystalline 

 lens and shadows of the retinal blood vessels obscure the field of 

 vision. And these difficulties which, as will be shewn in the 

 following pages, are neither few nor unimportant, are inherent in the 

 constitution of the eye ; whilst the only circumstances which 

 favour the accuracy of images are (first), that the plan of anatomical 

 construction admits of accommodation of lens to varying focal dis- 

 tance and of movement of the eye on its axes ; (secondly), that the 

 eye reduces the image instead of magnifying. 



We may, in the first place, consider the question of visual 

 angles and magnitudes. Mr. Sorby quotes an experiment of Dr. 

 Pigott's, by which, this gentleman determined a datum of six 

 seconds, from which a magnifying power is inferred when a 

 microscope of i-OAP power is used, and this magnifying power is 

 considered as indicating what the distinguishing power of the 

 retina might be if not obstructed by diffractive effects. 



Now, assuming the physiological value of the acuteness of 

 vision to be an angular distance, it should not be forgotten that 

 this smallest angle is determined ly the illumination. " To 

 illuminated points on a dark ground there are scarcely any 

 boundaries. Small as such a point may be, its image has, on 

 account of the imperfection of the dioptric system of the eye, a 

 certain extent ; and the only question is whether this produces on 

 one or more percipient elements of the retina a sufficiently distinct 

 impression of light to be perceived. Small dark points disappear, 

 on the contrary, very rapidly through irradiation on a bright 

 ground, which lessens the contrast of illumination of percipient 

 elements, and, if the illumination be strong, the contrast will be 

 much less perceptible. The question as to the smallest angle under 



