460 ON PHYSIOLOGICAL LI^^' rxS OF MICROSCOPIC VISION. 



which any ohject is still to be seen is thus governed completely ly 

 the degree of illumination, and i ; a physiological point of view it has, 

 therefore, no meaning. It is a very different thing to determine the 

 influence of illumination on the < ^idistinctness of objects. Nor is the 

 investigation with a definite illinnination devoid of importance in a 

 practical point of view (microscoical investigation), from which view 

 Harting especially has worked it."* 



The facts quoted in the not. conclusively show the inutility of 

 attempting to determine '' limits of visibility " by calculation 



* See Bonders on AnomaHes of Accommodation and Refraction of the Eye, 

 translated for Sydenham Society by Dr. Moore, 1864, page 195. ^ 



Dr. Figott's estimation of visual Mi!.:le (for his own eye; is cited without 

 any explanation of the various conditions which affect such observations. 

 The experiments of Hueck show: -1. That a normal eye, capable of 

 accommodating itself to near and distinct objects, finds small objects, whether 

 near or far off, disappear at the samo visual angle. (N.B. — The comparison 

 being made under same conditions of illumination and by the same eye). 2. 

 That a line is seen further off than a, pointy though both may have the same 

 thickness. 3. White objects on a darl: ground are seen at a greater distance 

 than hlach objects on xvliite ground. 4. At great distances the necessary 

 visual angle for recognition of objects slightly and gradually increases. _ o. 

 The smallest visual angle at which \\h ite points on black ground were visible 

 was 2' -6^ whilst white lines on black ground were seen with an angle of V'% 

 A cobweb thread with an angle of t ' 6, and a white hot wire at 0'-2. 



Volkmann found that he could di>t)nctly see a hair '002' diameter placed 

 30" distance. This gave '000033 inch for the dimension of its retinal image 

 according to his calculations. But he considered such observations unsuited 

 to determine the size of the smallest retinal images which could excite the 

 sensation of vision, firstly, because dispersion of light (from spherical 

 aberration of the crystalline lens) miglit affect a larger retinal surface than 

 would correspond to the calculated dimensions of the image ; and secondly, 

 because irradiation of the stimulus of light would likewise spread beyond the 

 precise area to which that stimulus was applied. And the following experi- 

 ment appears to shew that one at least of these two circumstances really 

 occurs : —Two parallel cobweb threads being set up with an interspace between 

 them of 00052'', Volkmann recognised them as double threads at V distance 

 but not further. But a friend who possessed acuter vision recognised the 

 double threads at 13" distance. The dimensions of the respective retinal 

 images Volkmann calculated, for his own short-sighted eye, at 0-00037'', and 

 for that of his friend at 0-00021". But by another experiment (two threads 

 0-016'' apart, seen at 27'' with aid of spectacles) Volkmann determined the 

 dimension of retinal image, appreciable by his own eye, at 0.00029''. From 

 a comparison of the figures in the two experiments he concludes that the 

 smallest magnitude which his eye could recognise was ten times 

 greater than the smallest recognisable retinal image, and that the focus which 

 his eye could form at smtable distance of vision occupied a space of 00029", 

 and lastly, that the reason why he could not see an object under any minuter 

 visual angle was because the light would then be too much dispersed. 



