490 



Mr. B. T. Lowne on some 



[Jan. IJ, 



astigmatic eye appears lighter than when accurately focused ; and this 

 appears to be the case in surfaces of considerable extent, so that it could 

 not be due to the formation of a slightly larger picture. A distant news- 

 paper scarcely differs in appearance from a corresponding sheet of white 

 paper, and two similar prints observed at suitable distances give dif- 

 ferent tints; the further one, when it no longer produces a distinct 

 picture of the individual line, appears lighter in tone. Still I did not feel 

 quite satisfied until I succeeded in having the accompanying diagram 



11 



llilllili 



The squares a and 6, c and d have respectively the same proportion of 

 black upon the surface : a and h are | black, c and d half black. At 

 suitable distances the following sensations result : — So long as all the 

 lines are distinct there are four distinct shades ; h and d appear darker 

 than a and c respectively. When the diagram is seen at a distance of 

 from 15 to 20 feet, c and d become identical in shade and can no longer 

 be separated, but h still appears much darker than a. At a still greater 

 distance there are but two shades ; and these remain distinct so long as 

 the diagram can be distinguished : the illumination is really different ; and 

 no distance makes the sensation the same. 



2. On the Time required to Produce or Obliterate an Image on the Retina. 



Much difference of opinion exists on this point, and many contra- 

 dictory statements have been made by the first authorities. Schafhaiitl * 



* Miinch. Abh. Tii. 465. 



