XIX, 1. Kheinberg: Common Basis of tbe Theories of Microscopic Vision. 5 



is simply the mean or average effect of tlie changing intensities, 

 and is apparently perfectly constant. 



In a great mauy instances tlie mean eflfect gives rise to pre- 

 cisely the same intensity of light throiighout a given area, as the 

 constant effect from a Single centre of distiirbance , but in others it 

 may cause a redistribution of light as was the case in the famous 

 experiments of Grimaldi and Young. -^ Not less than was the case 

 in these classical experiments is it essential to examine the possible 

 difference in etfect , when dealing with lenses , Harnes , and objects 

 which break up the light. 



Chapter II. 

 The Image of a Lens. 



Microscopic vision resolved to its simplest form, consists in the 

 reviewing of the enlarged Image of au object produced by a convex 

 lens, with the aid of another convex lens. 



Our first care must therefore be to understand the nature of 

 the Image projected by a convex lens. 



The magnified image of a tiny isolated disc of light of eqnal 

 intensity throughont , should , in order to be an absoliitely true re- 

 presentation , be another evenly illuminated isolated disc of larger 

 size. Instead of this the actnal image is a disc of light of greater 

 brightness at its centre, and fading off towards its edges. This disc 

 moreover is surrounded by rings of light, negligeable nnder ordinary 

 cirenmstances , because they are too close to the disc for the eye 



1) Grimaldi having observed the paradoxical effect, that two lights 

 superposed created partial darkness , passed sunlight into a darkened 

 Chamber through two tiny holes so close to one another that the dises of 

 light formed on the screen opposite partially overlapped. He thought 

 that when the edges overlapped this part was less bright than the single 

 edge was. It was however a mistake, and it was Young who performed 

 the experlment successfully by placing a screen with one hole a little 

 distance behind the screen with the two. The sunlight passed through the 

 Single hole first , and thence onwards through the separated holes , and a 

 number of overlapping coloured fringes of light with dark bands between, 

 was the result. The single hole was equivalent to a single centre of dis- 

 turbance, whilst in Grimaldi's e.xperiment no precaution to use light from 

 a Single centre onh^, had been taken. 



