Flat- Wavelet Resolution. 583 



reversal let the cover-glass, object, mounting medium, and 

 everything else below stratum S be suppressed, and let the 

 whole of the space below stratum S, which we may call 

 space Q, be filled with the same medium — air or oil — as 

 occupies stratum S and the space K. The light returning 

 downwards after reversal, will then form within space Q an 

 image, which we may call Image B, that will exhibit exactly 

 the same amount of detail as Image B' ; the chief difference 

 being that if the medium occupying S, K, and Q be air, 

 B will be located a little further down than B', with a similar 

 but smaller displacement if the medium be water or oil. The 

 slight displacement is of no significance, since it is un- 

 accompanied by any change in the amount of detail which 

 the images furnish. And there is now the great convenience 

 that only one medium — air, water, or oil — intervenes between 

 image B and the front lens of the objective ; whereas three 

 media — air or oil in stratum S, glass in the cover-glass, and 

 mounting material — had intervened between image B' and 

 the front iens of the objective. 



34. A further step in our investigation can be made by 

 supposing the light to undergo a second Reversal. For this 

 purpose, let the reversed light after forming image B, be 

 allowed to continue its downward course in space Q for 

 another minute and let it be then reversed. Let us imagine 

 that simultaneously with this Second Reversal, the micro- 

 scope-tube and the rest of the apparatus above stratum S are 

 allowed to resume their positions, while the cover-glass and 

 all subjacent apparatus are still absent. The following events 

 would then happen : — The light directing its course upward 

 after the Second ReAersal, would converge upon and form 

 the same standard image B as before, after passing which it 

 would continue to ascend as though it had been emitted from the 

 several puncta of that image. It would, accordingly, proceed 

 to fill air space S with precisely the same light as that which had 

 before the first Reversal been emitted by the real microscopical 

 objects. From this we learn that the light which in the 

 ordinary use of the microscope reaches the front of the 

 objective from objects under examination, after having passed 

 in succession through the medium in which the microscopical 

 objects are mounted, the cover-glass that protects them, and 

 the air space between it and the objective — that this light is 

 identical with what would come from Image B, pursuing the 

 latter part of it- course to the objective through the air 

 space S; and that therefore it will be legitimate to regard 

 the light that crosses the air space S as having emanated 

 from that image, and being incapable of presenting to us, in 



