Dr. G. J. Stoney on Microscopic Vision. 



521 



outwards, To make the following experiment, the lower 

 iris diaphragm should be opened to so moderate an extent 

 as will cause these maculae just to 

 come short of touching one another. 



When now the Davis's shutter is 

 gradually opened, which increases the 

 aperture of the objective, a succession 

 of images present themselves. If it 

 covers up all the maculae except the 

 central one, the diatom is indeed seen, 

 but with no detail upon it. In all 

 subsequent enlargements of the aper- 

 ture, the honeycomb structure of the 

 diatom is visible ; but with the ad- 

 dition of spurious effects which vary * while the grasp of the 

 objective is being enlarged. In fact, they depend on what 

 spectra are permitted to pass and what spectra are excluded. 



Let us consider three of these images, I, 2, and 3, of 

 which 2 is formed with a larger aperture than 1, and 3 with 

 a larger aperture than 2. If we start with image 1 and open 

 the Davis's shutter so as to pass to image 2, we may regard 

 2 as being image 1 modified by adding to it the new rulings 

 which are formed by the newly admitted light cooperating 

 with a part of the dioptric sheaf. And on the other hand, if 

 we start with image 3 and close the Davis's shutter, we may 

 regard image 2 as being image 3 with the rulings added to it 

 which would result if light equal in intensity but opposite in 

 phase to the excluded light were allowed to cooperate with 

 part of the dioptric light. In fact, these rulings fill the gaps 

 between whatever rulings 3 has and which 2 has not. 



Sometimes the one of these processes and sometimes the 

 other helps us to a better conception of what is going on. 

 The first process needs no elucidation, and of the second the 

 following is a very neat illustration. 



Keep the same diatom on the stage; then look at image #, 

 and open the Davis's shutter till the inner 

 hexagon of six spectra is seen. On re- 

 placing the eyepiece a few intrusive mark- 

 ings will be perceived, including a bright 

 patch of light occupying the middle of each 

 honeycomb cell. This is image 1. Next 

 look again at image x, and open the Davis's 

 shutter farther until the succeeding hexagon of twelve 

 spectra is also allowed to pass. The image which is then 



* With the half-inch apochromatic, two, three, and four intercostal 

 markings can be produced along- each side of each hexagon, the last 

 number when we employ black-field illumination. 



