522 Dr. G. J. Stoney on Microscopic Vision. 



seen on replacing the eyepiece is image 3. One of its 

 honeycomb cells is represented in the diagram, from which 

 it will be seen that there are 19 deceptive markings upon 

 it. The central one is bright, but by a slight change of 

 focus becomes dark. 



Adjust the focus so as to make it dark, and then close the 

 Davis's shutter into an intermediate position, which shuts out 

 the whole of the red of the outer twelve spectra, while it 

 allows their blue halves to pass. This produces a new image — 

 image 2 — which will be seen on replacing the eyepiece. It 

 will be found to be nearly like image 3, but with a red central 

 speck instead of a dark one. 



The reason of this is that the hexagon of twelve spectra 

 produces, along with some of the dioptric light, rulings which 

 formed the dark central speck, as is proved by its being- 

 present in image 3 and not in image 1. ISTow when w T e shut 

 out the red of those spectra, we produce the same effect as if 

 we added red light in the opposite phases, and half of this with 

 half the same dioptric light (which has not been excluded) 

 produces rulings the same as before, except that the bright 

 and dark bands are interchanged. These accordingly change 

 the dark into a bright speck at c. The effect is somewhat 

 startling, since we change the specks from dark to red by 

 shutting out red light ! This gives one a lively picture of 

 how it comes to pass that the exclusion of light from image C 

 can lead to the intrusion of unauthorised markings. 



Another matter may here be adverted to. As explained 

 in § 39, these spectra need not be equally intense for every 

 wave-length : sometimes the less refrangible light pre- 

 ponderates in them and sometimes the more refrangible. 

 Accordingly some of the intercostal markings to which they 

 give rise will have a preponderance of red, others a prepon- 

 derance of blue. The first will seem buff-coloured, the others 

 whiter than the lamplight we use as our illuminant ; and 

 upon scrutinizing the fictitious markings this difference in 

 tint between them is plainly visible. 



When the dioptric macula is placed excentrically the 

 resulting intrusive markings become in some degree both 

 different and differently situated. Hence when we give a 

 large aperture to the dioptric sheaf of beams by opening the 

 iris diaphragm below the condenser, its macula becomes 

 large ; one part of it is central, others are excentric in 

 different degrees and different directions. Each part gives rise 

 to its own set of false markings, and these where they overlap 

 interfere and may occasion a general illumination at the place 

 where they are situated instead of distinct markings. This 



