Dr. G. J. Stoney on Microscopic Vision. 527 



of the object which is furnished by the microscop9 when the 

 card is removed. 



Note that the ruling as seen does not extend across the 

 whole field of view, but only a little beyond the boundary of 

 the diatoms. This is because the ruling we see is made by 

 sheafs of beams instead of by individual beams. The more 

 we can reduce the size of the holes without making the ruling 

 too faini, the more diffuse will the image of the diatom 

 become, and the farther out will the ruling extend ; until at 

 the limit it would extend over the whole field of view and be 

 perfectly uniform everywhere "*. 



It is instructive to make this experiment with a specimen of 

 Actinocyclus Ralfsii, selecting one which is blue when seen 

 through a small aperture. Here we found the diffracted 

 light to be red, see § 39. This red light cooperating with 

 some red out of the dioptric light produces a ruling which if 

 seen alone would consist of alternate red and dark bands. 

 But there is an excess of dioptric light beyond what is 

 employed in contributing to form this ruling, and this excess 

 throws a wash of blue light over everything. Where it falls 

 on the red it turns it white, where it falls on the dark parts 

 it turns them blue. Accordingly what is seen is a ruling 

 of bands which are alternately white and blue. Similarly 

 on other diatoms the ruling is found to consist of white and 

 coloured bands instead of merely light and dark. The colour 

 which takes the place of the dark bands is in each case what- 

 ever colour the diatom, or the part of the diatom, presents 

 when seen through a very small aperture. 



* What has happened may be clearly apprehended from the following- 

 considerations: — If instead of three individual beams with puncta at a, b, c 

 to produce a ruling, we suppose two such sets as in the figure, and all under 

 such circumstances that they can interfere, then a and a' produce a coarse 

 ruling with its luminous bands and dark 

 intervals lying vertically. Rulings in exactly 

 the same position are produced by b and b', 

 and by c and c', so that these all reinforce 

 one another — they together produce one 



from aa', bb' , cc' is extinguished at the 

 middles of the dark bands of this ruling. At 

 the same time the beams with puncta at a, 

 b, c produce a fine ruling which lies hori- 

 zontally, and a', b', c' produce an identical 

 ruling which, reinforces it, and all light from Inuv 



aa', bb', cc' must disappear at the middles 

 of the dark bands of this ruling. Hence we have two rulings that 

 co-exist, a coarse vertical ruling and a fine horizontal one; and the 

 outcome is that we see a horizontal fine ruling, which however is visible 

 only across the bright bands of the coarse vertical ruling, and which 

 fades out in the intermediate dark spaces. 



