Dr. Gr. J. Stoney on Microscopic Vision. 519 



consisting of from two to five little points within each of the 

 specks which are visible with the half-inch. 



Now a study of image x enables us to trace the cause of 

 every one of these effects. Focus the diatom with the half- 

 inch. Take out the eyepiece and look at image x. Nearly 

 close the iris diaphragm under the condenser, so as to reduce 

 the incident light to a narrow central sheaf of beams. Then 

 the macula of this sheaf of beams will be seen in image x 

 as a central spot of bright light. Faint light 

 is visible about it, which is scattered pretty 

 generally over image x [this is light which 

 helps us to see the larger features of the 

 object],, and at about the distance from the 

 centre where (7 = 0'5 there is a ring of much 

 more intense light which has been diffracted 

 in all directions to that distance. This ring with X scale 

 of diffracted light is mainly red, owing to the 

 unequal distribution of colour spoken of in § 16, p. 345. 

 As so much red has been thrown into this spectrum, there 

 is an equal deficiency of red in the light which forms the 

 central macula and the faint diffracted beams by which 

 the larger features on the object are seen. Hence it is 

 that when the marginal ring is shut out by partially closing 

 the Davis's shutter, the diatom will appear blue : blue 

 being the colour which lamplight becomes when much of 

 its red is withdrawn from it. The exclusion of the diffracted 

 red light has another effect — it prevents the formation of 

 a number of the rulings which are necessary for the 

 formation of a good image ; and accordingly much of the 

 detail on the diatom which is visible when the Davis's shutter 

 is open, is lost to sight when it is sufficiently closed to render 

 the image blue. 



That the image is not quite colourless with the full aperture 

 of the half-inch is because there also exist other diffracted 

 beams which lie beyond the reach of that objective. We 

 know that they exist and that they are coloured, because the 

 image seen with the immersion objective is colourless, and 

 because more detail is seen in it. It has rendered the image 

 colourless by adding some coloured beams to the slightly- 

 tinted image which the half-inch, fully open, presented, and it 

 has brought out further detail by transmitting these additional 

 beams in directions which furnish new rulings. 



Return now to the half-inch objective. When the central 

 stop is put over its back lens, the central macula of image x 

 is covered up. This shuts out the dioptric beams. The image 



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