1900] MICROSCOPICAL JOURNAL. 9 



quite so luminous), which proceed from the point of the 

 obstacle at various angles to this direction. The whole 

 forms what is known as a diffraction fan. 



In ordinary vision we need take but little account of 

 these diffracted rays, but in vision with optical instru- 

 ments and especially with the microscope they play a 

 large and fundamental part, for one of the functions of 

 the objective is to collect these rays, starting from a point 

 of the object at different angles, and bring them alto- 

 gether back into one focus at that spot where an image of 

 that particular point of the object is formed. A peculi- 

 arity about diffraction fans is that the smaller the struc- 

 ture which causes them, the more spread out are the com- 

 ponent rays of the fan, and that at least two rays must 

 be taken up by the objective to show up the structure at 

 all, except as a sort of indefinable blur. 



To return to the color discs, we can now comprehend 

 how any ray (R, fig. 8), gets split up into rays R 1, d 1, 

 d2; the first one ?of which passes through the blue part 

 of the disc before being brought to a focus up near the 

 eyepiece, the others through the red part of the disc. The 

 greater number get the best of it, and so the most of the 

 structure appears in red. Of the whole object, only the 

 very coarse structure would appear in blue, because the 

 rays of the diffraction fan produced by this are so crowd- 

 ed together that they all pass through the blue centre of 

 the disc. 



Incidentally I may say that by having color discs ground 

 in a peculiar manner, it has been found possible to get the 

 separate images of an object formed by the blue and red 

 portions of a color disc side by side, the one of which 

 shows only the coarsest structure, the other all the finer 

 structure. 



Refraction of light also plays some part in determining 

 the color of the object in this third method of illumina- 

 tion, but having dealt with this already in the other 



