702 Note. 



shut up on one side and roll out upon the other.) When the 

 focus is raised, the " ripples " always advance towards the incident 

 light. 



If, therefore, Grayson's photomicrograph was taken with the 

 focus slightly raised, then the light was coming in a direction from 

 right to left, but if depressed, in the opposite direction. 



Whether the image of the lines or stria? is stronger with an 

 elevated or depressed focus will depend upon the correction of the 

 object-glass, i.e. tube-length. Now let it be assumed that Grayson 

 had such a length of tube that the stronger image was obtained 

 with a slightly depressed focus, in which case the light will be 

 coming from left to right, and the " ripples " running away from it, 

 as at c (pi. XIV. fig. 1). 



At a the lines will be rolling up, and it may be assumed that 

 they have rolled up as far as the right-hand edge of the left-hand 

 border of the absorption image of the band. 



The absorption image of this edge will consist of a black line 

 with its complementary white lines on either side of it.* 



We have then an absorption black line of the border of the 

 band, and on top, and rather to the right of it, the first diffraction 

 line of the ruling ; these two reinforce each other and originate 

 the wide black line seen at a and b (pi. XIV. figs. 1, 2). The 

 second line of the ruling falls on top of the white border of the 

 absorption line, and, being weakened, appears as a faint line to 

 the right of the broad black line. 



At c (pi. XIV. fig. 1), on the other side of the band, are the 

 "ripples," which are running off the lower right-hand absorption 

 band, and the farther off they get the weaker they become. Also 

 at e a rather wide white line followed by a blacker line may be 

 detected ; these are due to the influence of the border of the 

 absorption band underneath them. I do not know the reason why 

 the same phenomena on the right hand at c should be so much 

 less pronounced than they are on the left hand at a (pi. XIV. 



If a photomicrograph of ruled lines is taken by a direct axial 

 cone of wide angle — that is, if the illumination is critical — the 

 print will truly represent the original. Thus pi. XIV. fig. 3. repre- 

 sents one of a band containing 34 ruled lines ; but the moment 

 oblique light in one azimuth is used the image may no longer 

 reproduce the original, the rate of ruling only excepted. In other 

 words, while it is possible in an image of rulings obtained by 

 oblique illumination to determine the rate per inch of the ruling, 



* Another example of an interference phenomenon. -These white lines are of 

 necessity not easy to detect, because they are viewed upon a bright field, but they 

 are often to be found in photomicrographs on account of their exaggerated con- 

 trasts. But even in the Microscope, when looked for with attention, they may 

 be seen ; at the same time, it is quite easy to overlook them. 



