701 



NOTE. 



Grayson's Photomicrographs of his Rulings. 

 By Edwaed M. Nelson. 



Plate XIV. 



In Grayson's beautiful photographs, exhibited at the Meeting on 

 June 15, there are points of interest which deserve special notice. 

 PI. XIV. fig. 1 is a reproduction of his photomicrograph of his own 

 90,000 band, amplified 1930 diameters, and pi. XIV. fig. 2 is a copy 

 of his enlargement of his own negative, in which the total amplifi- 

 cation amounts to 7750 diameters.* 



It will be apparent to everyone that by any examination of the 

 photomicrograph (pi. XIV. fig. 1) it is quite impossible to determine 

 the number of the lines Grayson has actually ruled in that band, 

 because no one can tell where the ruling begins or where it ends. 

 The band has been photographed by oblique light ; on the left-hand 

 side, at a, fig. 1, there is a very dark broad line, and next to it a 

 thin faint one, and upon the other side of the band, at c, the lines 

 fade off in three or four " ripples." In pi. XIV. fig. 2 Grayson has 

 cut off the " ripples " at d. 



It does not appear that these phenomena have been noticed by 

 anyone, but, be that as it may, no account has been published 

 either of them or of the source of their origin. Both the strong 

 and the faint lines and the " ripples " owe their presence to oblique 

 illumination in one azimuth, and are examples of the " diffraction 

 phenomena " usually present with that kind of illumination. When 

 ruled lines and diatomic strise are examined by oblique light in one 

 azimuth, two images can be produced (these are usually perceived 

 at once, if not, a little special manipulation will always cause them 

 to appear) : the one image is that of the band or diatom unresolved, 

 the other is that of the resolved lines or stria?, which seemingly 

 float over the unresolved band. In brief, they are examples of 

 Professor Abbe's absorption and diffraction images. Now the law 

 in respect to these images is this : upon focal alteration the upper, 

 or diffraction image will roll over the lower, or absorption image. 

 (The word roll precisely describes this phenomenon, which is 

 exactly like that of the roll-top of an escritoire : the lines appear to 



* These amplifications refer to the reproductions which differ but slightly 

 from the original prints. 



