SOME DIFFRACTION PHENOMENA; SUPERPOSED 



FRINGES.^ 



By CHARLES F. BRUSH. 

 (Read April ig, 1913.) 



Fresnel observed that diffraction fringes, outside the shadow, are 

 not affected by the thickness or shape of the diffracting edge so long 

 as the latter is smooth and straight ; and cited, as an instance, the 

 back and edge of a razor, which gave identical fringes under the 

 conditions of his experiment. Presumably he observed the fringes 

 as developed several decimeters, or even meters, from the diffract- 

 ing edge in the usual wa}'. 



I have found, however, that when the fringes are observed within 

 a millimeter or two of the dift'racting edge, by means of a microscope, 

 they are very greatly influenced in brightness and sharpness by the 

 contour of the edge. 



In most of my experiments I have used cylindrical edges in order 

 that their shape and curvature might be accurately known. I have 

 used fine wires grading up from 0.02 of a millimeter in diameter to 

 line needles, thence to medium and large needles, and small, medium 

 and large brass rods and tubes, always with a smooth surface. The 

 line wires and needles were screened on one side to confine diffrac- 

 tion to the other side only. 



In the diagram of my apparatus A represents the source of light, 

 which conveniently may be a short section of a tungsten lamp fila- 

 ment ; 5 is a spectrometer slit parallel with the lamp filament and 

 very nearly closed. C is th.e diffracting screen located 15 or 20 cm. 

 from the slit, with its edge adjusted parallel with the slit by turning 

 the stage of the microscope D. Z? is a microscope provided with a 

 5.0 or 2.5 cm. objective and a strong eyepiece giving a magnifying 



^ Presented in preliminary form before The American Asociation for the 

 Advancement of Science, December 30, 1912. 



276 



