294 McClellan — Note on Interference with the Bi-Prism. 



Art. XXYII. — A Note on Interference with the Bi-Prism ; 

 by William McClellan. 



In most descriptions of interference with the bi-prism the 

 trouble which diffraction is likely to cause receives scant atten- 

 tion. It is true, reference is usually made to the fringes that 

 are commonly found to border the field, but not as if they 

 could seriously detract from the result. Some time ago, on 

 setting up a prism on an optical bench, a colored center was 

 obtained, and apparently no amount of adjusting would make it 

 appear white. This, of course, proved the presence of something 

 other than interference, that is, diffraction. The matter was 



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studied a little, and an explanation found which is quite simple. 

 It is obvious that the condition of the field as obtained from 

 a given prism depends upon the relation of the distances 

 between the prism, screen and light. The fact is well known 

 to many experimenters, but no detailed note has ever been 

 observed by the writer, in any text. This is probably due to 

 the fact that the bi-prism is a lecture-room piece rather than 

 one for the laboratory. The following explanation is suggested. 

 Let L (fig. 1) be a source of light, and I 1? I 2 its two images 

 formed by the by-prism P x P 2 . MN is a screen. We may 

 treat the diagram- as if I 15 I 2 were separate sources, as they 

 might be if we could arrange constant phase relations. Light 

 coming from I x will light the screen from S x to T 1 if we con- 

 sider the prism set in an opaque holder HH. Similarly, light 



