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



small angles, which give wider lines but a very narrow clear 

 space. 



Figures 2 and 3 will illustrate the above discussion. They 

 are reproduced from photographs taken on the optical bench, 

 using the eye-piece as an objective. Figure 2 shows the two 



sets of lines superimposed ; the center band could be made any 

 color desired, by changing the distances a and b. Figure 3 

 shows a half field with complete separation. On the left the 

 two sets of lines can be seen superimposed, while on the right 

 the interference lines alone are seen. Another photograph, 

 not reproduced, shows a whole field with very complete separa- 

 tion, the central portion covered with very fine interference 

 bands. 



Randal Morgan Laboratory, 



University of Pennsylvania. 



