Manchester Memoirs, Vol. Iv. {i^\\\ No,%\. 15 



the rings will be laterally displaced a distance d. If r be 

 the angle of refraction we have : 



tan r = tan / - d-t cos /'. 

 and /.I - sin //sin r 

 from which ju the index of refraction can be calculated. 

 Thus if /=45^, / = 47-6, and d=^\^-<, then \x=v^ nearly. 



A B 



Fig. 8. — Displacement produced by glass slab. 



The calculation being rather involved, a graphical method 

 may be applied as shown in Fig, 8, the ratio OO/OP 

 giving the value of /x. 



If the slab of glass be replaced by a lens, the apparent 

 distortion of the circles will vary with the angle i. A 



