536 Wright — Explanation of Interference Phenomena. 



Art. XLIX. — A Device to Aid in the Explanation of Inter- 

 ference Phenomena ; by Fred. Eugene Wright. 



Students of crystal optics, on taking up the subject of bire- 

 fringence, frequently encounter difficulty in forming a clear 

 conception of the exact course of the light waves through the 

 crystals and the resultant interference phenomena when polar- 

 izer and analyzer are used (crossed nicols). The small apparatus 

 of fig. 1 has been found serviceable 

 as a model in this connection, and 

 facilitates to a certain degree the 

 explanation of several of the phe- 

 nomena of plane-polarized light. 



The device consists essentially of 

 a brass rod divided into three parts, 

 a, b, c, which are so connected that 

 each one is revolvable for itself about 

 the common axis ; into each section, 

 moreover, longitudinal slits have been 

 cut and plates of thin, transparent 

 celluloid inserted. The celluloid plate 

 A represents the plane of vibration 

 of light waves emerging from the 

 lower nicol of the microscope ; B is 

 a celluloid model of the crystal sec- 

 tion with its ellipsoidal axes at an 

 angle of 45° to the plane of vibra- 

 tion of the lower nicol; C 3 and C 2 

 represent the planes of the two waves 

 emerging at right angles to each 

 other from the crystal plate, the light 

 waves C 1 being a definite distance 

 ahead of C 2 as a result of the une- 

 qual velocities of the two waves in their passage through the 

 crystal B. On entering the upper nicol, these two waves are 

 again reduced to the common planes of vibration D 1 and D 2 , 

 the waves vibrating along D 2 , however, being destroyed by 

 total reflection and those along T> 1 only passing through. By 

 the use of this model, it is not difficult to prove : (1) that two 

 waves emerging from a refracting crystal at a distance apart of 

 one or more whole wave lengths (phase difference zero) interfere 

 mutually when reduced to the common plane of vibration, Dj 

 (2) while two waves one-half wave length apart (in opposite 

 phase) mutually strengthen each other when reduced to the 

 common plane of vibration, D l? of the upper nicol ; vice versa, 

 if the plane D 2 be considered, the phenomena are exactly 

 reversed — facts which are difficult to represent clearly without 

 the aid of some such model. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C. 



