IGO J. V. UABFOED ^POLARISATIOJT OF LIGHT. 



also shown all round. This kind of polarisation is termed circular, 

 and the crystal quartz exhibits the property in a remarkable way. 

 Of the same character is also what is termed elliptical polaiisation. 

 The subject altogether is too extensive for hasty treatment, but 

 the object of the lecture has been to bring under notice its most 

 prominent featui'es, and it is hoped that the eli'ort will not have 

 proved unavailing. 



EXPLANATION OF PLATE II. 



Fig. 1. Section of beam of ordinary light. 



Fig. 2. Sections of beams {a b) of polarised light. 



Fig. 3. Polarisation by reflexion and refraction : a, ray of ordinary light ; 

 i, portion of ray polarised by reflexion from sm-face of glass plate ; c, portion of 

 ray polarised by refraction through it. 



Fig. 4. Polarisation by double refraction: a, ray of ordinary light; b, ordinary 

 ray, and c, extraordinary ray, polarised in planes at right angles to each other by 

 transmission through prism of Iceland spar [d d). 



Fig. 5. Snell's Law of Sines : as the sine, a b, of the angle of incidence (in 

 air) is to the sine, a b', of the angle of refraction (in water), so is Ij to 1. 



Fig. 6. Nicol's Prism : a, ray of ordinary light ; b, ordinary ray reflected 

 from Canada balsam and thrown out of the prism ; c, extraordinary ray trans- 

 mitted through it ; d, Iceland spar ; e, Canada balsam. 



Fig. 7. Propagation of waves : the lines denote summits. 



Fig. 8. Wave motion: the arrows show the direction of the particles of 

 the wave and their upward and downward movement. 



Fig. 9. Two waves coinciding. 



Fig. 10. Two waves totally interfering. 



Fig. 11. Two waves partially interfering. 



Fig. 12. Newton's Rings (curve of upper glass plate exaggerated) : Nos. 1, 

 2, 3, 4, 6, 6, and 7 denote repetitions of tlie same colour ; i, ii, iii, iv, etc., equal 

 intervals of the passage of light. 



Fig. 13. Films (thickness exaggerated): a, reflected from upper surface; 

 a ', reflected from lower sui-face ; b, refracted direct ; b ', refracted by reflexion 

 from upper surface. 



Fig. 14. Production of colour by polarised light: a, polarised beam; 

 b, selenite film; c c', beams separated but not visibly; d, Iceland spar; 

 ee', and//", the two foregoing beams each doubly refracted; i/ ff', two 

 colours complementary. 



