Professor Airy on the Double Refraction of Quartz. S'Si 



of rays. The same want of time I hope will be allowed as an 

 excuse for the want of accurate measures : without which, no 

 theory, however satisfactory in general explanations, can be con- 

 sidered as firmly established. 



Note A. — It is not easy to make this experiment in a satis- 

 factory manner. If the axes of the crystals are not precisely 

 adjusted, several images will be seen. ' I have not succeeded in 

 obtaining two only, though I have made the others much more 

 faint than the two principal. 



Note B. — As I cannot appreciate the mathematical evidence 

 for the nature of circular polarization, I shall mention the expe- 

 rimental evidence on which I receive it. 1st, The light when 

 received on an analyzing plate or tourmaline presents the same 

 appearance in whatever direction the analyzing plate is turned 

 round the incident ray. 2d, The phenomena of depolarization 

 are the same in whatever direction the analyzing plate is turned. 

 3d, If the polarized light passes through two such rhombs 

 placed in similar positions, the plane of polarization is shifted 

 90°. 4>th, If they are placed in crossed portions the plane of 

 polarization is unaltered. 5th, The phenomena of depolariza- 

 tion agree with the calculations founded on this supposition : in 

 uniaxal crystals, where the plane of polarization of one ray is 

 changed {360° in going round the axis, the alternate quadrants 

 are pushed in and thrust out one quarter of a tint : and in bi- 

 axal crystals, where the plane of polarization is changed only 

 180° in going round the axis, the alternate semicircles arc al- 

 tered in the same manner. 



Note C. — If I might venture to fix on the discovery of Fres- 

 ncl, which among all his wonderful additions to optical science 

 appears likely to possess the greatest practical value, I should 

 select his invention of the mode of producing circularly-polariz- 

 ed or elliptic-ally polarized light by internal reflexion of plane- 

 polarized light in glass or water. lie has given us the power 

 of producing light whose laws are as well kr.ov n as those of 

 plane-polarized light, and which is more manageable, inasmuch 

 as it admits of degrees in its cllipticily. The beautiful geome- 

 try of Mains is forgotten when we think of the discovery of po- 

 larization : the I valuable theoretical di covi ries of I 



