Double Refraction of Quartz. 83 



refraction commonly recognized. All the phenomena of colours 

 which I have observed agree perfectly with the results of my 

 hypotheses. 



I may mention that I have found observations corresponding to 

 many detached parts of the phenomena which I have viewed 

 assembled, in the early memoirs of Arago and Biot. But the 

 method which these philosophers used, (particularly the latter), 

 of examining a small part only at a time, does not appear to 

 be well adapted to the discovery of the laws of light. In the 

 experiments which I am about to describe, every thing depends 

 on the form of the coloured curves ; and to attempt to dis- 

 cover this from observation of detached parts would be perfectly 

 hopeless. These coincidences I have recognized only since I 

 made my own observations. 



It must be observed that all the phenomena mentioned be- 

 low are described as they appear when examined with an ana- 

 lyzing plate of unsilvered glass. If a plate of tourmaline be 

 used, the right and left parts of the image will have the same 

 relative position, but the upper and lower will be interchanged : 

 the observer's eye being supposed to turn in such a manner 

 that the axis of the tourmaline appears up and down. 



PHENOMENA. 



I. If a plate of calc spar cut perpendicular to the axis be 

 examined with the polarizing and analyzing plates crossed, the 

 system of rings is that represented in fig. 1. If the analyzing 

 plate be turned less than 90° either way round the incident ray, 

 the system of rings is that represented in fig. 2 : and if turned 



ultimate state, or not. From all that I could extract from his Memoirs, I was always in doubt 

 whether both the ordinary and the extraordinary ray in the neighbourhood of the axis ought 

 not to be considered as divided each into two circularly polarized rays. 



L2 



