222 . APPENDIX. 



transverse to each other, and the field of view conse- 

 quently dark,) if it be turned round in one plane, it is 

 seen to grow four times luminous and four times dark in 

 the course of one revolution. This I have found to be 

 universally the case with all the substances which I have 

 tried, and it also is in accordance with theory. 



2. In the case of an acicular crystal one of the optical 

 axes always coincided with the axis of figure, or length 

 of the crystal ; so that if a crystal of this sort appears 

 unilluminated, all the others that are either parallel to it 

 or perpendicular to it are likewise dark, 



3. It results from the above, that a circle composed of 

 acicular crystals diverging from a point must present the 

 appearance of a black cross, and that the crosses on all 

 the circles will be parallel. 



4. With respect to the rings of colour, they are a con- 

 sequence of the variable thickness of the crystalline circle 

 at different distances from its centre. Their being visible, 

 and indeed very conspicuous, upon a body of such small 

 diameter, arises from the very energetic action of boracic 

 acid upon polarized light. 



SECT. 4. I have remarked that the circular crystalli- 

 zation of boracic acid is frequently entirely superseded 

 by other modes of crystalline formation ; which circum- 

 stance appears to be chiefly owing to the presence or 

 absence of combined water. Some of these variations 

 deserve to be particularly specified. 



1. Instead of circles there often occurs a formation 



