364 It Dr. Brewster^s Memoirs. 



moved by correcting the primitive forms of the minerals 

 which then stood as exceptions. 



The present paper is principally occupied by a compar- 

 ison of Haiiy's Primitive forms with the fundamental forms 

 of Mobs and the Optical System of Dr. Brewster, 



The following Table will exhibit the results of this com- 

 parison. 



Haiiy's Primitive > Moh^' Fundamen- Brewster's Opti^ 



Forms. 5 tal Forms. cal System. 



Rhomboid ^ ^ 



Reg. Hexahed. | I 



Prism )-Rhomboidal System | Crystals with 



Bipyramid. Dode- I )»one axis of 



cahed. J Double Refrac- 



Octohedron ) Base „ • j i o ^ tion. 



T) I i D • ? ryramidal System 



lti2;ht rnsm ) square •' •' ^ 



Right Prism-Base"^ 



not square f Crystals with 



Oblique Prism I Prismatic System .' two axes of 



Octohedron-Base ,' "^i Double Refrac- 



a rectangle | ^ tion. 



or rhomb. J 



f Crystals with three 



Cube \ I rectangular axes in 



Reg. Octohedron I Tessular Sys- J a state of equilib- 



Rhomb. Dodeca- ,' tem ] rium and therefore 



hed. J I producing no Doub- 



l^le Refraction. 



3. On the distribution of the colouring matter and on cer' 

 tain peculiarities in the structure and optical properties of the 

 Brazilian Topaz. (Trans. P. S. Camb.) — The first part 

 of this paper, on the distribution of the colouring matter, we 

 could hardly render intelligible without the aid of the very 

 beautiful accompanying figures. The colours are distribu- 

 ted with great delicacy, and are all referable to the red, 

 yellow and blue rays, with the exception of two specimens 

 which exhibited the green rays. The crystals in almost 

 all Dr. B's specimens are tesselated in a peculiar manner, 

 not hemitrope or turned half way round or any determinate 

 portion of a circle, hut polytrope, the principal sections of 

 different laminae forming different angles with one another. 



