Mr Pritchard on forming Diamond Lenses. 1 47 



1. To examine the right and left handed crystals of quartz. 

 % To examine the structure of amethyst^ and compare it 

 with that of quartz, 



3. To examine the pyramidal sulphate of potash, and com- 

 pare it with the uniaxal sulphate. 



4. To examine sulphate of lime and glauberite at different 

 temperatures, so as to determine if the axes of elasticity change 

 while these crystals pass from their biaxal to their uniaxal state. 



5.' To examine plates of rapidly cooled glass possessing re- 

 gular axes of double refraction. 



6. To examine apophyllife, analcime, and other crystals in 

 which the doubly refracting structure is so singularly dis- 

 tributed. 



7. To examine crystals such as ice, or in which no clea- 

 vage planes have been discovered, and compare them with crys- 

 tals having the same doubly refracting structure, but posses- 

 sing regular cleavage planes. 



8. To examine the two classes of pyramidal crystals, in one 

 of which the double refraction is negative, and in the other 

 positive, — properties which seem to be related to the existence 

 or non-existence of cleavage parallel to the base of the pyramid. 



Art. XXII. — On the Art of forming Diamonds into Single 

 Lenses for Microscopes. By Andrew Pritchard. Com- 

 municated by C. R. Goring, M. D. 



Of the various improvements in microscopes originated by Dr 

 Goring, that which he conceives to be the most important is 

 the construction of single magnifiers from adamant. The de- 

 tails relative to this novel class of instruments I have been in- 

 duced to lay before the public. Single microscopes, naturally 

 aplanatic, or at least sufficiently so for practical purposes, pos- 

 sess an incontestable superiority over all others, and must be 

 recognized by the scientific as verging towards the ultimatum of 

 improvement in magnifying glasses. The advantages obtained 

 by the most improved compound engiscopes over single micros- 

 copes, resolve themselves into the attainment of vision without 

 aberration, with considerable angles of aperture ; but against 

 this must be set the never-to-be-forgotten fact, that they only 



