NATURAL PHILOSOPHY. 145 



Diamond in Ilunterian Museum, Glasgow . 3'53 Thomson.* 



Specific gravity, aa stated by Mohs . . 3'52 Mohs.f 



Brazilian diamond 3-44 } Brisson 1 



Another variety of the same . . . . 3 - 52j 



Mean specific gravity of a " beautiful collection ) o..„ Lowrv S 

 ofdia ...... $ '"* 



••Star of the south" 3-53 { g-fjgQ"* 



Borneo diamond 3"49 Grailich.*j 



compact .... 3 P 41 



■25 j 



Rivot. 



Diamond used in Jacquelain's experiments . 3"33 Jacquelain.tt 



Specific gravity, as given by Henry . . . 3'55 Henry.Jt 



Well-orystaUized Brazilian diamond, weighing^ „.,„ -pi *• m 



0-57(31 gramme, in the Edinburgh Museum . j riajlair.fcfc 



Mean sp. gr. . ■ 3 - 461 



If we reject the second Borneo diamond of Rivot, which has too 

 low a specific gravity, we have a mean sp. gr. of 3*48, which is the 

 same number as that found by Wilson Lowry for the mean specific 

 gravity of "his beautiful collection of crystallized diamonds" 

 (Thomson's Mineralogy, vol. i. p. 46). 



It is to be expected that the experimental determination of the 

 specific gravity of diamonds should be rather above than below the 

 truth ; for we are aware that they all leave a minute quantity of ash 

 on burning, and that this ash, according to Petzhold, contains silica 

 and iron. — Dr. Lyon Play f air ; Proceedings of the Royal Society of 

 Edinburgh. 



MICROSCOPIC VISION, AND NEW MICROSCOPE. 



Sir David Brewster has read to the British Association a paper 

 on Microscopic Vision and a New Form of the Microscope. In this 

 the worthy Principal again, after an interval of more than a quarter 

 of a century, and notwithstanding all the disappointments which 

 have intervened, recommends gems as material for lenses instead of 

 glass. He objects even on the ground of truthfulness to object- 

 glasses with large apertures, and suni3 up thus the other improve- 

 ments which he suggests : — 1. The first step, we conceive, is, to 

 abandon large angular apertures, and to use object-glasses of mode- 

 rate focal length, obtaining at the eye-glass any additional magnify- 

 ing power that may be required. 2. In order to obtain a better 

 illumination, either by light incident vertically or obliquely, a new 

 form of the microscope would be advantageous. In place of direct- 

 ing the microscope to the object itself, placed as it now is almost 

 touching the object-glass, let it be directed to an image of the object, 



* Thomson's Mineralogy, vol. i.p. 46. 



t Mohs' Mineralogy, vol. ii. p. 306. 



j Brisson, as quoted by Bottger, Specifizche Oewicht., p. 32. 



§ Lowry, as quoted in Thomson's Mineralogy , vol. i. p. 46. 



fi Dufrenoy, Comptet Rendus, vol. xl. p. 3. 



•f Grailieh, Ball. Geo!. [2], vol. xiii. p. 542. 

 •• Kivot, Ann. des Mines, vol. xiv. p. 423. 

 tt Jacquelain, Ann. de Ck. et Phyt. [2], vol. xx. p. 459. 

 XX Henry's Mineralogy, vol. iv. p. 19. 

 §§ Experiment made lor this paper. 

 K 



