INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



851 



Mr. Attwood adds that instead of a clip-lens he uses a watch- 

 maker's eye-glass, fixed with a screw into a light pair of steel 

 sjiectacle frames, which he thinks the botanist, also, will find a very 

 useful arrangement. This suggestion was, however, anticipated by 

 the late Robert Brown, whose set of spectacles were some years 

 since (1874) presented to the Society by Dr. Gray. Fig. 75 shows 



Fig. 75. 



one of the spectacle frames fitted with two double-convex lenses 

 in a short brass tube forming a doublet ; one lens being broken, the 

 focus cannot now be determined. Fig. 76 shows another arrangement 

 which allows the magnifier to be turned aside when it is desired to 



Fig. 76. 



use the naked eye. Two other pairs of spectacles also accompanied 

 the preceding, with lenses of about 3 inch and 4 inch focus, tho lens in 

 one unscrewing. 



Process for Microscopical Study of very minute Crystalline 

 Grains.* — M. J. Thoulct" imbeds the particles of mineral in a cement 

 which, when set, he slices and polishes for microscoitical examination. 

 Tho mineral powder to be examined is mixed with ten times its 

 volume of oxide of zinc, and enough silicate of soda (or preferably, of 

 potasli) is added to make a thick paste. This paste is then transferred 

 to a niouM, made by laying a thin ring of ghiss upon a sheet of paper. 

 * ' Bull. SfK-.. Mill. Fninri>,' ii. (1880) p. 7. 



3 K 2 



