350 THE AMERICAN MONTHLY [Nov., 



Several devices have been suggested for tlie conveni- 

 ence of those microscopists that do not possess a me- 

 chanical stage. These consist of lines photographed or 

 ruled on a glass slip, their number usually being great 

 and the spaces between them small, each of the latter 

 bearing one or more figures. The best known of these 

 finders is Maltwood's, a glass plate bearing twenty-five 

 hundred squares, so numbered that the position of an ob- 

 ject may be recorded by recording the numbers within 

 the space over which it may be when in the field of a 

 certain objective. 



The object is brought into the centre of the field of 

 view, when the slide is removed and the Maltwood finder 

 substituted. The numbers on the square now occupying 

 the position previously occupied by the object are noted, 

 and whenever this special square is again brought into 

 the field of that objective, the stage will be in a position 

 to bring at once the desired object into that field. 



EDITORIAL. 



Sawing Rock Sections. — The Geological Survey in Wash- 

 ington uses a band saw which consists of an endless steel wire I 

 incli thick running at a high rate of speed over two fl}^ wheels. 

 Water and emery are fed upon the wire when in motion and the 

 hardest rocks yield to it. 



With grinding machines sections one-thousandth of an inch 

 are made and they are of course perfectly translucent, under 

 proper illumination. After one side has been ground flat, a 

 stone is cemented to a piece of glass by means of Canada bal- 

 sam. A slice can then be sawn off and ground down to within 

 the least possible distance from the glass. It is then removed 

 from the glass by softening the Canada balsam. Mineralogists 

 now regard this method of identifying rocks as favorably as the 

 chemical method in the measurement of crystals. 



The Ruby. — A pure, limpid, fiery red piece of corundum is 

 called a ruby. It crystalizes in many different shapes. By 

 submitting it to polarized light its structure is seen to be very 



