134 THE AMERICAN MONTHLY [Aug 



of thiGhnesses of box cloth, and a screw working through 

 two eyelet holes, as shown in Fig. BB, may he used; this 

 slipped round the fine-adjustment and tightened up and 

 connected with the long brass handle will enable fine 

 focussing to be perfected. A -is the box-cloth and BB 

 the tongues with screw. 



The question as to whether an eye-piece should be 

 used or not depends to a great extent upon individual 

 taste, but it has this great advantage, that when an eye- 

 piece is used, the extension of the camera is considerably 

 less for any given magnification, in comparison with that 

 required when no eye-piece is used. The eye-pieces in 

 general use are the Huyghens, Eamsden, orthoscopic, 

 compensating and projection oculars. Eor those who 

 intend to do really good work in photo-micrography, the 

 projection oculars should he obtained. They are made 

 by several firms, Eeichert, Zeiss, Swift, Powell and Lea- 

 land, and Beck, and usually they are made-iu four sizes, 

 2 and 4 for Continental tube length, and 3 and 6 for 

 English tube length. 



Ordinary ground glass as supplied by camera makers 

 is utterly useless for focussing upon. It should be re- 

 placed by a dry plate treated as follows : — Place the dry 

 plate without exposure to light in a clean solution of 

 hypo.(l : 4), allow it to remain for fifteen minutes, then 

 .wash thoroughly for an hour in running water, allow it 

 to drain, and immerse for ten minutes in 5 per cent, sol- 

 ution of sulphuric acid. Finally rinse and immerse for 

 the same time in 2^ per cent, solution of barium chloride, 

 and wash well and dry. This treatment precipitates a 

 very fine deposit of barium sulphate in the gelatin which 

 is easy to focus on. Instead of this, a sheet of plate glass 

 about ith inch thick may be used, on one surface of 

 which, (that nearer to the microscope), fine lines crossing 

 each other at right angles in about half inch squares 

 have been ruled with a diamond. When an eye-piece is 



