THE MICROSCOPE, ETC. 31 



top combination when using the eighth and higher 

 powers ; it not only prevents the passage of any rays 

 except those actually concerned in the direct illu- 

 mination of the object and the subsequent formation 

 of the image on the focusing screen, but also if 

 brought into view with a low power objective enables 

 us to make the optic axis of the condenser coincident 

 with that of the objective, and therefore with any 

 other substituted for it. This will be more fully ex- 

 plained when treating of the arrangement of the 

 microscope prior to taking a photograph. 

 This pin hole cap is easily made as follows. 

 Procure a piece of tubing, that shall fit the top of 

 the condenser accurately, lead will do, but brass is 

 to be preferred. Cut it so that it projects slightly 

 above the surfaces of combination, and while in this 

 position press a piece of sheet zinc, previously cut to 

 fit it, down into the tube till it just touches the glass. 

 Having filed down the part of the tube projecting 

 above the zinc, we find the centre of the latter and 

 pierce it with a hole about the diameter of an ordin- 

 ary small pin. The cap is now blackened both ex- 

 ternally and internally, with oxide of copper or any 

 other material giving a dull black. 



It has been found that the nearer the diaphragm 

 approaches the back combination of the condenser, 

 the more brilliant the illumination. The following 

 arrangement which enables us to place it in immedi- 

 ate contact, besides possessing several advantages in 

 practice, lessens the spherical aberration and improves 

 the illumination, though it decreases the angle of 

 aperture. 



