MICROSCOPE, ETC. 



31 



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 follow- 

 ing arrangement which enables us to place it in 

 immediate contact, besides possessing several advan- 

 tages in practice, lessens the spherical aberration 

 and improves the illumination, though it decreases 

 the angle of aperture. 



To the largest hole of the diaphragm a short tube, 1, 

 fig. 16, lined with cloth is attached, another tube 2 of 

 sufficient length to reach the under surface of the 

 condenser, slides within it, at the upper extremity 

 of the inner tube and a short distance from its ori- 



