ARRANGEMENT OF APPARATUS; ETC. 109 



of a ring B, fig. 25, is substituted for the tube fitting 

 into the substage aperture of the microscope. About 



an eighth of an inch from its upper edge, we place a 

 cardboard disc with a slit C in it; all the interior of 

 the tube is blackened except a narrow strip, corres- 

 ponding in size and position to the slit. This part 

 of the ring should be flattened. An opening D is 

 made in the front, to enable the angle at which the 

 light falls on the object to be changed. The ring B 

 is covered on its outside with black cloth, so that it 

 may slip easily up and down in the aperture, in the 

 centre of the stage. 



The light is concentrated by the bull's eye on the 

 narrow strip corresponding to the slit, and from 

 thence on the object. 



The next method is the only one possible, when it 

 is desired to photograph objects in fluid, with arti- 



