ARRANGEMENT OP APPARATUS. 119 



Now if uncertain as to the proper amount of ex- 

 posure required, or desirous of photographing an- 

 other portion of the object ; we remove the card, 

 re-examine the image, and correct the focus if neces- 

 sary. The card being replaced, the dark slide is 

 again substituted for the ground glass, care being 

 taken that the plate farthest from our person while 

 the slide was in the pocket, is placed facing the 

 microscope ; the sliding door is withdrawn ; a slight 

 rest allowed, and the requisite exposure given, the 

 same precautions being observed. 



Having exposed the required number of plates, 

 we carry them to the dark room, and either put 

 them carefully away till a more convenient time, or 

 proceed at once to develope by one of the processes 

 suitable for dry plates ; preference being given to 

 the ferrous oxalate, on account of its giving perfect 

 results, where the time of exposure, fastness of the 

 plate, and intensity of the light are previously 

 known. 



With the same arrangement, it is quite possible 

 to photograph opaque objects, and those illuminated 

 by dark ground or oblique illumination. Wenham's 

 parabolic illuminator, a spot lens, or what is the same 

 thing, a central stop in the condenser, are all used 

 for the latter purpose. We, however, prefer the 

 following simple arrangement. 



A piece of electro-plated copper, bent in the form 

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

 into the substage of the microscope. About an 

 eighth of an inch from its upper edge, a carboard 



