116 ARRANGEMENT OF APPARATUS. 



stand. The focussing glass previously adjusted, is 

 applied to that part of the image on the ground 

 glass, requiring the most accurate definition. The 

 best focus is obtained by observing the appearance 

 of the structure through the focussing glass during 

 the revolution of the milled head attached to the rod 

 connected with the fine adjustment. 



The chemical focus is found when the object 

 appears clearly defined, although its markings are 

 surrounded by a red areola, (see previous pages'). A 

 black cloth is now hung over camera and stand, but 

 not reaching lower than the stage of the microscope ; 

 the blue glass, sulphate of copper cell, or plate of 

 Alum placed between the bull's eye condenser and the 

 mirror ; and an opaque card between the two latter, 

 to enable the exposure to be given at the proper time. 



We now proceed to the dark room, and having lit 

 the lamp, observe the condition of the trays and 

 measures, their position, and that of the various 

 bottles containing solutions. 



The barrel should contain a good supply of fresh 

 and pure water; the tin box containing the dry 

 plates is placed on the floor or a shelf, on the oppo- 

 site side of the dark room, to that occupied by the 

 washing trough and lamp. 



We recommend the use of dry plates twice as fast 

 as ordinary collodion in this case, as the intensity 

 of the light is insufficient and the position of the 

 camera unsuitable for wet plates. 



The dark slide is examined ; the side on which the 



