AREANGEMENT OF APPARATUS. 113 



bull's eye condenser is placed from J to 2 inches in 

 front of and from the flame of the lamp, the convex 

 side, if the condenser be a single plano-convex 

 lens, should face the microscope, and be centred by 

 observing the position of its image in the microscope ; 

 this will be known by its appearing as a brilliant 

 disc of light without flaw or spot, in fact a regular 

 sun, in the centre of the field of vision. 



We now remove the condenser from its tube ; this 

 is best done by racking back the objective, and un- 

 screwing the lenses from the diaphragm tube at the 

 object side of the stage, without disturbing the 

 diaphragm or any apparatus except those requiring 

 removal. If on account of their position they cannot 

 be removed in this manner; the substage tube H, is 

 unscrewed without altering the position of either the 

 diaphragm or mirror. The condenser can then 

 be easily taken out and the substage tube replaced, 

 the same precautions being observed as for its 

 removal. 



All the apparatus required are now perfectly cen- 

 tral with each other. It might be thought that 

 keeping the condenser in the substage would be 

 superfluous, but by its aid the images of the dia- 

 phragm, lamp, and bull's eye condenser are ob- 

 served through the microscope, and all made coin- 

 cident with its optic axis. Perfect accuracy of 

 adjustment in this respect, is one of the greatest aids 

 to the production of good and truthful negatives. 



The object, chosen by the rules laid down in the 

 following pages, is placed on the stage and brought 



