28 THE GENERAL PRINCIPLES OF THE METHOD 



for use, the Nernst lamp being kept lighted as well as 

 the gas one during any experimentation in order that 

 a photograph can be taken at a moment's notice. Of 

 course, so long as the mirror is in its usual position no 

 light reaches the specimen from the Nernst lamp; 

 swing the mirror out of its position, and the light is 

 instantly changed from that of the gas-burner to the 

 powerful one from the Nernst burner. The distances 

 between the Nernst lamp, aplanatic condenser, and the 

 substage condenser, are of great importance. It must 

 be determined at the outset by trials which distances 

 give the best results. The presence of the water-tank 

 renders it difficult to make a rule. 



When a cell or other object comes under observa- 

 tion which it is desirable to photograph, the working 

 eye-piece is removed from the microscope draw-tube; 

 the camera is allowed to slide down the beam until its 

 shutter is about an inch from the mouth of the draw- 

 tube, when it is clamped to fix its position. The 

 projection eye-piece, which is already attached to the 

 camera-shutter by means of the flexible velvet collar, 

 is inserted into the microscope draw-tube. The mirror 

 is now swung on its gimbals out of the focal axis, 

 thus allowing the light from the 300-candle-power 

 Nernst burner to replace that of the gas-burner; and 

 the former, after being cooled by transmission through 

 the intervening water-trough, is projected directly 

 through the two condensers. The image of the field 

 of the specimen will then be seen on the ground- 

 glass screen at the back of the camera, where it can 

 be rapidly focused. 



