252 PROJECTION MICROSCOPE \CH. IX a 



These are used without a projection ocular. The iris in each 

 objective should be wide open for micro-projection. 



FIG. 205 D. Projection apparatus showing the metal hood for the 

 lamp. The microscope, the stage and the water bath have been 

 removed. 



A. Screw for adjusting the upper carbon along its axis. 



B. Wheel by which the lamp may be regulated by hand. It is 

 also used in putting the carbon holders in the proper position for 

 adding new carbons. 



C. Screw for adjusting the lamp side-wise. 



D. Screw for raising or lowering the lamp. 



In the door of the metal covering is a transparent window made 

 of a piece of red and a piece of blue glass. This kind of a window 

 enables one to look at the arc without hurting the eyes. 



FIG. 205 B. Half tone reproductions of the crater of the Thomp- 

 son 90 degree automatic arc lamp. The lower carbon shows only its 

 tip. In the center of the one at the left is shown a faint shadow. 

 This is in the depression of the soft core. The image of the crater 

 was thrown on the screen by a 64 mm. projection lens (B) and this 

 image was then photographed. 



400. Blackening the Apparatus. The metal parts of the en- 

 tire apparatus should be dead black to avoid the reflections. Reflec- 

 tions dazzle the eyes of the operator, and spreading over the room, 

 injure the brilliancy of the image on the screen. The hint to blacken 

 the mountings of the objectives was given by Dr. Coplin. It was 

 found by personal experience that not only the objective mountings 

 but every metal part about the apparatus should be blackened. If 

 necessary one can do this himself by using thin shellac mixed with 

 lampblack. This should be filtered through two or three layers of 

 gauze to avoid lumps. It can be put on with a soft brush like a camel's 

 hair duster. If the shellac is too thick or if not enough lampblack is 

 used the surface will be shiny. It should be a dead black. 



401. Radiant for the Projection Microscope. No light for 

 projection has been found so generally satisfactory as the electric arc 

 lamp used with the constant current. The alternating current can be 

 used, but it is very unsatisfactory. For demonstration purposes a truly 

 automatic lamp is more convenient than a hand feed lamp. In start- 

 ing the latter the carbons must be brought in contact and then slightly 

 separated. With the automatic lamp 'the separation is attended to by 



