CH.IXa] PROJECTION MICKOSCOPE 257 



core seems to furnish a kind of guide. In some lamps the two carbons 

 are of equal size while in others they differ in diameter. In such 

 cases one must learn from the makers which carbon is to be positive 

 and which negative. In the lamp shown in Fig. 204, B, C, D, (A. T. 

 Thompson & Co.'s), the upper or positive carbon is the smaller (i. e. 

 upper carbon, n mm.; lower carbon 13 mm. in diameter). 



406. How to Make the Upper Carbon Positive. In most 

 cases one does not know which of the wires in a circuit is positive and 

 which negative. The simplest way to proceed is to make the connec- 

 tions as shown in Fig. 206 without regard to which is positive and 

 which negative. When all is complete turn on the current and allow 

 it to pass through the lamp for a few moments and then open the 

 switch. The carbons will glow for some time and the positive one will 

 be much brighter than the negative one. If the upper carbon is the 

 bright one then the current passes through the lamp in the right direc- 

 tion, but if the lower carbon is the bright one then the current passes 

 through the lamp in the wrong direction and the wires must be 

 changed. That is, in the diagram, the wire marked ( 2) is in place 

 of the one marked (+ i). Very often the simplest way to make the 

 correction is to change the position of the wires passing from the bind- 

 ing posts of the switch to the lamp. If one uses a plug to connect 

 with the main line all that is necessary is to turn it the other side up if 

 the lower carbon is positive, or if the plug is non-reversible then the 

 wires must be reversed. It is well to mark the wires in some way after 

 one determines which is positive and which negative. 



407. Stage of the Projection Microscope. This should be 

 large (about 10 x 12 cm.) and independent of the rest of the appa- 

 ratus. It should be freely movable toward or from the lamp conden- 

 ser so that objects of various sizes may be completely illuminated (see 

 Fig. 209). It should also be adjustable vertically so that it may be 

 centered with the other pieces of apparatus (Fig. 208 C.). There 

 should be available an attachable mechanical stage with a large range 

 of movement both vertically and horizontally (Figs, 67 69, 204 B. 

 and D.). A specimen cooler in addition to the large water bath is 

 also desirable (Fig. 204 B, C, D, Fig. 208 G.). 



408. Openings in the Stage. For the projections desirable 

 in a modern laboratory of Histology and Embryology it is necessary 

 to be able to project objects varying greatly in size, i. e. from less 

 than i millimeter to 50 or more millimeters. In order to accomplish 



