200 



THE PROJECTION MICROSCOPE 



[Ch. VI 



(8) For all but the highest powers a substage condenser is not 

 needed; and one can light objects up to 50 or 60 mm. in diameter if 

 the object is placed in the right position in the cone of light (fig. 124). 



(9) For objectives of higher power than 4 mm. a substage con- 

 denser should be used, and if an ocular is used as well as an objective 

 then the substage condenser is advantageous for powers above 8 mm. 

 equivalent focus. For lighting see § 104, 107, 352. 



Fig. i2i. Illuminating Objects of Various Sizes in Micro-projection 

 with the Main Condenser Only. 



(From Optic Projection). 



The object must be put in the cone of light at a point where it will be fully- 

 illuminated. 



For high powers it will be at or very near the focus (/). For larger objects and 

 low powers at 2 or 3, or even closer to the condenser face. 



Arc Supply The right-angled carbons of the arc lamp. 



L\ Lo The first and second elements of the triple condenser. 



Water Cell The water cell for absorbing radiant heat. It is in the parallel 

 beam between the first and second elements of the condenser. 



Axis The principal optic axis on which all the parts are centered. 



(10) One of the most important points is to have a very white 

 screen. A cloth or wall screen painted with Artist's Scenic White 

 gives a very perfect screen which does not yellow with age, and its 

 primitive whiteness is restored by an occasional coat of fresh white. 

 Semi-mirror screens are successful only in narrow rooms. 



For short screen distances (1 or 2 meter screen distances) white 

 cardboard or a sheet of very white bristol board gives excellent 

 results. 



The apparatus, in contrast to the screen, should be dull black. 



§ 312a. Masks for demonstration slides. — The paper to use should allow 

 the red and orange to pass, but cut off the green-blue end of the spectrum. An 



