714 



SUMMARY OF CUERENT RESEARCHES RELATING TO 



Fig. 154, 



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A (fig. 154) shows the effect produced by centering or placing the 

 edge of a flame (from 1/2-in. paraffin wick) in the exact focus of a 

 plano-convex bull's-eye P. 



It is necessary to explain the meaning of the word " effect," for if 



a piece of card were held in 

 the rays proceeding from P, 

 the picture as shown at A 

 would not be seen ; but, in- 

 stead of it, an enlarged and 

 inverted image of the edge 

 of the flame. Then, one will 

 naturally ask, How do you 

 get the picture A ? By 

 simply putting your eye in 

 the rays and looking at the 

 bull's-eye. 



As this is often disa- 

 greeable, by reason of the 

 strength of the light, a more 

 pleasant way of examining 

 the picture is by placing in 

 the rays a condensing lens 

 (the field-glass of a 2-in. eye-piece) and focusing the image on a card. 

 It should be noticed particularly that the diameter of the disk A 

 depends on the diameter of the bull's-eye P ; but the intensity of the 

 light in A on its focal length. The shorter the focus the more 

 intense the light. In making these experiments the condensing lens 

 is presumed to be at a fixed distance from the bull's-eye P. 



B represents the picture when the edge of the flame E is centered, 

 but within the focus of P. 



C the picture when E is centered, but without the focus of P. 



D the picture when E is focused, but not centered. 



Fig. 155 shows an error often perpetrated, viz. that of putting a 



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concave mirror C at the back of a bull's-eye P, to increase its effect. 

 The rays are brought to a focus and then scattered. 



The method of obtaining a critical image with transmitted light 

 by objectives of 1/2-in. focus and less is shown at fig. 156, where E 

 is the edge of the flame from a 1/2-in, paraffin wick, S substage 

 condenser, and P the object, S is centered to, and the image E 

 focused by S on, P. Fig. 157 shows the same thing with the addition 



