PRACTICAL PHOTO-MICROGRAPHY. 



require to be racked down more or less. The use of the bull's- 

 eye is so important that diagrams and quotations from papers 

 by Mr. E. M. Kelson, in the English Mechanic >, 1884, shall be 

 given in explanation. In the first place the light must be in 

 the focus of the bull's-eye, and the latter is to be fixed so that 

 the edge of the flame if a wick is used, or the surface of the 

 lime if the limelight is used, is in the focus of the bull's-eye. 

 In order to ascertain whether the bull's-eye and light are in 

 proper relation to each other, Mr. Nelson recommends either 

 that the eye be placed in the rays proceeding from the bull's- 

 eye, or that a condensing lens be placed in the rays and the 

 image thrown upon a white card there examined. 



In fig. 28, E represents the edge of the flame, P the bull's- 

 eye and A the image as it ought to be seen on the card. 



6 



FIG. 28. 



B represents the appearance when the bull's eye is too near 

 the edge of the flame. 



C represents the appearance when the bull's eye is too far 

 away from the flame. 



D shows the appearance when the bull's eye is focused, but 

 out of centre. 



In order to get the proper use of the bull's-eye, it should not 

 be nearer to the substage than a distance of, say, 12 inches. 



The same papers by Mr. Nelson contain information regard- 

 ing the substage condenser, so useful that the author ventures 

 to copy some further figures. 



