ELEMENTARY PHOTO-MICROGRAPHY. 49 



the bull's-eye spindle, so that it can be removed 

 and replaced in exactly the same position, to 

 facilitate the occasional use of the lamp flame 

 without any bull's-eye. For instance, in getting 

 " critical light," after the objective has been 

 focussed on the slide, an image of the edge of lamp 

 flame must also be focussed through the substage 

 condenser on the object to be photographed, and 

 to do this the bull's-eye is taken away, but is 

 afterwards interposed when the edge of the flame 

 has been shown as a streak of light in the centre 

 of field. 



A plano-convex lens should be so placed that 

 parallel rays either enter the convex surface or 

 emerge from it, and its focus is approximately the 

 diameter of curvature, or twice the radius of its 

 convex surface. 



One advantage of an oil lamp is the absence of 

 that intense heat which gives the user of limelight 

 so much trouble. 



When electric current is obtainable, the Nernst 

 lamp is very handy and efficient, or with a gas 

 supply the incandescent mantle can be usefully 

 employed. Both these lights reduce the exposure 

 considerably. 



MAGNESIUM. 



Magnesium ribbon burns at the rate of about 

 2ft. per minute. It is purchased in coils, and can 



