ELEMENTARY PHOTO-MICROGRAPHY. 45 



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. 



MAGNESIUM. 



Magnesium ribbon burns at the rate of about 

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

 be fed through a tube, though the smoke from this 

 is objectionable in ordinary households, where it 

 is presumed the reader will usually work. 



If care be taken to get uniform illumination by 

 means of ground-glass, then much time is saved by 

 this light, which is rich in actinic rays, but the be- 

 ginner is not advised to try it until he has a good 

 knowledge of all the points to be observed in the 



