i58 THE AMERICAN MONTHLY [May 



In the centre of the broad end of the shutter a piece of 

 brass tube with an aperture of about 3-16ths of an inch 

 is screwed, and this tube when the shutter is in position 

 is exactly opposite the optical axis of the microscope. 

 This adjustment is easily made by means of a low power 

 object glass and eye-piece before the shutter is fixed, and 

 when this is done in my case with a fixed microscope, all 

 work is ended. Now, when about to take a photo-micro- 

 graph, the microscope and camera are moved along the 

 base-board, with the lamp behind the one-inch aperture, 

 the shutter being swung aside on its nail out of the way, 

 until with the object glass used one is satisfied with the 

 magnification and has obtained a sharp focus. The front 

 of the camera is then pinched firmly down and so is the 

 board on which the microscope is carried. The focus is 

 again examined to see that all is right, a plate is put in 

 the dark slide, this end of the camera covered with a 

 dark cloth, the lamp removed and the small shutter is 

 turned into position, the shutter of the dark slide is with- 

 drawn, and we are ready for an exposure. I do not turn 

 down the lights in the room. Suppose I am using a Zeiss 

 35 mm. apochromatic as the objective with a camera ex- 

 tention such as to get 30 inches between the object and 

 the sensitive plate, I push about three-quarters of an inch 

 of magnesium ribbon through the aperture in the small 

 brass tube in the shutter and set fire to the ribbon by 

 means of a match, and this three-fourths of an inch of 

 magnesium ribbon, which will barely take three seconds 

 to burn, will give me a dense negative. I use no con- 

 densers with low powers. With this illuminant they are 

 not necessary. The light proceeds from a small point 

 and the rays reaching the object are nearly parallel. Sir 

 Henry Roscoe, in his book on ''Spectrum Analysis," states 

 that this light is rich in purple and violet light, the very 

 qualities which affect the sensitive plate most. I have 

 used magnesium now for over ten years, and have con 



