54 PRACTICAL PHOTOMICROGRAPHY. 



slide shutter in the usual way, then taking a square of blackened 

 cardboard in (say) the right hand and holding it close behind, 

 but not touching the substage condenser so as to shut all light 

 from the object, we open the nap with the left hand and 

 swiftly raise and lower the card in the right hand, thereafter 

 immediately closing the flap. Where the hand is unable to make 

 a sufficiently quick exposure, we rig up a photographic instan- 

 taneous shutter between light and condenser, and proceed with 

 this shutter on the same principles as we did with the card- 

 board. 



There is a fine field open to the mechanic in the designing of 

 apparatus for photo-micrography ; but the writer can only say 

 that after long and varied work with the apparatus, figured 

 No. 16, which was put together chiefly by Mr. Baker of 

 London, he is at a loss to suggest any improvement on it. 



A cell or trough of wood with plate-glass sides contains a 

 saturated solution of common alum or in certain cases a solu- 

 tion of cupric ammonio-sulphate. These solutions, or the water 

 of them, should be well boiled to drive off air, which, if the 

 boiling is omitted, rises in bubbles from the bottom when the 

 solution begins to get heated as it does with the oxyhydrogen 

 limelight. This alum cell is necessary when the limelight or 

 any other illuminating arrangement generating considerable 

 heat is used. In high-power work changes of temperature 

 have a very marked effect on the sharpness of image in the 

 negative. (See Fig. 16, 2.) 



