ON PHOTO-MICROGRAPHY. 191 



cell is then removed, a photograph is taken, and if 

 it shows any want of sharpness, the cell is again 

 interposed, and by means of the fine adjustment 

 screw, the object glass is approached a little nearer 

 to the object, until the image as seen with the eye 

 is about as indistinct as that on the photograph. 

 Another plate is then exposed and developed, and 

 will probably be quite sharp. If not, another slight 

 alteration of focus must be made, and the process 

 repeated if necessary, until at last the requisite 

 sharpness is obtained. It should be then noticed 

 how much the fine adjustment screw has to be 

 turned, to bring back the proper visual focus, when 

 the copper cell is used, and this amount should be 

 recorded and applied as a constant correction when- 

 ever the lens in question is used. 



So much for apparatus. Now we must say a few 

 words about the photographic processes employed. 

 Gelatine dry plates are the best suited to this work 

 on account of their rapidity, the little apparatus 

 required, the cleanliness of the process of develop- 

 ment, and their being always ready for use. There 

 are many excellent dry plates of various degrees of 

 rapidity, and still more various prices in the market, 

 and it is very much better to buy them than to 

 make them for oneself. A rapid plate is most 

 suitable, but there is no occasion for excessive 

 rapidity. Gelatine plates must be exposed to none 

 but deep red light, and the less of this they are 

 exposed to the better. When unpacked they should 

 be stored in grooved light-tight boxes. The smallest 

 ray of white light finding access to the dark room 

 will be fatal to success. If the operator works by 

 night only, he will not need a special dark room. 

 Any ordinary room with a closely fitting door, to 



