224 PHOTOMICROGRAPHY 



sure correct development. There are many excellent tanks 

 on the market, such as those of Griffin, Houghton, Kodak, 

 and Watkins,but a covered enamelled iron box with grooves 

 down each side to hold the plates may be used instead. 



Factorial Development. When it is possible to watch 

 the development of a plate, the rapidity with which the 

 image appears is a valuable indication of the correctness 

 of the exposure given. Over-exposure is followed by a 

 very quick appearance, while under-exposure determines 

 a slow appearance with a hanging back of the dark parts 

 of the object. Mr. Watkins has found that if the time 

 required for the first appearance of the salient part of 

 any image is noted, the best result in regard to density 

 and contrast will be obtained if the plate is kept in the 

 developer for a fixed multiple of that time ; the multiple 

 depending on the developer and type of negative required, 

 and being independent of the temperature. Factors are 

 given with each developer in the appendix ; they are 

 correct for the majority of photomicrographs, but may 

 require to be increased or decreased at the judgment of 

 the operator if a more or less contrasty negative than 

 usual is desired. The method is a great favourite with 

 many workers, but is not easily carried out with colour- 

 sensitive plates on account of the difficulty of seeing the 

 first appearance of the image. 



Fixing and Washing. It is hardly necessary to insist 

 that fixation be complete as indicated by the absence of 

 any trace of white, such as is seen on looking at the back 

 of imperfectly fixed plates. It is advisable to rinse 

 plates after removal from developer and before placing 

 in the fixing bath, particularly with pyro, otherwise the 

 fixing bath rapidly becomes discoloured and may stain. 

 To prevent any discolouration and stain an acid fixing 

 bath is advantageous. Potassium meta-bisulphite is 

 generally mixed with the hypo for this purpose. A 

 clearing bath is useful for brightening negatives and 



