282 PBACTICAL PHOTO-MICBOGKAPHY 



of aesculin ; 0'5 c.c. of solution A is then added, 1 c.c. of 

 solution B, and 12 c.c. of a 10-per-cent. gelatin solution 

 in distilled water. The whole is made up to 25 c.c. with 

 distilled water, filtered hot, and flowed over the glass in the 

 proportion of 5 c.c. per square decimetre of surface. 



These screens are suitable with slight variations for either 

 the Autochrome, Thames, or Dufay plates, but may require some 

 slight alteration to suit the particular illuminant in use. The 

 Autochrome plate is of much finer grain than any other at present 

 on the market ; but, fine as it is, lantern-slides made by this 

 process still show, when projected on the screen, some evidence of 

 granularity. The Thames plate is of coarser grain ; but as in the 

 Autochrome plates the starch-grains of a particular colour tend 

 to form themselves into groups, the actual advantage of the 

 Autochrome method over the Thames-plate method is from 

 this point of view not perhaps so great in practice as might 

 be expected from theoretical considerations. The Thames or 

 Dufay plate certainly has somewhat greater transparency ; but 

 with either method, when projecting the colour positives, an 

 illuminant must be used of high power, otherwise a brilliant 

 image on the screen cannot be obtained. At present no 

 method is available commercially for producing prints in colour, 

 the lantern-slide or larger positive on glass being all that is 

 practicable. 



