SCKEENS PLATES EXPOSUEE 219 



performed. After coating they keep for perhaps a week or so, 

 and give extremely good results if the exposure is correctly 

 timed. 



For negative-making, however, it will almost invariably 

 happen that gelatin plates will be chosen, and the description 

 of collodion processes will therefore be left until the making 

 of lantern-slides is considered, since it is particularly for 

 the latter that collodion is now best suited and most largely 

 employed commercially. 



Gelatin plates, as sent out by the makers, usually have 

 some name attached to them, which is supposed to indicate 

 their speed, although such names rarely convey any definite 

 information to a purchaser. Formerly their speeds were 

 estimated in reference to the speed of a wet collodion plate ; 

 but latterly a more exact method of determining the speed 

 has been employed, and although it is still open to discussion 

 whether the description is strictly accurate, it is at least 

 sufficiently so for the purpose in view. 



For photo-micrography a plate of medium or slow speed 

 will be found the most satisfactory, since if properly developed, 

 the grain of the image will be as fine as it is possible to get it in 

 a gelatin plate. 



Gelatin plates are sometimes described as ' ordinary,' 

 ' special rapid,' or 'extra rapid.' Such plates are usually most 

 sensitive to the blue and violet end of the spectrum that is, the 

 visually brightest part of the spectrum does not have the 

 greatest photographic action on them. Therefore, in photo- 

 graphing preparations in which yellow, green, or red predomi- 

 nate, or in cases where a yellow or green colour-screen is used, 

 an ordinary plate would require abnormally long exposure, 

 and even then the gradation of tone obtained would not 

 faithfully represent the object. To overcome this inequality 

 of sensitiveness, plates are now bathed in a suitable aniline 

 dye, or the dye is incorporated with the emulsion in the process 

 of the manufacture of the plate. The result is that plates may 

 be obtained more highly sensitive to the yellow and green 

 region, in which case they are generally referred to as ' isochro- 

 matic.' They may be obtained still further sensitised for 

 practically the whole of the visible spectrum, and are then 

 described as ' panchromatic,' 



