COLOUR SCREENS 165 



the light used to the object under examination, the colour 

 screen is not the only factor : the nature of the illuminant 

 must also be taken into consideration, as different radiants 

 have different spectra, or emit the various coloured rays 

 in different proportions. The nearest approach to sun- 

 light that can conveniently be used is the electric arc ; 

 the majority of artificial illuminants differ from it in 

 the greater proportion of green, yellow, and red rays 

 compared to the blue which they emit. That this is so 

 can very easily be seen by examining a colourless object 

 under the microscope, using a light blue screen, first with 

 daylight, and then with oil. With daylight it appears 

 blue, with oil-light green, owing to the excess of yellow 

 over blue rays in the radiant. Similarly a double- or 

 triple -stained specimen has an entirely different appear- 

 ance when examined with artificial light and daylight. 

 Consequently it is essential to treat the illuminant and 

 screen together as a complete light source : anj r change in 

 the illuminant necessitates a complementary change in 

 the screen if the same result is to be obtained. 



Colour Sensitive Plates. The examples given of the 

 use of colour screens introduce the colour sensitiveness 

 of the plate as a factor in the case, as it is obviously useless 

 to attempt to photograph with red light on a plate that 

 is only sensitive to blue. If a red screen is required to get 

 the best results from a certain preparation, it may be 

 possible to get a better photograph on a yellow sensitive 

 plate with the screen in position than can be obtained 

 without the screen, but the full advantage offered by the 

 red screen cannot be made use of unless a red sensitive 

 plate is employed. If taken on an orthochromatic plate 

 the exposure will be much lengthened, and is really taken 

 by the blue and green light that passes the screen, and 

 not by the red at all. 



Plates are broadly divided into four classes according 

 to their colour sensitiveness. 



