1>44 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



in the vertical plane ; thus four exposures could he made on the same 

 plate, and therefore an accurate comparison was possible between them. 

 In the figure A is the spectroscope ; B the back of the camera, carrying 

 screen and plate-holder ; C, supports upon which the back B may be 

 moved ; D, shutter ; E, colour-screen in colour-screen-holder. The 

 plates examined were grouped, according to their degree of perfection, 

 as follows : — 



Group i. — Characterised by a very high degree of sensitiveness, a 

 little above the line D, falling off abruptly on either end, and only 

 slightly sensitive to greens and blues. 



Group ii. — Characterised by two distinct maxima, one a little above 

 the D line, and the other in the blue-green. Between these two maxima 

 the sensitiveness falls very considerably. 



A BC 



Fig. 53. 1 



Group iii. — Characterised by having its maximum sensitiveness in 

 the blue (as with ordinary plates), with lesser bands of sensitiveness 

 extending below the D line. 



Group iv. — Characterised by bands of sensitiveness extending below 

 line D, with greatest intensity in the yellow-green, and falling off at 

 the violet end before H 2 . 



Group v. — This group most nearly approaches perfection. It is 

 characterised by a sensitive band well below line D, and somewhat 

 below the red end of Groups iii. and iv. This plate gives an almost 

 uuiform degree of sensitiveness with a maximum intensity in the green. 



In fig. 58 the upper curve shows the visual intensity of the spec- 

 trum (from Fraunhofer). Curves i.-v. represent the photographic in- 

 tensity of the spectrum taken on plates corresponding to the groups of 

 the same number. Group i., Cramer isochromatic (slow) ; Group ii., the 



