PLATE 17. 



Colour Screens for control of contrast and improvement 

 of definition. 



A, B, and C STEM OF Clematis vitalba. Transverse Section x 28 



Stained by immersion in picro-aniline blue. The lignified cell walls 

 are coloured yellow (Y), and the cellulose walls blue (B). 



Objective Swift 1". 



Ocular Swift No. 2. 



Plates Imperial Special Sensitive Ortho. 



Light Incandescent gas. 



A No screen. Exposure 1 second. The yellow appears darker 

 than the blue. 



B Wrattcn's G (blue-violet) screen. Exposure 10 seconds. Screen 

 nearly complementary to the yellow stain. The contrasts are 

 increased, the yellow becomes darker and the blue lighter. 



C Wrat ten's B and E screens, giving monochromatic yellow- 

 green light of wave length 5600-6000 Exposure 10 minutes. 



This figure gives a fairly correct representation of the visual 

 appearance of the section, the blue being darker than the yellow, 

 with a notable improvement in definition and detail, due to the use 

 of monochromatic light of wave length suited to the corrections of 

 the objective. 



D LEAF OF Cycas revoluta. Transverse Section x 25, stained red 

 and blue. 



Objective Leitz No. 3 Achromat. 



Ocular ... ' ... Leitz No. 2. 

 Condenser ... Abbe. 

 Illuminant ... Incandescent gas. 



Screen 1 Left side, without screen. 



2. Right side, yellow screen (5 times). 



Plate Imperial Special Rapid Ortho. 



Exposures ... 2 sec. and 10 sec. 



The section is stained mainly blue. The illustration shows the 

 advantage of a yellow screen in giving the blue portions their correct 

 depth of tone, and due contrast. 



These four photomicrographs all taken on Orthochromatic plates. 



Photos. W. B. R. i To follow page 152 



