PLATE 20. 



The use of Colour Screens for Contrast and Detail. 



FAIRY FLY, Gonotocerus <5 x 25. 



Objective Zeiss Planar. 25mm. 



Condenser ... Back combination of Watson's Universal. 

 Illuminant ... Liliput 4 amp. arc, alternating current. 

 Aux. Condense?- Bull's-eye, for parallel beam. 



Plate Wratten's "M" Panchromatic. 



Screen A Wrattcn's A+D deep red light 6400 to red end. 



B Wratten's D + H violet light 4200-4600. 

 Exposure A 40 sec. 



B 10 sec. 



The contrast in transparency between the delicate wings and the 

 yellowish-brown body, antennae, and legs of the Fly is considerable, 

 although the preparation is a very fine one. If no screen is used it 

 is difficult to get detail in the body without almost obliterating 

 the wings through over-exposure. 



A To reproduce the detail in the coloured portions as well as 

 possible without over-exposing the wings, the photomicrograph is 

 taken with monochromatic red light. 



B To get the maximum contrast between the body and wings 

 and background, monochromatic violet light approximately com- 

 plementary to the yellow-brown of the body is used. 



Photos. H. L. H. 



Prep. F. ENOCK. [To follow page 162. 



