ELEMENTARY PHOTO-MICROGRAPHY. 95 



CAMERA-LUCIDA. A prism or reflector attached to the 

 eyepiece of microscope which enables the observer to 

 sketch the enlarged image on paper. 



COMPLEMENTARY COLOURS. Pairs of colours which, when 

 mixed, produce white or grey. With pigments the 

 primary colours are taken to be red, blue, and yellow ; 

 but with coloured light, red, blue, and green. If one 

 primary colour be selected, its complementary will be 

 that made by the union of the remaining two colours. 

 Thus yellow would have for its complementary a 

 mixture of blue and red, or violet. 



COLOUR SCREEN. A coloured glass or liquid used as a 

 filter to intercept colours not required. 



CHROMATIC ABERRATION. Unequal refraction of various 

 colours which do not focus at one point, but give 

 prismatic colouring of the image. 



CRITICAL LIGHT. Critical light is given by the substage 

 condenser when both flame of lamp and objective are 

 focussed on the slide in the same optical axis. It is 

 essential for high power work. 



DAVIS SHUTTER. An iris diaphragm used like a nose- 

 piece at the end of body tube, by means of which 

 aperture can be modified from the back of objective. 

 It is useful for getting penetration and in dark ground 

 illumination. 



DEFINITION. Sharpness and clearness of image, which 

 must be free from blur or fog. 



DIAPHRAGM. A plate with circular hole to cut off mar- 

 ginal beams of light. 



DIFFRACTION. Caused by an opaque body in the path of 



light, which deflects the rays from their course and 



causes them to interfere with one another, 



EYEPIECES, DEEP AND SHALLOW. A deep eyepiece has 



greater magnifying power than a shallow one, but the 



