ELEMENTARY PHOTO-MICROGRAPHY. 1 31 



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. 



CORRECTION COLLAR. A contrivance for separating the 

 lenses in an objective or bringing them nearer together 

 to correct for different thicknesses of cover glass. 

 For thick covers the lenses are brought nearer, and 

 for thinner ones separated. Or the correction may 

 be made by shortening the tube for thicker covers, 

 or lengthening it for thinner ones. 



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 

 terms deep and shallow refer to the amount of cur- 

 vature of lenses, and not to the distance between them. 

 Eyepieces are also called high and low, the last giving 

 a lower amplification of the image than the first. 



