GLOSSARY. 



ACHROMATIC OBJECTIVES do not unite all the coloured 



rays in an image, but leave a secondary spectrum. 



The spherical aberration is corrected for rays of only 



one colour. 

 APOCHROMATIC OBJECTIVES give images free from colour, 



and are corrected for spherical aberration for different 



parts of the spectrum. 

 ANGULAR APERTURE. The angle of a cone of light that is 



received by the objective from a point on the object. 

 APLANATIC. Free from spherical aberration, or the quality 



that brings all rays to one focus. 



APLANATIC APERTURE. The N.A. of the largest solid cone 



of light which can be transmitted by the condenser. 

 BULL'S-EYE. A plano-convex lens, usually mounted on a 



pillar with ball and socket joint. Sometimes called 



an auxiliary condenser. 

 COMPENSATING EYEPIECES correct colour defects in the 



objectives. 

 CONDENSER. A system of lenses for the substage to 



collect the light and focus it upon the specimen. Not 



to be confounded with BULL'S-EYE. 

 CAMERA LUCID A. 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. 



