94 ELEMENTARY PHOTO-MICROGRAPHY,. 



RULE FOR FINDING DISTANCE OF LANTERN 

 FROM SCREEN. 



Add one to the number of times the slide is to be 

 enlarged, and multiply by the focus of lens in inches. 

 The answer will be in inches. 



EXAMPLE : What distance should lantern, with Sin. 

 focus lens, be from screen to give a loft, picture ? 



If the lantern slide has a clear opening of 3in., then 

 the magnification of a loft, picture would be forty times. 

 Therefore, 40 -{- i x 8 = 328in., or 27ft. 4in. 



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. 

 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. 



