134 ELEMENTARY PHOTO -MICROGRAPHY. 



The penetrating power of an objective decreases with 

 the increase of both numerical aperture and magnify- 

 ing power, inversely as the numerical aperture and 

 also inversely as the square of the magnifying power. 

 POWER. Magnifying powers of lenses are estimated by 

 comparing their focal lengths with the distance of 

 distinct vision, and this has been generally agreed 

 should be arithmetically expressed by reference to 

 visual magnitude seen at loin, distance. Thus loin, 

 divided by the focal length will be approximately the 

 magnifying power of the lens, e.g., a lin. objective 

 should magnify 10 diameters, |in. 20 diameters, Jin. 

 40 diameters. More nearly correct if the magnifying 

 power of a lens at any tube length is obtained by 

 dividing the distance of projection by the focus and 

 subtracting one. Thus a lin. lens at loin, distance 



will magnify 9 times. i = 9. The total power 



of the microscope is found by multiplying the initial 

 enlargement of the image by the further magnifying 

 power of the eyepiece. Thus an objective magnifying 

 20 diameters used with the A eyepiece, usually mag- 

 nifying 5 diameters, will give 20x5 = 100 diameters. 



POLARISCOPE. An instrument for polarising and analysing 

 the light. Generally consisting of Iceland spar prisms 

 mounted for easy attachment to the microscope. The 

 polariser underneath the object and the analyser 

 above the objective. 



PROJECTION, OCULAR. An eyepiece used for photo-micro- 

 graphy or for demonstration purposes, to throw the 

 enlarged image on to a screen. 



REFLECTED LIGHT OR OPAQUE ILLUMINATION. Light that 

 passes into the microscope by reflection from the 

 front of the object viewed. 



