98 ELEMENTARY PHOTO-MICROGRAPHY. 



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 



10 

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



i 



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. 



REFLECTED LIGHT OR OPAQUE ILLUMINATION. Light that 

 passes into the microscope by reflection from the 

 front of the object viewed. 



REFRACTION. The deflection or change of direction of rays 

 of light by a transparent body through which they 

 pass. 



RESOLVING POWER. The power to separate component 

 parts of a body dependent on, and directly propor- 

 tional to, the N.A. 



SPHERICAL ABERRATION exists when marginal and central 

 "rays do not meet at one point, or focus, but give a 

 blurred and indistinct image. 



SPECTRUM. A coloured band resulting from the decom- 

 position of light. The colours are red, orange, yellow, 

 green, blue, indigo, and violet. 



