ELEMENTARY PHOTO-MICROGRAPHY. 135 



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. 



SELENITE. A transparent mineral used in thin plates for 

 varying the colours with a polariscope. 



SHARPNESS OF IMAGE AND CIRCLE OF LEAST CONFUSION. 

 An image would be sharp if the rays were correctly 

 focussed; but as lenses are not free from error, a 

 permissible amount of aberration called the circle of 

 least confusion is recognised, viz., T ^in. diameter. 

 That is, a point may be blurred by the lens to the 

 extent of y^m. without spoiling the picture. 



SPOT LENS. A lens for the substage with blacked centre 

 and clear marginal ring, used for dark ground illu- 

 mination. 



STOPS. Discs of metal of various shapes for insertion in 

 the condenser. 



STANDARD SIZES. Certain standards have been adopted 

 for eyepieces, objectives, and condensers, so that the 

 products of different makers may be interchangeable. 

 Eyepiece, students' size, .9173111. diameter (23.3 mm.) 

 Larger stands, i.2yin. (32.258mm.) Objective, 

 .7967111. (thirty-six threads). Condenser, 1.52710. 

 (38.786 mm.) 



