ELEMENTARY PHOTO-MICROGRAPHY. 99 



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., i-aooin. diameter. 

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

 extent of i-2Ooin. 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. 



APPENDIX. 



To find magnification with given camera length and 

 focus of lens. No eyepiece 



Divide the length of camera by the focus of 

 lens, and subtract one from the result, which will 

 give the enlargement in diameters. 

 With given focus to find camera extension for required 

 magnification 



Add one to the number of diameters wanted, 

 and multiply by the focus of lens. 

 To find what focus lens is required for given magnifi- 

 cation and camera length 



Add one to the number of diameters wanted, 

 and divide the camera length by this number, 

 which will give the focus of lens required. 



