132 ELEMENTARY PHOTO-MICROGRAPHY. 



FIELD. The actual area presented at once to the view of 

 the observer. 



Focus OF OBJECTIVE must not be taken to mean that the 

 focus is the distance between the objective and the 

 cover glass. It refers to the magnification of a single 

 lens of that focus, and is used for comparison only. 

 Focus of single lens is distance from optical centre to 

 point where rays meet. 



IMMERSION OBJECTIVES are used with a fluid, usually 

 cedar oil, to connect the lens of the objective with the 

 cover glass of the specimen under examination, and 

 have greater aperture than would be possible with 

 dry lenses, therefore giving greater resolving power. 



IRIS DIAPHRAGM. A series of small shutters arranged to 

 give a circular opening, the size of which may be 

 varied to give different apertures at will. 



INTENSITY OF LIGHT diminishes as the square of the 

 distance from the radiant point. 



MONOCHROMATIC LIGHT. Light of one colour and uniform 

 wave length. Images formed by waves of different 

 lengths may destroy each other. The longest wave 

 is red, and the shortest violet. Every colour has a 

 different wave length, and the separating power of 

 objectives increases as the length of light waves 

 diminishes that is, if the wave length be reduced, it 

 is equivalent to increase of aperture. 



MALTWOOD FINDER. A series of squares with vertical and 

 horizontal numbers photographed on a glass slip 

 to be placed on microscope stage for recording and 

 finding the position of any desired object on a slide. 



MECHANICAL TUBE LENGTH. The distance from the end 

 of tube receiving the eyepiece to the shoulder against 

 which the objective will be screwed. The Conti- 

 nental pattern is usually 160 mm. (about 6 inches), 



