96 ELEMENTARY PHOTO-MICROGRAPHY. 



terms deep and shallow refer to the amount of cur- 

 vature of lenses, and not to the distance between 

 them. Eyepieces are also called high and low, the 

 last giving a lower amplification of the image than 

 the first. 



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 one 

 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 wave 

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

 it is equivalent to increase of aperture. 



MICROMETER. A slip of glass ruled to scale, generally 

 hundredths or thousandths of an inch. Made for use 

 on the stage of microscope and also for the eyepiece. 



