CH. XIV] 



EFFECT OF APERTURE IN PROJECTION 



611 



increase of the diameter of the cone of 

 light will do no good because the light 

 falls outside of the opening of the lens. 

 The conclusions from this are: 



1. If there is a cone of directed 



light from a given point it is of no ad- FIG. 344. THE "CLOSING AN- 

 , . , . ., , GLE" OF LIGHT FORMING A 



vantage to use an objective with SCREEN IMAGE. 



lenses of larger diameter than this The shaded portion shows 

 cone. the closing angle when an or- 



TTT-.I . . , . ,. , dinarv magic lantern is used 



2. With a given size objective, when with an ar lamp as a sol?rce 



the object is illuminated by directed of light; and the outside lines 

 v t, r , v show the closing angle when an 



light an increase in area of the source extended source of light, like a 

 beyond that which will fill the aper- gas or acetylene flame, is used, 

 ture of the objective is of no advantage. 



856. Method of determining the aperture of the objective 

 which is used. The simplest way is to look directly into the 

 objective when in use; of course, using a colored glass or smoky 

 mica to protect the eyes. Do not hold the head too close to the 

 objective. If the whole of the back lens of the objective is filled 

 with light the aperture is filled. If the bright light is only in the 

 center of the back lens the bright spot is the part of the aperture 

 which is used. It often occurs that different parts of the objective 

 are used by the light from different parts of the object. This can 

 be determined by looking at the objective and moving the head 

 from side to side. 



FIG. 345. 



THE CLOSING ANGLE OF LIGHT 

 TO FORM AN IMAGE. 



In order that the images i and j, shall be equally bright with opaque pro- 

 jection, the closing angle of light from the objectives must be the same. 

 As the distance bj, is twice as great as ai, the diameter of the lens (b) must be 

 twice as great as that of (a) and its area must be four times as great. 



