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is thrown upon the screen will be ascertained, and a more correct 
idea will be obtained of what is happening. 
Fig. 2 (b) shows that the so-called parallel light emitted by a 
condenser when the light is in its focus, as shown in fig. 1 ( b ), is 
far from being parallel if the light source has any finite size, and 
that when it meets the screen it illuminates a large area. In the 
cases ( c ) and (ft) in which the light is focussed to form an image 
of the source, the size of the image is much larger in the case of 
( c ) than in that of (d), so that although the lens in the case ( c ) 
collects more light, it spreads it over so much larger an area that 
the brilliancy on the screen will be far less. 
There is no arrangement of lenses or reflectors by which the 
brightness of a source of light can be increased. All that can be 
done is to concentrate upon an object a larger number of rays than 
it would otherwise receive. If daylight falls upon an object in all 
directions from an unobscured sky, no arrangement of lenses or 
reflectors can collect more, and, therefore, a bull’s-eye will not 
increase the brightness of an object illuminated by an unobscured 
sky. It is only of use in daylight in a room into which the light 
enters from a window which forms a small source of light, where 
light which would pass into other portions of the room can be 
s 
Fig. 1. 
