SOIOPTICON MANUAL. 



of radiation would lose intensity with distance, and so a 

 lens of short focus, being nearer, would bring more light 



Fig. 14. 



to bear on the image. But the light in fact proceeds 

 from I, and what passes through the condenser and 

 picture becomes a cone of light, with its apex in the 

 objective. 



This cone of light must be regulated by the position 

 or power of the condenser so as to fall within the compass 

 of the objective. Except for some reflections from the 

 surfaces of the glasses, the light would not illuminate 

 the room at all in its passage, and there would not be 

 even the small need there is of the hood t (Fig. 15). 



ADVANTAGES OF A CONCENTRATED EIGHT. 



Could the light proceed from a mathematical point 

 behind a faultless condenser, the pencils a! V d (Fig. 14) 

 would be without marginal rays, and there would be 

 nothing for the objective to do, for its office is to bring 

 the marginal rays of each pencil to coincide with the 

 axial ray. Were we to adjust the aperture of our 

 window shutter (Fig. 1), to the apex of the cone, there 

 would be no rays for the shutter to cut off. The imago 

 would be formed anywhere within reach of the light, 

 either with or without lens or stop. But in reality even 

 the most concentrated light occupies some space with 

 countless radiant points, so near together however that 



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