306 IN STARRY REALMS. 



chief recommendation of the reflecting telescope is found 

 in the circumstance that the rays of light of every de- 

 scription are all brought to the same focus. Thus if the 

 plate be placed at the correct point for visual purposes, it 

 is also correctly placed for the photographic rays. There 

 is here no troublesome question as to the difficulty of 

 securing a confluence of all the rays at a single point 

 where their united action shall be devoted to engraving a 

 mark on the plate. On the other hand, it has been 

 customary to believe that the support of the mirror, and 

 the precautions necessary to prevent the distortion of its 

 figure by flexure, went far to neutralize the advantage of 

 the useful indiscriminateness with which all rays were 

 conducted to the same focus. The remarkable achieve- 

 ments of Mr. Roberts and of Mr. Common have, however, 

 been accomplished by reflectors, in a way which proves 

 that the difficulties attendant on this form can be sur- 

 mounted. 



For most of the great photographic enterprises which 

 are now proposed to be undertaken refractors are being 

 erected, and here a difficulty of a peculiar kind is en- 

 countered. A glass lens of accurate figure, when it 

 receives a parallel beam of any homogeneous light, will 

 direct all the rays of the beam to concentration at a focal 

 point. To this extent the action of the lens and the 

 action of the reflector are identical. By homogeneous 

 light we mean light which we may with sufficient 

 accuracy describe as being one of the prismatic colours. 

 Thus a beam of pure red falling in parallel rays on the 

 lens are all brought to the same focus. So also are the 

 rays of a blue beam ; but the point to which the blue rays 



