308 IN STARRY REALMS. 



convex lens of crown, tends towards different foci. But 

 in contact with, or very close to, the crown lens is a con- 

 cave lens of flint glass, which proceeds to undo the bend- 

 ing which the beam has received from the crown. It is 

 possible by a combination of two lenses to produce a single 

 objective which shall bring the foci for any two desired 

 hues into coincidence. If, for example, we arrange 

 the proportions of the lenses appropriately, the red rays 

 and the blue rays will be conducted to a common focus, 

 and all the other visual rays of the intermediate hues will 

 be brought to foci so close to the main focus that the 

 telescope will be practically perfect for optical purposes. 

 Such is the modern achromatic object-glass. 



When an objective is to be employed for photography a 

 new class of considerations arises. The rays most specially 

 potent in their action on the salts of silver are not visual 

 rays. The focus to which they would be brought by a 

 single lens is much nearer the glass than is the focus of 

 the extreme violet. In the ordinary adjustment of the 

 achromatic objective for visual purposes the photographic 

 rays are, as the optician says, allowed to go wild, for there 

 would be no object in leading them to the common focus, 

 and the attempt to do so would seriously impair the visual 

 performance of the telescope. Hence we see the important 

 fact that an achromatic telescope, however perfect for the 

 ordinary purposes of astronomy, would be unsuited for the 

 photographer, If a plate be placed at the ordinary 

 optical focus of such an instrument, the visible rays from 

 a star are no doubt brought to a point on that plate, but 

 the photographic rays, not having the same focus, will be 

 spread over a little circle instead of a point, and the 



