THE SUN'S TRVE ATMOSPHERE. 105 



Instead of this being the case, however, there is a limit 

 for every object-glass, beyond which no separation is 

 possible ; for this reason, simply, that the star-images 

 formed by the object-glass are not points of light, as 

 they would be if they correctly represented the stars of 

 which they are the optical images. The larger the 

 object-glass (assumed to be perfect in construction) the 

 smaller is the star-image ; 1 but it has always a definite 

 size, and if this size is such that the two images of the 

 stars forming a pair actually touch or overlap, we can- 

 not separate them by using highly-magnifying eye- 

 pieces. 



Now what is true of a star is true of" every point of 

 any object we examine with a telescope. The image 

 of the point is always a circle of light, which, though 

 minute, has yet appreciable dimensions. The image 

 of the object is made up of all these circles, which 

 necessarily overlap. Nor let the reader suppose that 

 on this account telescopic observation is untrustworthy. 

 Precisely the same peculiarity affects ordinary vision. 

 There is no such thing as a perfect optical image of an 

 object; though neither eyesight nor telescopic vision 

 need be regarded as deceptive, on this account. Our 

 power of seeing minute details is limited by this 

 peculiarity, but we are not actually deceived. If 



1 A curious illustration of this is given by the fact that a certain 

 astronomer of old, having reduced the aperture of his telescope to a 

 mere pin-hole, announced that he was thus enabled to measure the real 

 globes of the stars, for, instead of seeing the stars through his telescope 

 as minute points of light, he now saw them with discs like the planets. 

 He thought he was improving the defining qualities of his telescope, 

 instead of altogether destroying them. 



