104 LIGHT SCIENCE FOR LEISURE HOURS. 



an atmosphere relatively so shallow as this. Let it be 

 remembered, in passing, that the average height of the 

 sierra may be set at about five thousand miles ; so that 

 the atmosphere we are dealing with would be at the 

 outside but one-fifth as high as that fine rim of red 

 light with saw-like edge which astronomers detected 

 around the eclipsed sun in the total eclipses of 1842, 

 1851, and 1860. Still it might be thought that 

 patience only would be needed to detect the signs of 

 such an atmosphere, shallow though it be. But there 

 is a peculiarity of telescopic observation which renders 

 the recognition of such an atmosphere, if of less than a 

 certain depth, not difficult merely, but impossible. It 

 may be well to exhibit the nature of the peculiarity at 

 length, because it is of considerable interest to all who 

 possess or use telescopes. I take an illustrative case, 

 which seems, at first, to have little connection with my 

 subject. 



Every reader of this work has heard of the double 

 stars, and I dare say most of those who read this parti- 

 cular article have seen many of these beautiful objects. 

 It is known that some double stars are much closer 

 than others, and we commonly hear it mentioned as a 

 proof of the excellence of a telescope that it will divide 

 such and such a double star. But it might seem that 

 if a telescope of a certain size were constructed with 

 extreme care, it should be capable of dividing any 

 double star ; because we might use an eye-piece of any 

 magnifying power we pleased, and so, as it were, force 

 apart the two star-images formed by the object-glass. 



