106 LIGHT SCIENCE FOR LEISURE HOURS. 



microscopic writing be shown us, for instance, we may 

 find ourselves, after poring over it for some time, 

 unable to make out its meaning, the letters seeming 

 all blended together ; but we know what our failure 

 really means, and do not fall into the mistake of 

 concluding that there are no details because the actual 

 details are inscrutable. 



Let us apply this consideration to the sun, and more 

 particularly to the appearance presented by the edge 

 of the sun's disc. The image of every point of this 

 edge is a small circle ; the combination of all these 

 small circles must produce a ring of light all round 

 the true outline of the disc. If the sun's atmosphere 

 did not reach beyond this ring, then no contrivance 

 whatever could render the atmosphere discernible, let 

 the telescope be ever so perfect and the observer 

 ever so clear-sighted or skilful. Now, the actual ex- 

 tension of this ring will be greater or less according as 

 the object-glass of the telescope is less or greater. It 

 may readily be shown that neither Mr. Lockyer's tele- 

 scope nor Fr. Secchi's could possibly show any signs of 

 a solar atmosphere under two hundred miles in depth, 

 while in all probability an atmosphere four or five 

 times as deep would escape their scrutiny. 



Are we then to remain altogether in ignorance of 

 such an atmosphere, supposing that it actually exists, 

 and that the dark lines in the solar spectrum are due 

 to its absorptive power ? Is there no way of obviating 

 the difficulty which has just been dealt with ? 



So far as the method of observing the sun when 



