8 MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



lar motion. The warm air below will be rising through the cold strata 

 above and the air of the latter will be falling. These motions are 

 necessarily irregular and complex. If a strong wind is blowing in 

 these regions, the rapidity and complexity of the changes may be in- 

 creased. A ray of star-light will pass through such a mixture in a 

 zigzag line with a thousand small irregularities, and these will produce 

 variations in the image seen by a telescope. Let us first consider the side- 

 wise motions of the star-image. To the naked eye these may appear 

 quite considerable. In the telescope they will be multiplied by the 

 magnifying power used. 



Beside th^, sidewise motion of the star-image produced as described, 

 the motions of the layers of atmosphere give rise to other effects. 

 They virtu?\lly change the focus of the observing telescope, as 

 follows : Tb-e object-glass of the telescope is a lens which grasj^s paral- 

 lel rays and brings them to a definite focus. The eyepiece is placed so 

 as to see the image at the focus as sharply as possible. A change of a 

 few thousandths of an inch in the position of the eyepiece may be fatal 

 to good definition of the image. If we should suddenly change the 

 object-glass of the telescope and replace it by another one of slightly 

 different focus, say a few tenths or even hundredths of an inch differ- 

 ent, leaving everything else the same, it is clear that accurate vision 

 would be destroyed. A perfect image of the star would be formed in 

 the iovas of the new object-glass, but the eyepiece would no longer be 

 in the correct position with reference to the new image, and the vision 

 would be unsatisfactory. 



An effect precisely similar to the sudden changing of object-glasses 

 is frequently produced by the sudden changing of the curvature of the 

 layers of air in front of the telescope. These layers, which were, let us 

 say, at first horizontal, are suddenly bent by air currents so as to have 

 a decided curvature and so that they act like lenses upon the incident 

 star-light. 



The ray from the star which at first came to the true focus of the 

 glass-lens of the telescope is suddenly brought to a new focus, whose 

 position is fixed by the combination of the air-lens, so to say, and of 

 the glass-lens. Measures of the curvature of such atmospheric strata 

 have been made, and their radii of curvature have been shown to be 

 at least as small as 6600 feet. An air-lens of this curvature in front of 

 the object glass of a large telescope will change the place of the image 

 by several hundredths of an inch. The eyepiece, which remains at 

 one place, can no longer give an accurate image, and the definition is 

 thus spoiled. 



The foregoing elementary explanation supposes the change of focus 

 to take place with some regularity. In practice the changes are usually 



