i2 4 SCIENCE PROGRESS 



planetary disc, brilliantly illuminated as in Mars : required, 

 the aperture and magnifying power which will best reveal fine 

 detail upon its surface. It is necessary to digress at once to 

 inquire what happens when we turn the telescope upon a star. 



The star disc seen in the telescope is a diffraction effect 

 produced by the lens. It is sufficient for the present purpose 

 to recall the fact that the larger the aperture of the lens, the 

 smaller is this diffraction disc ; but besides the disc there are 

 concentric rings surrounding it arranged in order of brightness, 

 the faintest visible being the outermost. 



Now let us suppose that we wish to separate two bright 

 stars which are very close together. In a large telescope they 

 appear perhaps as two discs either in contact or overlapping 

 with their respective systems of diffraction rings interlacing. 

 The confusion apparent to the eye in this picture is further 

 increased by any unsteadiness in the air between us and the 

 star, which causes the two images to swim and flicker ; the rings 

 break and mingle, so that the observer is unable to see anything 

 clearly, the stars appearing as a single pool of boiling light. 



The nature of the movements of the air must therefore be 

 considered. These consist of a series of ripples or waves 

 passing across the field of view, whose size may be estimated 

 from the nature of the disturbance they produce. An analogy 

 may illustrate the point. 



Any one who has been out in a boat has seen the sea bottom 

 in the shallows on a calm day and noticed how the small objects 

 on the bottom — shells and stones— appear to swing about below 

 on account of the waves. This swaying does not disturb the 

 outlines of the small objects that are visible but merely produces 

 a general rhythmic motion. But if a little breeze ruffle the 

 surface of the water, the minute ripples immediately shatter the 

 image of shells and rock, leaving nothing visible but a confused 

 mass of colour. 



Now the analogy between the watery ocean on the earth's 

 surface and the airy ocean above it leads us to expect kindred 

 disturbances ; whether we look down through the one or up 

 through the other, like Newton we may learn something from 

 the pebbles which fringe their mutual margin. 



In looking through water — if the attention be confined to 

 a small area — no perceptible distortion of bottom detail is 

 produced by big waves. And so it is with the air also. 



