THE PLANET MARS 125 



Telescopic vision is only concerned with those vibrations 

 which produce disturbance in its field. 



Since aerial waves may be of any size up to many yards 

 long, it is obvious that their disturbing effects may be best 

 avoided by the use of a small telescope. 



In practice it is found that when a telescope three inches in 

 diameter is used these disturbances are generally negligible. 

 Contrasting this small instrument with a three-foot telescope, 

 we see at once how much more we may expect to suffer. If it 

 be assumed that the air waves at the moment are a foot across, 

 then to the smaller instrument they are big waves of which 

 only part of one is in the field at any moment. They will 

 therefore produce general motion but being intrinsically small 

 the motion may well be imperceptible, both on account of its 

 minuteness and extreme rapidity. 



The case is very different however in the larger instrument. 

 Here are waves much shorter than the diameter of the lens and 

 since every part of the lens contributes light to form the image 

 there are at the focus the integrated effect of three waves or 

 at least six different phases of disturbance superposed upon 

 one another and producing inextricable confusion. 



In this case there is no general motion but instead a con- 

 tinuous blurring of the image. It therefore appears that since 

 air disturbance is inevitable it is best to seek that which is 

 longest and that which is least in amplitude. If the wave be 

 very big, it will produce only an occasional swaying motion of 

 the image which in no way disturbs the integrity of its parts. 



We are now in a position to remove the first difficulty there 

 is in viewing the supposed double star — by stopping down the 

 telescope until the image is free from blurring and subject only 

 to general motion. 



We accordingly stop down the telescope and the star now 

 presents the appearance of a peaceful, oblong patch of light, 

 somewhat fainter it is true and perhaps a little bigger but 

 something which will give our eye a chance. 



The stars are not yet separate. The observer is still balked 

 of his aim — by reducing the aperture he has increased the star 

 discs, which now overlap the more and he seems to be in the 

 quandary of Alice in Wonderland when she had reduced herself 

 with the aid of the magic cake so as to get through the little 

 door in the passage and found that she could not then reach up 



