52 The Royal Observatory of Scotland. 



closeness with which a very small quantity may be measured 

 to, — the improvement of the exactness of observations has 

 always been the great cynosure of practical astronomers. 

 But much more is it the case now, as in addition to such 

 motives, there is the further one, that so many observations 

 of long discovered bodies having been already accumulated 

 in the world, there is little advantage to be obtained by re- 

 observing those same objects again, unless it can be done 

 better than on former occasions. Accuracy, therefore, still 

 accuracy, and accuracy above all things, must be the ruling 

 idea of modern practical astronomy. 



" This being confessed, it will be found that the greatest 

 impediment to the desired accuracy is the atmosphere ; an 

 ever present obstacle, and producing, with well-made modern 

 instruments, far more untoward effects than all other sources 

 of error whatever. Putting out of the question actual clouds 

 preventing any view of the sky, and even not stopping to 

 consider the effect of the diffusion of general day-light, 

 though that is a consequence of the atmosphere, and very 

 prejudicial, too, in eclipsing the fainter objects in the sky, — 

 yet if we only take account of the smaller undulations shewn 

 by the telescope to exist in the medium, when apparently 

 to the naked eye it is very clear and tranquil, — we yet find 

 them there so excessive and so lawless, that seldom or 

 never can the highest, or even anything like the highest, 

 magnifying power be applied, which the object-glass is ac- 

 tually provided with, and would otherwise bear with advan- 

 tage. Thus, telescopes may be increased in size and accuracy, 

 but, when under such drawbacks, without any benefit result- 

 ing therefrom ; while the bad effects of the atmosphere are 

 even more hopelessly obstructive in a large than a small 

 apparatus. 



" The atmosphere, then, being so determined an opponent, 

 every effort should be made to eliminate its effects as far as 

 possible ; and this can only be accomplished by rising above 

 its grosser parts, as when placing the telescope on a high 

 mountain. Such was Newton's recommendation more than 

 a century ago, when trying the first little reflecting telescope 

 that had ever been made. And yet, though he recommended 



