1 82 SCIENTIFIC APPARATUS. 



pendulum be permanently recorded, but the beat of a standard 

 clock may be made to control the beats of others (No. 1844). 



When we pass from the mechanical to the physical side of 

 astronom)', the means of getting a great quantity of light, and of 

 utilising it in various ways, are the first things to be considered. 



In this place, of course, we can only consider this question from 

 the instrumental point of view. We need not enter into the ques- 



The Telescope, as used in Galileo's time for observing the Sun. 



tion of the actual invention of the refracting telescope. It is 

 quite clear, however, that Galileo, whose instrument has been for- 

 warded from Florence for exhibition, was the one who used it in the 

 very early days with the greatest amount of success. In his time 

 we find it composed of an object-glass rarely exceeding one inch 

 in diameter. In some of the lenses used by Galileo himself it did 

 not even reach these dimensions, and it was soon found convenient, 

 in order to correct the chromatic aberration of the glass, to make the 

 focus as long as possible. The first telescopes, therefore, were of 

 the dimensions of walking sticks. The next great advance made 



