i8o SCIENTIFIC APPARATUS. 



It will be seen, then, that great progress had been made in mea- 

 sures of space. Equal progress had been made in the measure of 

 time ; for in Tycho's observatory the dial by day and the clepsydra 

 by night had given place to clocks not clocks as we now know 

 them, regulated by pendulums, but clocks controlled by the oscil- 

 lations of weighted bars, such as the Dover clock. 



The introduction of the refracting telescope and pendulum, in 

 the i yth century, marks the most important epoch in the history 

 of astronomical instruments. 



In mechanical astronomy the use Of a telescope, instead of the 

 cylinders of Ptolemy and the plain sights to be seen on Tycho's 

 quadrant and on the various astrolabes, at once placed the 

 determinations of positions, and therefore of motions, which are 

 simply changes of position, on a new basis. Nor was this all. 

 In the telescope itself, at the common focus, was soon placed, 

 independently by Huyghens and Gascoigne, and afterwards by the 

 Marquis Malvasia, an apparatus for measuring small angles. The 

 difficulty of doing this, without such an apparatus, is very strongly 

 indicated by Grant, in his admirable History of Physical Astro- 

 nomy, who tells how Tycho had been so misled by his measure- 

 ments of the sun and moon, that he had come to the conclusion 

 that a total eclipse of the sun was impossible. 



The strip of metal inserted in the eye-piece by Huyghens, is 

 now represented by the modern micrometer, which allows mea- 

 surements to be made to the hundredth of a second of arc. 



Since the time of Hall and Dollond (to which reference will be 

 made further on) refracting telescopes have been constantly 

 growing larger, more perfect and more compact ; at the same time 

 the division of the circle into equal parts has been growing more 

 perfect and more minute. Hence the tables are turned, and in- 

 stead of a small sight on a gigantic arc, we have a large sight (the 

 telescope) on a comparatively small circle. This state of things 

 has necessitated a change in the method of mounting. The tele- 

 scope is now the first thing to be considered, and it is generally 



