ASTRONOMICAL INSTRUMENTS. 



ASTRONOMY, so far at all events as instruments are concerned, is 

 an applied science, and the history of practical astronomy is the 

 history of the adaptation of apparatus which had already been 

 used in other fields to the special purpose of studying the heavenly 

 bodies. We begin by measurement of angles, we end with a 

 wide range of instruments illustrating the application of almost 

 every branch of physical as well as of mathematical science. 

 In modern Observatories applications of the laws of Optics, 

 Heat, Electricity, Chemistry, and Dynamics, are met with at 

 every turn. 



Each introduction of a new instrument, or of a new method of 

 attack, has by no means abolished the pre-existing one ; accretion 

 rather than substitution has been the rule. Measurement of 

 angles goes on now more diligently than it did in the days of 

 Hipparchus, but the angles are better measured, because the 

 telescope has been added to the divided arc. Time is as neces- 

 sary now as it was in the days of the clepsydra, but now we make 

 a pendulum divide its flow into equal intervals and electricity 

 record it. The colours of the stars are noted as carefully now as 

 they were before the spectroscope was applied to the telescope, 

 but now we study the spectrum and inquire into the cause of the 

 colour. The growth of the power of the telescope as an instru- 

 ment for eye observations has gone on, although now almost all 

 phenomena can be photographically recorder!. 



