i70 HISTORY OF PHYSICAL ASTRONOMY. 



CHAPTER VI. 



THE INSTRUMENTS AND AIDS OF ASTRONOMY DURING THE NEWTO- 

 NIAN PERIOD. 



Sect. 1. Instruments. 



SOME instruments or other were employed at all periods of astro- 

 nomical observation. But it was only when observation had at- 

 tained a considerable degree of delicacy, that the exact construction of 

 instruments became an object of serious care. Gradually, as the pos-. 

 sibility and the value of increased exactness became manifest, it was 

 seen that every thing which could improve the astronomer's instru- 

 ments was of high importance to him. And hence in some cases a 

 vast increase of size and of expense was introduced ; in other cases new 

 combinations, or the result of improvements in other sciences, were 

 brought into play. Extensive knowledge, intense thought, and great 

 ingenuity, were requisite in the astronomical instrument maker. In- 

 stead of ranking with artisans, he became a man of science, sharing 

 the honor and dignity of the astronomer himself. 



1. Measure of Angles. Tycho Brahe was the first astronomer who 

 acted upon a due appreciation of the importance of good instruments. 

 The collection of such at TJraniburg was by far the finest which had 

 ever existed. He endeavored to give steadiness to the frame, and ac- 

 curacy to the divisions of his instruments. His Mural Quadrant was 

 well adapted for this purpose ; its radius was five cubits : it is clear, 

 '.hat as we enlarge the instrument we are enabled to measure smaller 

 arcs. On this principle many large gnomons were erected. Cassini's 

 celebrated one in the church of St. Petronius at Bologna, was eighty- 

 three feet (French) high. But this mode of obtaining accuracy was 

 soon abandoned for better methods. Three great improvements were 

 introduced about the same time. The application of the Micrometer 

 to the telescope, by Huyghens, Malvasia, and Auzout ; the application 

 of the Telescope to the astronomical quadrant ; and the fixation of the 

 centre of its field by a Cross of fine wires placed in the focus by Gas- 

 coigne, and afterwards by Picard. We may judge how great was the 

 improvement which these contrivances introduced into the art of ol>- 



