212 HISTORY OF SCIENCE. 



To Huyghens is due, also, the first idea of another instrument, which 

 has been of vast service to astronomy. Every one knows that at the 

 focus of the object-glass of the astronomical telescope there is an 

 image perfectly similar to the object, and proportional in magnitude 

 to the angle which the object subtends to the eye. The eye-piece 

 of the telescope may be regarded as a lens for magnifying this image. 

 Huyghens placed at that part of the telescope ^.t which the image is 

 formed a diaphragm with a circular opening, and he determined the 

 angular magnitude corresponding with this opening by observing the 

 time occupied by a star in traversing the diameter. When he wished 

 to measure the angular magnitude of any interval, the diameter of a 

 planet for instance, he introduced at the same part of the telescope a 

 slip of metal which would just cover the interval. The width of the 

 piece of metal, compared with that of the circular opening, would give 

 the angular measurement required. The Micrometer was improved 

 first by the introduction of fine threads, and finally the screw micro- 

 meter was introduced by Auzout in 1667. In this instrument there 

 are two parallel threads, one of which is fixed and the other is attached 

 to a sliding frame. The frame is moved by turning a fine screw and 

 an index, and by the number of turns given to this the distance be- 

 tween the parallel threads is accurately known. 



Another great improvement in the instruments of astronomy was 

 made about this period, and its idea is due to an Englishman named 

 GASCOIGNE. It consisted in applying the telescope to the graduated 

 arcs or quadrants by which altitudes, etc., were measured. There are 

 several advantages obtained by substituting the telescope for the plain 

 sight. The unassisted eye cannot appreciate an angular space in the 

 sky less than 30"; but if a telescope magnifying only thirty times is 

 used, the position of an object may be determined to i". Many con- 

 trivances have been employed for obtaining accurate readings of the 



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indications of graduated instruments, but for this purpose the Vernier, 

 which was invented about this period, has proved the most effective. 

 We specially mention these inventions for the accurate measurements 

 of angles and lengths, as this accuracy has been one of the causes of 

 the rapid advance of astronomy and other branches of science which 

 has distinguished modern times from all previous ages. The vernier 

 is a contrivance applied to the scales of all divided instruments which 

 are required to be accurately read off. Its principle is very simple, and 

 the mode of reading its indications may be easily described. Let the 



