A HISTORY OF SCIENCE 



ed out that the unaided eye is unable to appreciate an 

 angular space in the sky less than about thirty seconds. 

 Even in the best quadrant with a plain sight, therefore, 

 the altitude must be uncertain by that quantity. If 

 in place of the plain sight a telescope is substituted, 

 even if it magnify only thirty times, it will enable the 

 observer to fix the position to one second, with pro- 

 gressively increased accuracy as the magnifying power 

 of the telescope is increased. This was only one of the 

 many telling arguments advanced by Huygens. 



In the field of optics, also, Huygens has added con- 

 siderably to science, and his work, Dioptrics, is said to 

 have been a favorite book with Newton. During the 

 later part of his life, however, Huygens again devoted 

 himself to inventing and constructing telescopes, 

 grinding the lenses, and devising, if not actually mak- 

 ing, the frame for holding them. These telescopes 

 were of enormous lengths, three of his object-glasses, 

 now in possession of the Royal Society, being of 123, 

 1 80, and 210 feet focal length respectively. Such in- 

 struments, if constructed in the ordinary form of the 

 long tube, were very unmanageable, and to obviate 

 this Huygens adopted the plan of dispensing with the 

 tube altogether, mounting his lenses on long poles 

 manipulated by machinery. Even these were un- 

 wieldy enough, but the difficulties of manipulation 

 were fully compensated by the results obtained. 



It had been discovered, among other things, that in 

 oblique refraction light is separated into colors. There- 

 fore, any small portion of the convex lens of the tele- 

 scope, being a prism, the rays proceed to the focus, sep- 

 arated into prismatic colors, which make the image 



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