CHRISTIAN HUYGENS 73 



lenses himself, assisted in part by his brother Constantine, 

 and introduced the preliminary examination of the glass to be 

 used as regards its freedom from striae (a process later 

 brought to the greatest refinement some two hundred years 

 later by Topler in Dresden); he also improved the eye pieces 

 of the Kepler telescope, the 'Huygens eye piece' being still 

 in use to-day. It consists of two convex lenses, the inner of 

 the two working together with the objective, so that the real 

 image is formed between the two lenses of the ocular. The 

 whole of these investigations were first published after 

 Huygens' death in his Dioptrics in 1703; but he had already 

 quite early made a remarkable application of his study of the 

 telescope, by discovering a moon of Saturn and the free rings 

 of this planet, an account of which he published in 1669 in his 

 Systema Saturnium. Galileo had only been able to see the ring 

 in the somewhat blurred form of an apparent multiplication of 

 Saturn into three, and this appeared to vanish later; subse- 

 quent observers saw other inexplicable phenomena about 

 this planet. Huygens was able, after years of observation 

 with his better telescope, to explain everything: the ring 

 surrounding Saturn but not connected with it is plane and 

 thin, and has a strong inclination to the plane of the earth's 

 orbit, but remains parallel to itself; thus it comes about that 

 twice during a revolution of Saturn in its orbit (that is, 

 about every fifteen years) it is illuminated on the narrow 

 edge only, and thus becomes quite or almost invisible, 

 whereas in the intermediate period it arrives at full visibility. 

 Huygens made these discoveries with a telescope con- 

 structed from lenses he had ground himself; its objective, 

 which still exists in Utrecht, was only about five and a half 

 centimetres in diameter, but had a focal length of three 

 metres; the ocular was a small lens of seven centimetres 

 focal length.^ After the telescope, Huygens also worked on 



^ See Oeuvres completes de Christian Huygens. The only English 

 translation of Huygens is of the Treatise on Light (S. P. Thompson, 1912). 



