TEE REFRACTING TELESCOPE. 171 



with great exertion and expense, completed one magnifying thirty di- 

 ameters, which we now know to be the greatest power possible with 

 the form of lenses that he used, viz., a double-convex lens for the ob- 

 ject-glass and a double-concave lens for the eye-piece. 



With such crude instruments as these, Galileo made his well- 

 known discoveries, which, coming just when they did, proved of great 

 importance in giving an additional impulse to the then rapidly awak- 

 ening intellect of Europe. 



Soon after the death of Galileo the telescope was further perfected 

 by Huygens, who, in the first place, invented the form of eye-piece 

 which still bears his name, and gives a large, flat field with very sharp 

 definition. Many variations of form, but no improvement in the see- 

 ing quality of telescopic eye-pieces, have since been made, so that from 

 this time all improvements in the telescope have been necessarily con- 

 fined to the object-glass. 



Huygens next enlarged the single-lens object-glass to its greatest 

 possible power. His largest telescope had an object-glass five inches 

 in diameter, and a focal length of one hundred and twenty feet ; this 

 enormous focal length being absolutely necessary to reduce the blur- 

 ring effect of the prismatically colored fringes, as well as spherical 

 aberration, to such moderate limits that a magnifying power of up- 

 ward of two hundred diameters could be employed. 



To have watched Huygens at work with this telescope must have 

 been an amusing sight. Its great length precluded the use of a tube, 

 and therefore an assistant was obliged to slide the object-glass up and 

 down a vertical pole, one hundred feet high, by a cord, while Huygens 

 pointed the eye-piece at the object-glass by sighting along a string 

 connecting the two, meanwhile steadying himself by resting his 

 elbows on a two-legged wooden horse. A more difficult and unsatis- 

 factory contrivance to use can hardly be imagined, yet, with this 

 telescope, in 1655, he discovered the rings of Saturn, and one of its 

 satellites. 



Newton, about this time, hastily concluded, from experiments of 

 his own, that refraction without prismatic color was out of the ques- 

 tion, and that the refracting telescope was incapable of further im- 

 provement ; he therefore abandoned the study of the refracting tele- 

 scope, and turned his attention to the construction of reflectors, and 

 thus narrowly escaped making that most important discovery — the 

 achromatic object-glass — which, only two years after his death, act- 

 ually was made by Dollond, who, in 1757, constructed one two and a 

 half inches in diameter, corrected both for prismatic color and spherical 

 aberration. 



From that day the power of the refracting telescope rapidly in- 

 creased, and up to the present moment has only been limited by the 

 ability of the glass-makers to furnish large pieces of optically perfect 

 glass. 



