36 PHILOSOPHICAL TRANSACTIONS. [aNNO 1665* 



Of the Optic Glasses of M. Hevelius and M. Huygens; and other 

 Improvements in Telescopes. N" 6, p. 98. 



That eminent astronomer of Dantzic, M. Hevelius,* writes to his corre- 

 spondent in London as follows : — 



What has been done in the grinding of optic glasses in your parts, and how 

 those beginnings, mentioned by you formerly, do continue and succeed, I am 

 very desirous of hearing. It is now ahove ten years since I myself invented a 

 peculiar way of grinding such glasses, and reduced it to practice ; by which it 

 is easy, without any considerable danger of failing, to make and polish optic 

 glasses of any conic section, and, what is of most consequence, in any dish 

 of any section of a sphere. This invention I have as yet discovered to none, 

 my purpose being, for the improvement of natural knowledge, to describe the 

 whole method in my celestial machine, and to propose it to the examination and 

 judgment of the Royal Society ; not doubting at all but they will find the way 

 true and practicable, myself having already made several glasses by it, which 

 many learned men have seen and tried. 



Monsieur Huygens inquiring also in a letter, newly written by him to a 

 friend of his in England, of the success of the attempts made by an ingenious 

 Englishman for perfecting such glasses, and urging the prosecution of the same, 

 so as to show, by the effects, the practicability of the invention, says, that he 

 intends very shortly to try something of that kind, of the success of which he 

 has good hopes. 



M. du Son, that excellent mechanician, is also now employing himself in 

 London, to bring telescopes to perfection, by grinding glasses of a parabolical 

 figure ; by means of which he hopes to enable the curious to discover more by a 

 tube of about one foot long, furnished with glasses thus figured, than can be 

 done by any other tubes of many times that length. . 



* John Hevelius was born at Dantzic in 161I, and died there in 1687. His education was liberal 

 and his studies extensive, but mathematics were his chief pursuit. He built an observatory for the 

 purpose of astronomical observations, the result of which he published in l6i7, under the title of 

 Selenographia, sive Lnnoe descriptio, SfC. His Cometographia , which appeared in 1668, occasioned 

 a controversy between himself and Dr. Hook, on this question, " Whether distances and altitudes 

 could betaken with plain sights nearer than to a minute ?" This dispute was carried on with great 

 warmth and considerable enmity. The principal of his numerous works is that entitled Machina 

 Calestis. 



