NOTES ON THE EARLY EVOLUTION OF THE REFLECTOR. 71 



duly polished yet did pretty well, but was under charged. It was 

 ordered to be perfected against the next meeting." 



At the next meeting the following: — " The 4 foot telescope of Mr. 

 Newton's invention was produced again, being improved since the last 

 meeting. It was recommended to Mr. Hooke to see it perfected as 

 far as it was capable of being." So far as the annals record it never 

 again appeared on the scene. 



While it was not definitely stated by whom this telescope was made 

 an entry of March 1st of the same year in the Journal de S^avans 

 definitely ascribes this 4 foot telescope to Newton, although the ac- 

 count is rather vague. It would look therefore as though Newton 

 himself or some of his friends had tried out his invention on a larger 

 scale, and had fallen into exactly the trouble that might have been 

 expected* 



Perhaps Newton's aversion to the paraboloid, of which he knew 

 perfectly well the properties, may have been partly due to the fact, 

 as stated in one of his letters, that there is no strictly geometrical 

 method of grinding it. About the same time Hooke proposed to stamp 

 up the specula out of silver and before the end of the year he was 

 working on a 15 inch mould for a speculum of 10 feet focal length, of 

 which nothing has since been heard. 



Meanwhile enters upon the scene the personage known in the 

 histories of science as " Cassegrain, a Frenchman." A communication 

 from M. DeBerce to the Royal Society describes his invention and 

 gives a very rough sketch of it. This was a translation of a letter sent 

 by DeBerce to the Academy of Sciences from Chartres, and read at the 

 seance of April 16, 1672. DeBerce says that this invention had been 

 communicated by Cassegrain to him some three months previously, 

 and how much before this time Cassegrain had been working on the 

 problem will probably never be discovered. At all events the Casse- 

 grainian telescope was disclosed to others than the inventor at sub- 

 stantially the same time as Newton's and was certainly an independent 

 invention, although one letter to the Royal Society vigorously berates 

 the Frenchman for stealing "Our Newton's" thunder, and explains 

 how an ingenious friend of the writer is making a still further im- 

 provement on Cassegrain's form by using a flat secondary speculum. 



Newton's comment on DeBerce's letter plainly shows that he did 

 not enjoy a rival in the field, for it is somewhat discouraging in tone 

 and incidentally his specific criticisms were consistently wrong, as for 

 instance he had the erroneous idea that speculum metal reflects much 

 better at 45° incidence than at normal incidence, whereas in fact the 



