J42 On ihe Application of Telescopes 



the trouble to trace back my steps. I endeavoured to re-« 

 member where I had seen these points discussed, and I re- 

 collected at length, that ii was in Morin's Science des Lon- 

 gitudes, 



In the celebrated conference which he had on this sub- 

 ject, the 30lh ot "March 1 634, with the commissioners named 

 by cardinal Richlieu, Morin (when answering to the objec- 

 tion against the size of the instruments, which did not per- 

 mit theni to be used at sea,) among a number of methods 

 which he used to preserve their accuracy whilst diminishing 

 their radius, mentioned the application of telescopes to the 

 index, instead of sights. These are his words: — "The third 

 method is the application which I have found means ot 

 making of a telescope tcj the index instead of sights, to 

 measure more readily and more exactly the distance of 

 the moon from the stars *." For in the method he used 

 of deriving the longitude from the observed distance of ihu 

 moon from the stars, Morin had found that he could not 

 expect sufficient truth in the collimation, for a star to tra- 

 verse the sights; and he therefore thought of using tele- 

 scopes, which had not then been invented more than about 

 25 years- 

 He had for this purpose placed a telescope on the index 

 of his instruments : he then saw the stars with the greatest 

 clearness ; but it became necessary to determine the point 

 in the field of view of the telescope, on which to place the 

 star, so that the lino passing through the optic centre of 

 the object-glass might then be parall'J to the fiducial line 

 of the index. Morin had but one step further to proceed 

 in order to derive all possible utility from his invention, by 

 applying .cross wire's in the common focus of the two- 

 lenses f: but 1 know not by what fatality it happens, that 

 the most sin)ple means are commonly the last to present 

 themselves. He took another, less accurate method, which 

 brought with it an inconvenience: he thought of covering 

 the eye glass with a tiiin disc of copper or iron, pierced in 

 its middle with a small hole. By this means he obtained 

 indeed a part of what he, desired : for it is certain, that the 

 star once seen in (lie centre of the small hole was very 

 nearly in a constant line, which mi2;ht be made parallel to 

 the fiducial line of the index. We say nearly ; for, however 



• Astronomia jam a fundamontis integre et exacte restituta, complei-- 

 tcns 9 partes hactcnus optata; scientix longitudiimni, &c., autore J. B. Mori- 

 no, 16-10, in 4lo, pars i. pagt 18. This date in the frontispiece is later 

 than that of the first publication, which was in Uj^H. 

 I It was Huygeno who thought of this method — Systema Saturtiium, 1659- 



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