MEMOIR ON ACHROMATIC GLASSES. \ \f 



double refraction within certain limits, by means of the inter- double refrac- 



val between the glafles of differing refolding powers : the f ion increafcd by 

 r i r , . . * ,/ - / i iL mcreafing the 



leparatton of the images at the focus being to much the ; nterva ] between 



greater, as the interval is larger, when the flint glafs is the the object 



jffrft of the object glafTes • andlefs, when it is the fecond. flint"lafs!s 



Conformably to thefe new principles I have had two tele- firft; the re- ^ 

 fcopes with a double refracting medium conftructed under my ^ f e 7o n T '* '* 

 own infpection, which General Gantheaume will employ for 

 determining the pofition of his fhips, and to find whether he 

 be approaching any he may meet with at fea *. 



The ufes of an inftrument for meafuring very finall angles 

 with precifion are too well known for me to defcribe its ad- 

 vantages. The officers of the Englifh navy are fo fully aware 

 of them, that they have ufed for fome years Ramfden's eye Defects of 

 glafs micrometer, though this anfwers the end but imperfectly, Ramfden's e y« 

 becaufe it does not give the meafure of the angle, and becaufe ten 

 of the bad effect of the parallax produced by the decuffation of 

 the rays that enter the eye. This defect is more fenfible in 

 Ramfden's eye glafs micrometer than in Bouguer's heliometer. 

 The officers who have compared my inftrument with Ramf- 

 den's, of which there were feveral on board the Spanifh fhips 

 with our Breft fleet, agree that the celebrated Engliffi artift 

 has very imperfectly accomplifhed the object he propofed ; and 

 Bouguer's heliometer could unqueftionably be preferable for Bouguer's he^ 

 naval ufe, becaufe it has a lefs fenfible parallax, and gives the| lo ^ e , ter pre " 

 meafure of fmall angles, fo important for determining the dif- 

 tancesof mips from their known dimenfions. 



Table of the proportions which the magnitude of an objecl bears 

 to its diftance : calculated according to the rule, which is eafily 

 demonjlrated, that, in every right angled triangle, the tangent is 

 to the radim as the magnitude of an objecl is to its diftance from 

 the centre of the eye, ivhen, under an angle known, the dijlance 

 forms a right angled triangle with the object. 



* The General has made an advantageous report of this inftru- 

 ment, in his account of the chace of the Swiftfure, which he cap- 

 tured. This inftrument is fo difficult to execute, that I know only 

 one perfon, Citizen Narci, who is capable of giving rock cryftal only one perfon 

 the prifmatic form in the proper direction for obtaining the double y e * found capa- 



refi actions neceflary to the goodnefs of the micrometer. He has,,? ma 1D f_ 

 J ° '* this micrometer* 



made feveral, which have given me perfect fatisfaction. 



EXAMPLE. 



