Achromatic Lenfes rendereel mire itanfparettt, 233 



that an experimental refult with regard to thofe dimenfions might be obtained with as little 

 trouble as the mere data for computation. 



VIII. 



On thefuppofed Improvement of Achromatic Lenfes^ when rendered more tranfparent by cementing 



than together. 



A 



N achromatic glafs, confiding of one concave and two convex lenfes, has fix fur- 

 faci s at which the reflexion of light can take place. Two evils of confiderable magnitude 

 ar produced by this reflexion. The focal image becomes obfcure from the lofs of the 

 light fo diverted, and the ground or field of view is rendered mifty by the irregular illumi- 

 nation afford d by part of this light, which enters the tube. Thefe confequences aref 

 lirikingly obferved by holding an achromatic lens in fuch a manner as t6 throw the image 

 of a window upon a flieet of paper, at the fame time that a like image is afforded by a 

 fimple convex lens of the fame focal length. The latter image will be bright and clear,, 

 while the former is very faint. As the reflexion of light is ftrongelt at furfaces which con- 

 fine mediums moft different in denfity, it may naturally be imagined that the mifchievous 

 efFe£l of the four furfaces, at which the lenfes are applied to each other, might be in a. 

 great meafure removed by the interpofition of a denfer fluid than air. *Many years ago the 

 Abbe Rochon made fome experiments to improve achromatic lenfes by this invention, and 

 Mr. Grateloup afterwards made ufe of a tranfp-irent folid for the fame purpofe. He took a 

 very pure tear of maftic, and interpofeJ a piece ot fufHcient magnitude between the lenlesof an 

 achromatic glafs. Thefe being immerfed under water of which the temperature Was graduillyf 

 raifed, the mafi-ic at length became foft, and by prefTure of the glalTes was made to occupy 

 the whole of the two fpaces betv;een the concave and the exterior lenfes. After this pro- 

 cefs the compound lens was beautifully tranfparent ; the refledted images could fcarcely be 

 difcerned;. and upon trial in its tube the telefcope was- found to be greatly improved. 



My attention to this contrivance was excited by the approbation beftowed upon a tele- 

 fcope of this kind, of 27 inches long and aj lines aperture^ by Citizen Lalande,in the Con- 

 jioiflance de Terns for the fourth year of the French Republic, page 364. It appeared to 

 me, as it certainly will to the reader, to be a matter-of no fmall fiirprife, that an improvement 



♦Count CafKiii, Inthe Mtmoirs of the Pafifian. Academy for 1787, p. 19, informs us, that a memoiri 

 Jitr les moyens deperfeHtonner Us lunettes acbromaliques par I'iaterpqfiiwn d'un fiuick eitire Us objcBif<) - by the 

 Abbe Rochon, was read before that Academy in January 1774, and iiilcrted in ihe Rccueil des Memoires de 

 Mechsnique etde PhyfKjue, whieh 1 have not feen; and in a noteon tjiis paflage he quotes the. Report ofv 

 the Commiflioners of the Academy, who exprefs their fatisfaftion at the refult of their trials for correfting 

 great part of the dcfefti arifing from.- the furfacss of the gkfles by the iuterpofition of a fluid between them, . 

 but recommend trials with the telefcopes of aftronomers on celeftial objcfts; which it fliould therefore feein had; 

 n^ then been made. The fame author proceeds to acquaint us, that.Mf. Grateloup ufed maftic in 1785, afcer-- 

 w'hich the SieilrFutois made excellent lerifds ef thiskiiid ; ahd that 'tha maftic' anfiveied welf with compound, 

 knfes, whofeinterior furfaces were only ground (douciei.) but not pol'ilhed. In this laft experiment, however, 

 it can hardly be imag^ined that the lens would have (li-tiported an high power-; becaufe the maftic, though it-. 

 »ight afford tranfparency, muft have left the confines of the mediums as imperfeft as befcre. 



Vofct II.— August 179J5. ILli. «fc 



