JOURNAL 



OF 



NATURAL PHILOSOPHY, CHEMISTRY, 



THE ARTS. 



APRIL 1 797. 



ARTICLE I. 



The PrindpUi and application of a New Method of conjlrun'tng Achromatic Telefcopei. 

 By Robert Blair*-, A/. D. 



-iXFTER a fliort hiftorical retrofpcft to the difcoveries of the unequal refrangibility of 

 light by Newton, and the fubfequent corredion of its effects in optical inftruments by 

 Dollond, together with the remedy which is afforded in compound lenfes for the aberration 

 nrifing from fphericity, our author remarks, that it w^s expected by men of fcience that an 

 increafe of the aperture and power of the refrafting telefcope would be the neceffary confe- 

 quence of fuch important fteps towards the perfedlion of its theory. Thcfe expeftations 

 have not hitherto been fully anfwered : for it is certain, that no Achromatic Telefcopes 

 Lave yet been made of equal aperture with the fingle objeft-glafles of Huyghens and others, 

 nor in this refpeft comparable to reflectors, though the errors of workmanfliip are much 

 more noxious in thefe lad than in lenfes. 



The genera! anfwcr made by artifts to enquiries of this nature is, that the fault lies in the 

 imperfection of glafs, and particularly in the denfe glafs known by the name of flint-giafs. 



The imperfection of glafs, for optical purpofes, arifes partly from its opacity and colour, 

 but chiefly from irregularities in the refradtive dcnfities of its parts. The rcfearches of 

 chemiils and maniifa£turcrs to remove this defe£l have been confiderable, but hitherto 

 without much fuccefs. 



From the confideration that it is not impoITible to introduce a fluid medium to fupply the 

 place of one of ihe lenfes in the compound objeft-glal's, Dr. ISlair was led to the cxperi- 

 metital enquiry, whether nature alForded fluids poflijfll-d of the tequifite qualities. 



The original paper, from wliich I have made the aliove .abridgement or rdeclian of parts, u infcrtcd iu tiic 

 I 1 ijiW-linni of ihe Koyal Society of Edinburgh, vol. ii. It occupies 76 pages. N. 



VvL. I.— Ai'RiL 1797. B Iu 



