284 Superiority of'Dr Blair's Achromatic Telescopes. 



kind may be considered as too delicate and complex ; but, ad- 

 mitting in its full force the objection to them, drawn from the 

 change which either does or may take place in the fluid 

 media, their use in public observatories, and, particularly, 

 their application to fine graduated instruments, were objects of 

 primary importance to the progress of astronomy, even if the 

 lens required to be renewed every two or three years. 1 ' 



Since these observations were printed, we find that, at a meet- 

 ing of the Royal Society of Edinburgh, held on the ^\h De- 

 cember 1789, Dr Blair exhibited one of his telescopes to the 

 members who were present, and showed them its performance 

 when directed to double stars. Professor Playfair was one of 

 those members, and the following minute is inserted in his 

 hand- writing in the account of the proceedings of that evening: 

 '* Dr Robert Blair, Professor of Astronomy in the Univer- 

 sity of Edinburgh, produced before the society an achromatic 

 telescope of a new construction, in which the convex lenses 

 are of common glass, and the errors which arise from the dif- 

 ferent refrangibility of light, and from the spherical figures of 

 the lenses, are corrected by the interposition of transparent 

 fluids. The focal distance of the object-glass of this telescope 

 is twelve inches, the aperture is two inches, and the powers 

 belonging to it are two double eye-glasses, with one of which 

 it magnifies seventy-Jive times, and with the other nvnety-fitt 

 times. 



" There are also four double convex lenses, whose focal dis- 

 tance are one-fifth, one-tenth, one-twentieth, and one-thirtieth 

 of an inch, and whose powers are of consequence, sixty, one 

 hundred and twenty, ten hundred and forty, and three hun- 

 dred and sixty times. The fixed stars appear through this 

 telescope with round well defined disks ; sometimes surround- 

 ed with imperfect rings,* but free from radiation and glare. 

 Some double stars that happened to be in sight, were shown 

 through the instrument to the members present. The power 

 of two hundred and forty is found to perform best in separat- 

 ing; the close double stars." 





o 



* These rings are mentioned by Professor Amici, as produced by the 

 best achromatic telescopes, and also by reflectors, when directed to lumi- 

 nous points. See page 309 of this Number. 



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