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Transactions of the Society. 
of less density than the indispersive medium, an additional lens 
becomes necessary to remove the spherical aberration. The cavity 
which in this case is formed between this additional lens and the former 
may either remain empty or may be filled with spirits of wine or 
any transparent indispersive fluid, merely to prevent the loss of light 
which would otherwise arise from reflexion.” 
In 1791 Blair read before the Royal Society of Edinburgh a 
paper * entitled, “ Experiments and Observations on the Unequal 
Refrangibility of Light.” In this paper he gives a most elaborate 
account of his method for combining various dispersive fluids with 
crown glass lenses. 
Both Sir John Herschellt and Sir David Brewster held very 
favourable opinions of Blair’s invention. Herschell remarks that 
“ Could solid media of such properties be discovered, the telescope 
would become a new instrument and Brewster, { speaking”of Blair, 
says that he produced “ fluid object glasses in which the aherra - 
tion of colour was completely corrected. The telescopes which he 
made on this principle were so extraordinary, that Professor Robison § 
assures us that one of them, Fifteen inches in focal length, equalled 
in all respects , if it did not surpass , the best of Dollond’s Forty- 
two inches long.” 
Blair entrusted the manufacture of his instruments to George 
Adams the younger, who, however, was not very successful with 
them. An equally unsuccessful attempt to sell them was made in 
1827 by Archibald Blair, son of the inventor, who was an optician 
in Edinburgh. A biographical notice |] of Blair states that he died at 
Westlock, in Berwickshire, on December 22nd, 1828. I am indebted 
to Miss A. M. Clerke, the writer of the biographical notice, for much 
information concerning Blair. 
Dixon. 
In 1785, Hugh Dixon, a chemist of Clerkenwell, obtained a 
patent IF for “ Certain considerable Improvements in the Construction 
of Telescopes, Microscopes, Spectacles, and all other Instruments of 
Vision, either by Reflection alone or compounded of Refraction and 
Reflection, whereby the Field of View is greatly extended, and the 
Objects rendered much more Distinct than at present.” 
* Trans. Roy. Soc. Ediu., iii. (1794) pp. 3-76 (3 pis.). An abstract of Blair’s 
paper, under the title of “ The Principles and Application of a New Method of con- 
structing Achromatic Telescopes,” appeared in Nicholson’s Journ. Nat. Phil. Chem. 
and Arts, London, i. (1797) pp. 1-13; and as the “ Beschreibung einer neuen Art 
von achromatischen Fernrohren oder der sogenannten aplanatischen Telescope, und 
Entwickelung der Griinde, worauf sie beruhen,” in Gilbert’s Annalen der Physik, 
Halle, vi. (1800) pp. 129-148, pi. 3. 
t Encyc. Metropolitana, iv. (1845) art. “ Light,” pp. 427-9. 
i Encyc. Britannica, 8th ed., xvi. (1859) art. “ Optics,” pp. 584-6. 
§ Professor of Natural Philosophy at the University of Edinburgh. 
|| Diet. National Biog., v. (1886) pp. 166-7. 
1 Specification No. 1515, dated Dec. 14th, 1785, and Jan. 13th, 1786. Reprinted 
in 1856. 
