186 The Achromatic Telescope. 



crown glass, and which, therefore, in conjunction, would pro- 

 duce exactly the same effect with it. In some cases he en- 

 closed the separate fluids in distinct cells; in others, where 

 they would not decompose each other, he mixed them in one 

 cell between two convex crown-glass lenses. The object- 

 glasses thus formed were of such exquisite quality as to cor- 

 rection, that no trace whatever of colour remained ; and the 

 mathematical skill of the inventor enabled him so to remove 

 the spherical error (for which . purpose, however, a separate 

 glass lens seems to have been requisite with large apertures) 

 that he actually constructed, in 1791, an object-glass of excel- 

 lent definition, with only about nine inches of focus to three 

 inches of aperture ! while, with less exaggerated proportions, 

 there was no comparison between the performance of his tele- 

 scopes under very high powers, and that of the best ordinary 

 achromatics.* Little notice, however, was taken of this beau- 

 tiful discovery. Dr. Blair's attention was subsequently di- 

 rected to a still more important object, in which he was 

 equally successful — the preservation, during long voyages, of 

 lime-juice, on -which the health of our seamen materially de- 

 pends ; he instructed, however, his son in his processes, and 

 that gentleman publicly announced, in 1827, his intention to 

 commence the regular manufacture of these instruments, if 

 sufficient encouragement could be obtained. To the sole ob- 

 jection which could be raised — namely, on the score of want of 

 permanency — Mr. Blair was able to give a full reply, the object- 

 glasses having remained unchanged for twenty-one years ; but 

 I regret to add, that I have never heard of a single telescope 

 being constructed in consequence of this announcement. Let 

 us hope, that since those days we have made some advance in 

 scientific zeal, and, with the wish that something may yet be 

 done to give due effect to this admirable discovery, we will 

 close our lengthened examination of the construction of the 

 Achromatic Telescope. 



NEBULA. 



28. — In the Philosophical Transactions for 1789, Herschel 

 I. describes a star 9 mag. f 8 Geminorum, 9m. 6s. of R.A., 1° 1" 



* It is curious how narrowly this discovery escaped Newton. He says, in 

 one place, that " it did not seem to him impossible for contrary refractions so to 

 correct each other's inequalities, as to make their difference regular ; that, for this 

 purpose, he examined what may be done, not only by glasses alone, but more 

 especially by a complication of divers successive mediums, as by two or more 

 glasses, or crystals, with water, or some other fluid between them, all which toge- 

 ther may perform the office of the object-glass, on whose construction the perfec- 

 tion of the instrument chiefly depends ; and that the result of these trials, as well 

 as the results of theory, would probably be given on some future occasion." Yet 

 we find him, after all, acquiescing in the hopelessness of any such improvement 

 upon the old refractor. 



