184 Fluid Lenses. 



and the other slipped on one side so as to leave a small portion 

 of the interior of the ring open ; through this the fluid was 

 poured in, and the upper disc slipped over it into its place. 

 The subsequent contraction of the fluid in cooling would leave 

 a vacuum-bubble, which was kept out of sight by allowing 

 the ring an extra amount of aperture. Cement with tinfoil or 

 paper being applied round the edges, the lens was rendered 

 complete and permanent ; and being placed like the corrector 

 of the dialyte would have a "similar effect in all respects, 

 excepting the invariability of the spherical correction. In 

 1827 Barlow constructed a 6-inch telescope of this kind, and 

 in 1829 one of 7'8 inches, with a satisfactory result. A sub- 

 sequent attempt with eight inches, undertaken in 1833 for the 

 Royal Society, proved a failure ; and such might be considered 

 my own humble imitation in 1830, with four inches. It served 

 me, however, for four years, with tolerable achromaticity, but 

 much uncorrected spherical error. Whether the plan may 

 have been tried by others, I do not know. Several limpid 

 fluids have since been discovered whose properties might merit 

 investigation, especially chloroform, which from its density 

 seems to promise well. 



But we have not yet mentioned one very serious drawback, 

 affecting more or less every one of the constructions already 

 described. Not one of them answers to its name. They are 

 not " a- chromatic," i.e., colourless ; and no workmanship of 

 man can render them so. However carefully the correction 

 may be adjusted, there is invariably a fringe of colour sur- 

 rounding every bright object in focus ; and a single glance 

 with a high power at the planet Yenus will make it visible, or 

 even at the foliage of a dark tree against a white cloud or 

 bright blue sky. This uncorrected fringe, or " outstanding 

 colour," is a serious drawback to the beauty of the telescopic 

 image, and a waste of valuable light. Its cause, technically 

 termed the " irrationality " of the spectra of plate and flint 

 glass, may be easily understood by observing that not only is 

 the ivhole amount of dispersion into colour different in plate 

 and in flint glass, but their action on different parts of the 

 spectrum is not the same — the plate throwing the more lumi- 

 nous central rays of the spectrum nearer to one of its ends, 

 the flint nearer to the other. Hence, it is impossible to neu- 

 tralize the whole spectrum by lenses of two kinds of glass. If 

 we unite the extremities, the central rays will go on one side, 

 and be "outstanding" in focus; if we correct the colours 

 nearer the centre, the extremities will not balance each other. 

 The great art, therefore, of the optician is to neutralize the 

 most vivid and predominant tints; he can do no more; the 

 rest must take their chance. There is, however, some dis- 



