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SECOND REPORT- 1832 . 
this oversight Huygens failed in making that fine discovery 
which was reserved for M. Biot, that the double refraction of 
quartz differed from that of calcareous spar, only in its being 
regulated by a prolate in place of an oblate spheroid. 
After Huygens had established his law of double refraction, 
and even after he had drawn up his treatise on the subject, he 
discovered what he calls il a wonderful phenomenon” namely, 
the polarization of the two pencils of light formed by Iceland 
spar; and he confesses that other suppositions besides those 
which he has made will be required to explain it. He acknow¬ 
ledges, however, that he was unable to form any satisfactory 
conjecture respecting this new property of light, and therefore 
left the investigation of it to future inquirers. Sir Isaac New¬ 
ton followed Huygens in this difficult research; but he only 
stated the fact in another way, when he said that the different 
sides of the ray had acquired, in passing through the first 
crystal, different properties which either favoured or prevented 
its passage through the second. 
The subject of double refraction and polarization remained 
in the state in which we now leave it for one hundred and twenty 
years, without having received any accession of the least impor¬ 
tance. The current of optical discovery, however, was not stop¬ 
ped ; its direction only was changed, and during the next cen¬ 
tury it continued to flow in a more practical and useful channel. 
The discovery of the different refrangibility of light which 
caused Sir Isaac Newton to despair of the improvement of com¬ 
mon object-glasses, led himself and his contemporaries to perfect 
the reflecting telescope. The mistake which he had committed in 
supposing that all bodies produced spectra proportional to their 
mean refraction, was detected a few years after his death by Hall 
and Dollond, who discovered the different dispersive powers of 
bodies, and who were both led, by independent inquiries, to the 
invention and construction of the achromatic telescope. At a 
later period the discovery of the irrationality of the coloured 
spaces in the spectrum by Clairaut and Boscovich, furnished Dr. 
Blair with the general principle of the aplanatic telescope, and 
enabled him to construct fluid object-glasses, in which a perfect 
correction of colour was effected. These two instruments were 
doubtless the most valuable gifts which one science has pre¬ 
sented to another ; and the kindred subjects of navigation and 
practical astronomy exhibit in the perfection of their methods 
the great benefits which they have thus received. 
After having slumbered for a hundred years, Physical Optics 
began to revive about the close of the eighteenth century. The 
events of the French Revolution had summoned from a state 
