310 
SECOND REPORT -1832. 
The same discovery was undoubtedly made a few years after¬ 
wards by Mr. James Gregory; but though be never saw the 
MSS. of Snellius, as Descartes is believed to have done, neither 
he nor bis countrymen have endeavoured to disturb the laurels 
which have so long and so justly decorated the Flemish philo¬ 
sopher. 
The next important step in the history of Physical Optics 
was the discovery of double refraction by Bartholinus, a Danish 
Professor, who published an account of his experiments in 
1669. After a careful examination of the phenomena as they 
appeared in Iceland spar , he discovered that one of the refrac¬ 
tions was performed according to the ordinary law of Snellius, 
and the other according to an extraordinary rule which had 
not been recognised by philosophers. About nine years after 
the publication of this work, the celebrated Christian Huygens 
directed his attention to the same subject. He had long main¬ 
tained the doctrine that light consists in the vibrations of an 
elastic medium, and he had succeeded in applying it to explain 
the rectilineal propagation of light;—the equality of the angles 
of incidence and reflexion ;—the constant ratio of the sines ;— 
the total reflexion of light, and the relation between the reflec¬ 
tive and the refractive forces. While he was engaged in these 
researches, he became acquainted with Newton’s discovery of 
the different refrangibility of light; and though it had a tendency 
to unsettle his previous opinions, yet he viewed it with an un¬ 
favourable eye, and remained firmly attached to the undula- 
tory hypothesis. 
In order to explain the phenomenon of two separate refrac¬ 
tions, Huygens imagined an hypothesis before he had made a 
single experiment on the subject. He conceived that the ordi¬ 
nary refraction was produced by spherical emanations of light 
similar to those which take place in the elastic medium that 
pervades all transparent bodies, while the extraordinary refrac¬ 
tion was produced by spheroidal emanations propagated indif¬ 
ferently through the elastic medium and the solid particles of 
the body. This singular idea was immediately submitted to 
the test of experiment; and Huygens had the satisfaction of 
finding that it represented in the most accurate manner all the 
phenomena which had been observed by Bartholinus and 
himself. This law, confirmed by every subsequent inquiry, 
and certainly one of the most remarkable in physics, is now 
the law of double refraction for all crystals with one axis; and 
it deserves to be especially noticed, that it was discovered 
by the hypothetical method which Kepler had employed with 
such brilliant success. 
