Account ^Huygens’ Theory qf Double Refracticm, SS5 
Remarks on Huygens' Discoveries, 
The preceding analysis of the phenomena of double refrac-* 
tion as exhibited in Iceland spar^ will always be considered as 
an example of the fine genius and discrimination of Huygens. 
By means of the happy idea, that light was propagated through 
this mineral in spheroidal in place of spherical waves, he was led 
to a correct generalisation of the phenomena, which possesses a 
high value, independent of the hypothesis on which it is founded. 
Huygens, however, never intended, that the law to which he 
had referred the double refraction of Iceland spar, should be 
considered as the law of double refraction. On the contrary, he 
maintains, that he had found, by direct experiment, that the on- 
ly other doubly refracting crystal with which he was acquainted, 
namely. Rock Crystal^ had its double refraction regulated by 
an entirely different law^ the light being propagated through it 
in two spherical waves, one of which was a little slower than 
the other This result was obtained, by cutting prisms of 
rock-crystal in different directions, and is mentioned in his 
Preface, as a new remark which he had made after the first 
copy of his treatise had been communicated to some of the 
Members of the Academy of Sciences. Huygens, therefore, 
proposed two different laws of double refraction, the one for 
Iceland spar, and the other for Rock-crystal. The last of these, 
founded on incorrect observations, has no existence in nature, 
while the first represents the phenomena of Iceland spar, with- 
in the limits of the errors of observation. 
* As this passage is very interesting, we shall give it in his own words. 
“ La double emanation d’ondes de lumiere, que je m’estois imaginee, me devint 
plus probable apres certain phenomene que j’observay dans la crystal ordinaire 
(Rock crystal), qui croit en forme hexagone, et qui, a cause de cette regularite, 
semble aussi estre compose de particules de certaine figure, et rangees avec ordre. 
C’estoit que ce cristal a une double refraction, aussi bien que celuy d’lsland, quoy- 
que moins evideftte. Car, en ayant fait tailler des prismes bien polis, par des sec- 
tions differentes, je remarquay dans tons, en regardant la flame de la chandelle i 
travers, ou le plomb des vitres qui sont aux fenetres, que tout paroissoit double, 
quoyqu’ avec des images, peu distantes entre elles. 
“ Or, cette double refraction, suivant ma theorie cy dessus etablie, sembloit c?e- 
mander une double emanation d'ondes de Lumiere tout deux spheriques ( car les deux 
refractions sont regulieres et les unes seulement un peu plus lentes que les aufres^^ 
Traite de la Lumiere, chap. v. § 20, 21. 
VOL. Ill, KO, 6. OCTOBER 1820. U 
