1 68 Discoveries of Christopher Huygens respecting 
begin this investigation principally with the view of obviating 
any objection that might be drawn from the facts discovered by 
Bartholinus, against his own theory of ordinary refraction, and 
he was led to the particular views which he has published, from 
a desire to assimilate the two classes of phenomena. 
His researches on this subject form the ffth chapter of his 
Traite de la Lumiere^ which is entitled De Vestrange refrac- 
tion du Cristal dUIslande. This work was composed about the 
year 1678, and read to several of the learned individuals who 
then composed the Academy of Sciences ; but it was not publish- 
ed till the year 1690, when its author was resident in Holland. 
After a few preliminary observations, in which he gives Bar- 
tholinus the credit of having discovered some of the principal 
phenomena, he supposes ABEF, Plate IV. Fig. 1. to be a piece 
of Iceland cry>stal, and conceiving one of the three obtuse angles, 
which form the solid angle C, namely ACB to be bisected by CG, 
he calls the plane CGHF, which passes through this line, and 
the side CF, the Principal Section of the crystal. 
If the surface A B is now exposed to the sun, being all cover- 
ed but a small aperture K in CG, and if a ray IK is incident 
perpendicularly at K, it will be divided at the point K into two 
rays, one of which KL, will be a continuation of IK, while the 
other KM, will deviate from KL towards C, by an angle of 
6*" 40', but will still be in the plane CGHF. This ray will 
emerge at M in the direction MZ parallel to IK. Hence, since 
the point M, by the extraordinary refraction, is seen by the re- 
fracted ray MKI, the eye being at I, any point or aperture at 
L, by the same refraction, will be seen by the refracted ray 
LKI, LK being parallel to MK, if I is very distant. The 
point L will consequently be seen in the direction IRS, and as 
the same point is seen by the ordinary refraction in the direc- 
tion IK, it will necessai’ily appear double. 
If the ray now falls in the direction NO, in the plane CGHF, 
making an angle of 73° SO' with CG, or nearly parallel to CF, 
which makes with FH an angle of 70'" 57', it will be divided at 
O into two rays, one of which will be a continuation of NO, 
without refraction, while the other will be refracted in the direc- 
tion OQ, both rays being in the plane CGHF. This is true 
of all planes parallel to the principal section. 
