124 Account of Isaac Newton’s Investigations 
no less constant inconstancies by variation, were (and still con- 
tinue,) mysteries reserved to later posterities.” 
A^t. ^Wll.-^Histoi'icalAccount of Discoveries respecting the 
Double RefractioJi arid Polarisation of Light, (Continued 
from Vol. III. p. 285.) 
Period III. — Containing the Investigations qf Newton, 
Beccaria, Martin, Hauy, Wollaston, and La Place, 
Sect. I. — Account of the Investigations of Sir Isaac 
Newton. 
T- HE only observations which Sir Isaac Newton appears to 
have published on the subject of double refraction and polarisa- 
tion, are contained in the Queries printed at the end of the 8d 
Book of his Optics. As they are written with great perspicui- 
ty, and easily understood, we shall lay them before our readers 
in his own words. 
Query 25. Are there not other original properties of the 
rays of light, besides those already described An instance of 
another original property we have in the refraction of Iceland 
Crystal, described first by Erasmus Bartholine, and afterwards 
more exactly by Hugenius, in his book De la Lumiere. This 
crystal is a pellucid fissile stone, clear as water or crystal of the 
ixjckj and without colour ; enduring a red heat without losing 
its transparency, and in a very strong heat calcining without fu- 
sion. Steeped a day or two in water, it loses its, natural polish. 
Being rubbed on cloth, it attracts pieces of straws and other 
Sight things, like amber or glass ; and with aqua fortis it makes 
an ebullition. It seems to be a sort of talc, and is found in form 
of an oblique paralielopiped,with six parallelogram sides and eight 
solid angles. The obtuse angles of the parallelograms are each of 
them 101^^52' ; the acute ones 78° 8'. Two of the solid angles op- 
posite to one another, as C and E, (SeeTlate III. Fig. 8.) are com^ 
passed each of them with three of these obtuse angles, and each 
of the other six with one obtuse and two acute ones. It cleaves 
easily in planes parallel to .any of its sides, and not in any other 
planes. It cleaves with a glossy polite surface not perfectly plane. 
