extraordinary or irregular refraction of this Crystal. For, the point
H having been found and marked, as aforesaid, directly above the point
E, I observed the appearance of the line CD, which is made by the
extraordinary refraction; and having placed the eye at Q, so that this
appearance made a straight line with the line KL viewed without
refraction, I ascertained the triangles REH, RES, and consequently the
angles RSH, RES, which the incident and the refracted ray make with
the perpendicular.

15. But I found in this refraction that the ratio of FR to RS was not
constant, like the ordinary refraction, but that it varied with the
varying obliquity of the incident ray.

16. I found also that when QRE made a straight line, that is, when the
incident ray entered the Crystal without being refracted (as I
ascertained by the circumstance that then the point E viewed by the
extraordinary refraction appeared in the line CD, as seen without
refraction) I found, I say, then that the angle QRG was 73 degrees 20
minutes, as has been already remarked; and so it is not the ray
parallel to the edge of the Crystal, which crosses it in a straight
line without being refracted, as Mr. Bartholinus believed, since that
inclination is only 70 degrees 57 minutes, as was stated above. And
this is to be noted, in order that no one may search in vain for the
cause of the singular property of this ray in its parallelism to the
edges mentioned.

[Illustration]

17. Finally, continuing my observations to discover the nature of
this refraction, I learned that it obeyed the following remarkable