the line CD making a straight line with the remainder of that line which is outside the Crystal.  And then, marking on the surface of the Crystal the point H where the intersection E appears, this point will be directly above E. Then draw back the eye towards O, keeping always in the plane perpendicular through ab, so that the image of the line CD, which is formed by ordinary refraction, may appear in a straight line with the line Kl viewed without refraction; and then mark on the Crystal the point N where the point of intersection E appears.

13.  Then one will know the length and position of the lines NH, Em, and of he, which is the thickness of the Crystal:  which lines being traced separately upon a plan, and then joining NE and nm which cuts he at P, the proportion of the refraction will be that of en to NP, because these lines are to one another as the sines of the angles NPH, NEP, which are equal to those which the incident ray on and its refraction NE make with the perpendicular to the surface.  This proportion, as I have said, is sufficiently precisely as 5 to 3, and is always the same for all inclinations of the incident ray.

14.  The same mode of observation has also served me for examining the 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.

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