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Updated: May 10, 2025
as five-stress iambic, rhyming aa. The familiar measure of English ballad poetry, "The King has written a braid letter, And signed it wi' his hand, And sent it to Sir Patrick Spence, Was walking on the sand" is alternating four-stress and three-stress iambic, rhyming ab cb. The In Memoriam stanza,
For the triangles ACB, BNA being rectangular and having the side AB common, and the side CB equal to NA, it follows that the angles opposite to these sides will be equal, and therefore also the angles CBA, NAB. But as CB, perpendicular to CA, marks the direction of the incident ray, so AN, perpendicular to the wave BN, marks the direction of the reflected ray; hence these rays are equally inclined to the plane AB.
Supposing now a flat plate, BC = 12 in. wide move from DA to CB during 0.025 second, it will be readily seen that a drop of water starting from A will have arrived at C in 0.025 second, having been flowing along the surface BC from B to C without either friction or loss of velocity. This concave surface may represent one bucket of a turbine.
Movement will then be spread from the point A, in the matter of the transparent body through a distance equal to two-thirds of CB, making its own particular spherical wave according to what has been said before.
In order to show further that the surfaces, which these curves will generate by revolution, will direct all the rays which reach them from the point A in such wise that they tend towards B, let there be supposed a point K in the curve, farther from D than C is, but such that the straight line AK falls from outside upon the curve which serves for the refraction; and from the centre B let the arc KS be described, cutting BD at S, and the straight line CB at R; and from the centre A describe the arc DN meeting AK at N.
Since the sums of the times along AK, KB, and along AC, CB are equal, if from the former sum one deducts the time along KB, and if from the other one deducts the time along RB, there will remain the time along AK as equal to the time along the two parts AC, CR. Consequently in the time that the light has come along AK it will also have come along AC and will in addition have made, in the medium from the centre C, a partial spherical wave, having a semi-diameter equal to CR. And this wave will necessarily touch the circumference KS at R, since CB cuts this circumference at right angles.
For this point, having been found in this fashion, it is easy forthwith to demonstrate that the time along AC, CB, will be equal to the time along AD, DB.
And also, as this is a double curve, the point at which the curvature changes from one direction to the other: point C. By drawing lines CA, CB and noting the distances your curves travel from these straight lines, and particularly the relative position of the farthest points reached, their curvature can be accurately observed and copied.
Bartholinus, in his treatise of this Crystal, puts at 101 degrees the obtuse angles of the faces, which I have stated to be 101 degrees 52 minutes. He states that he measured these angles directly on the crystal, which is difficult to do with ultimate exactitude, because the edges such as CA, CB, in this figure, are generally worn, and not quite straight.
That is to say that if the ray AB in entering the transparent body is refracted into BC, then likewise CB being taken as a ray in the interior of this body will be refracted, on passing out, into BA.
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