[1] When the writer visited M. D'Arrest, the astronomer, at
Copenhagen, in 1872, he was presented by D'Arrest with one of several
bricks collected from the ruins of Uraniborg. This was one of his most
cherished possessions until, on returning home after a prolonged
absence on astronomical work, he found that his treasure had been
tidied away from his study.

[2] An ellipse is one of the plane, sections of a cone. It is an oval
curve, which may be drawn by fixing two pins in a sheet of paper at S
and H, fastening a string, SPH, to the two pins, and stretching it
with a pencil point at P, and moving the pencil point, while the
string is kept taut, to trace the oval ellipse, APB. S and H are the
_foci_. Kepler found the sun to be in one focus, say S. AB is the
_major axis_. DE is the _minor axis_. C is the _centre_. The direction
of AB is the _line of apses_. The ratio of CS to CA is the
_excentricity_. The position of the planet at A is the _perihelion_
(nearest to the sun). The position of the planet at B is the
_aphelion_ (farthest from the sun). The angle ASP is the _anomaly_
when the planet is at P. CA or a line drawn from S to D is the _mean
distance_ of the planet from the sun.

[Illustration]

[3] The ruled logarithmic paper we now use was not then to be had by
going into a stationer's shop. Else he would have accomplished this in
five minutes.