object. He ascribed an erect vision to an operation of the
mind, by which it traces the rays back to the pupil, where they cross
one another, and thus refers the lower parts of the image to the higher
parts of the object. He also explained the cause of long-sighted and
short-sighted vision, and shewed how convex and concave lenses enabled
those who possessed these peculiarities of vision to see distinctly, by
accurately converging the pencils of rays to a focus on the retina.
Kepler likewise observed the power of accommodating the eye to different
distances, and he ascribed it to the contraction of the ciliary
processes, which drew the sides of the eyeball towards the crystalline
lens, and thus elongated the eye so as to produce an adjustment of it
for near objects. Kepler wisely declined to inquire into the way in
which the mind perceives the images painted on the retina, and he
blames Vitellio for attempting to determine a question which he
considered as not belonging to optics.

The work of Kepler, now under consideration, contains the method of
calculating eclipses which is now in use at the present day.

The only other optical treatise written by Kepler, was his _Dioptrics_,
with an appendix on the use of optics in philosophy. This admirable
work, which laid the foundation of the science, was published at
Augsburg in 1611, and reprinted at London in 1653. Although Maurolycus
had made some slight progress in studying the passage of light through
different media, yet it is to Kepler that we owe the methods of tracing
the progress of rays through transparent bodies with convex and concave
surfaces, and of determining the foci of lenses, and of the relative
positions of the images which they form, and the objects from which the
rays proceed. He was thus led to explain the _rationale_ of the
telescope, and to invent the astronomical telescope, which consists of
two convex lenses, by which objects are seen inverted. Kepler also
discovered the important fact, that spherical surfaces were not capable
of converging rays to a single focus, and he conjectured, what
Descartes afterwards proved, that this property might be possessed by
lenses having the figure of some of the sections of the cone. The total
reflection of light at the second surface of bodies was likewise studied
by Kepler, and he determined that the total reflection commenced when
the angle of incidence was equal to the angle of refraction, which
corresponded t