h and
without lenses, and made great advances in this direction over his
predecessors. He was the first correctly to solve Aristotle's problem,
stated above, and to apply it practically to solar observations in a
darkened room (_Cosmographia_, 1535). Erasmus Reinhold has described the
method in his edition of G. Purbach's _Theoricae Novae Planetarum_
(1542), and probably got it from Maurolycus. He says it can also be
applied to terrestrial objects, though he only used it for the sun. His
pupil, Rainer Gemma-Frisius, used it for the observation of the solar
eclipse of January 1544 at Louvain, and fully described the methods he
adopted for making measurements and drawings of the eclipsed sun, in his
_De Radio Astronomico et Geometrico_ (1545). He says they can be used
for observation of the moon and stars and also for longitudes. The same
arrangement was used by Copernicus, Tycho Brahe, by M. Moestlin and his
pupil Kepler--the latter applying it in 1607 to the observation of a
transit of Mercury--also by Johann Fabricius, in 1611, for the first
observations of sun-spots. It is interesting to note this early
employment of the camera obscura in the field of astronomical research,
in which its latest achievements have been of such pre-eminent value.

The addition of optical appliances to the simple dark chamber for the
purpose of seeing what was going on outside, was first described by
Girolamo Cardan in his _De Subtilitate_ (1550), as noted by Libri. The
sun shining, he fixed a round glass speculum (_orbem e vitro_) in a
window-shutter, and then closing it the images of outside objects would
be seen transmitted through the aperture on to the opposite wall, or
better, a white paper screen suitably placed. The account is not very
clear, but seems to imply the use of a concave mirror rather than a
lens, which might be suggested by the word _orbem_. He refers to
Maurolycus' work with concave specula.

We now come to Giovanni Battista della Porta, whose account of the
camera obscura in the first edition of the _Magia Naturalis_, in four
books (1558, lib. iv. cap. 2), is very similar to Caesariano's--a
darkened room, a pyramidal aperture towards the sun, and a whitened wall
or white paper screens, but no lens. He discloses as a great secret the
use of a concave speculum in front of the aperture, to collect the rays
passing through it, when the images will be seen reversed, but by
prolonging them beyond the centre they would be s