s the simple camera
obscura and mentions the improvement some one had made in it by the use
of a double convex lens in the aperture; he also says that the images
could be made erect by reflection from any plane mirror.

Thus the use of the camera and of the lens with it was well known before
Porta published his second edition of the _Magia Naturalis_ in 1589. In
this the description of the camera obscura is in lib. xvii. cap. 6. The
use of the convex lens, which is given as a great secret, in place of
the concave speculum of the first edition, is not so clearly described
as by Barbaro; the addition of the concave speculum is proposed for
making the images larger and clearer, and also for making them erect,
but no details are given. He describes some entertaining peep-show
arrangements, possibly similar to Alberti's, and indicates how the dark
chamber with a concave speculum can be used for observing eclipses.
There is no mention whatever of a portable box or construction beyond
the darkened room, nor is there in his later work, _De Refractione
Optices Parte_ (1593), in which he discusses the analogy between vision
and the simple dark room with an aperture, but incorrectly. Though
Porta's merits were undoubtedly great, he did not invent or improve the
camera obscura. His only novelty was the use of it as a peep-show; his
descriptions of it are vague, but being published in a book of general
reference, which became popular, he acquired credit for the invention.

The first to take up the camera obscura after Porta was Kepler, who used
it in the old way for solar observations in 1600, and in his _Ad
Vitellionem Paralipomena_ (1604) discusses the early problems of the
passages of light through small apertures, and the rationale of the
simple dark chamber. He was the first to describe an instrument fitted
with a sight and paper screen for observing the diameters of the sun and
moon in a dark room. In his later book, _Dioptrice_ (1611), he fully
discusses refraction and the use of lenses, showing the action of the
double convex lens in the camera obscura, with the principles which
regulate its use and the reason of the reversal of the image. He also
demonstrates how enlarged images can be produced and projected on paper
by using a concave lens at a suitable distance behind the convex, as in
modern telephotographic lenses. He was the first to use the term _camera
obscura_, and in a letter from Sir H. Wotton written to Lord Baco