CAMERA OBSCURA

105

camera obscura, which was extensively used in sketching from
nature, before the introduction of photography, although it is
now scarcely to be, seen except as an interesting side-show at
places of popular resort. The image formed on the paper may
be traced out by a pencil, and it will be noticed that in this case
the image is real - not virtual as in the case of the camera
lucida. Generally the mirror and lens are combined into a
single piece of worked glass represented in section in fig. 2.
[Fig. 2 appears here inset on the left]
Rays from external objects are first re-
fracted at the convex surface a b, theu totally
reflected at the plane surface a c, and finally
refracted at the concave surface b c (fig. 2)
So as to form an image on the sheet of paper
d e. The curved surfaces take the place of
the lens in fig. 1, and the plane surface per-
forms the function of the mirror. The prism
a b c is fixed at the top of a small tent fur-
nished with opaque curtains so as to prevent the diffused day-
light from overpowering the image on the paper, and in the
darkened tent the images of external objects are seen very
distinctly.

Quite recently, the camera ohscura has come into use with
submarine vessels, the periscqpe being simply a camera obscura
under a new name. (C.J.J)

Fig. 2.

History. - The invention of this instrument has generally been
ascnbed, as in the nInth edition of this work, to the famous
Neapolitan savant of the 16th century, Giovanni Battista della
Porta, but as a matter of fact the principle of the simple camera
obscura, or darkened chamber with a small aperture in a window
or shutter, was well known and in practical use for observing
eclipses long before his time. He was anticipated in the improve-
ments he claimed to have made in it, and all he seems really to
have done was to popularize it. The increasing importance
of the camera obscura as a photographic instrument makes it
desirable to bring together what is known of its early history,
which is far more extensive than is usually recognized. In
southern climes, where during the summer heat it is usual to
close the rooms from the glare of the sunshine outside, we may
often see depicted on the walls vivid inverted images of outside
objects formed by the light reflected from them passing through
chinks or small apertures in doors or window shutters. From
the opening passage of Euclid's Optics (c. 300 B.C.), which
formed the foundation for some of the earlier middle age treatises
on geometrical perspective, it would appear that the above
phenomena of the simple darkened room were used by him to
demonstrate the rectilinear propagation of light by the passage
of sunbeams or the projection of the images of objects through
small openings in windows, &c. In the book known as Aris-
totle's Problems (sect. xv. cap. 5) we find the correlated problem
of the image of the sun passing through a quadrilateral aperture
always appearing round, and he further notes the lunated image
of the eclipsed sun projected in the same way through the
interstices of foliage or lattice-work.

There are, however, very few allusions to these phenomena
in the later classical Greek and Roman writers, and we find the
first scientific investigation of them in the great optical treatise
of the Arabian philosopher Alhazen (q.v.), who died at Cairo in
A.D. 1038. He seems to have been well acquainted with the
projection of images of objectsth through small apertures, and to
have been the first to show that the arrival of the image of an
object at the concave surface of the common nerve - or the
retina - corresponds with the passage of light from an object
through an aperture in a darkened place, from which it falls
upon a surface facing the aperture. He also had some knowledge
of the properties of concave and convex lonses and mirrors in as
formimg images. Some two hundred years later, between
A.D. 1266 and 1279, these problems were taken up by three
almost contemporaneous writers on optics, two of whom, Roger
Bacon and John Peckham, were Englishmen, and Vitello or
Witelo, a Pole.

That Roger Bacon was acquainted with the principle of the
camera obscura is shown by his attempt at solving Aristotle's
[column 2 starts here]
problem stated above, in the treatise De Speculis, and also from
his references to Alhazen's experiments of the same kind, but
although Dr John Freind,in his History of Physick, has given him
the credit of the invention on the strength of a passage in the
Perspective, there is nothing to show that he construceed any
instrument of the kind. His arrangement of concave and plane
mirrors, by which the realistic images of objects inside the house
oz in the street could be rendered visible though intangihle,
there alluded to, may apply to a camera on Cardan's principle or
to a method of aerial projection by means of concave mirrors,
which Bacon was quite familiar with, and indeed was known
long before his time. On the stregth of similar arrangements of
lenses and mirrors the invention of the camera obscura has also
been claimed for Leonard Digges, the author of Pantometria
(1571), who is said to hilve constructed a telescope from informa-
tion given in a book of Bacon's experiments.

Archbishop Peckham, or Pisanus, in his Perspectiva Communis
(1279), and Vitello, in his Optics (1270), also attempted the
solution of Aristotle's problem, but unsuccessfully. Vitello's
work is to a very great extent based upon Alhazen and some of
the earlier writers, and was first published in 1535. A later
edition was published, together with a translation of Alhazen,
by F. Risner in 1572.

The first practical step towards the development of the camera
obscura seems to have been made by the famous painter and
architect, Leon Battista Alberti, in 1437, contempoaneously
with the invention of printing. It is not clear, however, whether
his invention was a camera obscura or a show box, but in a
fragment of an anonymous biography of, him, published in
Muratori's Rerum Itaicarum Scriptores (xxv. 296), quoted by
Vasari, it is stated that he produced wonderfully painted
pictures, which were exhibited by him in some sort of small
closed box through a very small aperture, with great verisimili-
tude. These demonstrations were of two kinds, one nocturnal,
showing the moon and bright stars, the other diurnal, for day
scenes. This description seems to refer to an arrangement of a
transparent painting illuminated either from the back or the front
and the image projected through a hole onto a white screen in a
darkened room, as described by Porta (Mag. Nat. xvii. cap. 7)
and figured by A. Kircher (Ars Magna Lucis et Umbrae), who
notes elsewhere that Porta had taken some arrangement of pro-
jecting images from an Albertus; whom he distinguished from
Albertus Magnus, and who was probably L.B. Alberti, to whom
Porta also refers, but not in this connexion.

G. B. I. T. Libri-Carucci dalla Sommaja (1803-1869), in his
account of the invention of the camera obscura in Italy (Histoire
des sciences mathématiques en Italie, iv. 303), makes no mention
of Alberti, but draws attention to an unpublished MS. of Leonardo
da Vinci, which was first noticed by Venturi in 1797, and has
since becn published in facsimile in vol. ii. of J. G. F. Ravaisson-
Mollien's rcproductions of thc MSS. in the Institut de France at
Paris (MS, D, fol. 8 recto). After discussing the structure of the
eye he gives an experiment in which the appearance of the
reversed images of outside objects on a piece of paper held in
front of a small hole in a darkened room, with their forms and
colours, is quite clearly described and explained with a diagram,
as an illustration of the phenomena of vision. Another similar
passage is quoted by Richter from folio 404b of the reproduc-
tion of the Codice Atlantico, in Milan, published by the Italian
government. These are probably the earliest distinct accounts
of the natural phenomena of the camera obscura, but remained
unpublished for some three centuries. Leonardo also discussed
the old Aristotelian problem of the rotundity of the sun's image
after passing through an angular aperture, but not so successfully
as Maurolycus. He has also given methods of measuring the
sun's distance by means of images thrown on screens through
small apertures. He was well acquainted with the use of magni-
fying glasses and suggested a kind of telescope for viewing the
moon, but does not seem to have thought of applying a lens to
the camera.

The first published account of the simple camera obscura was
discovered by Libri in a translation of the Architecture of

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