_Formation of a Luminous Image by a Lens._--As the rays of light proceeding from a point are brought to a focus by means of a Lens, so are they when they proceed from an object, and in that case _an image of the object_ is the result. [Illustration] The above figure illustrates this. The size of the image varies with the distance of the arrow from the glass--being larger and formed at a point further from the Lens as the object is brought nearer. The refracting power of the Lens also influences the result--lenses of short focal length, _i. e._ more convex, giving a smaller image. In order that the course pursued by pencils of rays proceeding from an object may be easily traced, the lines from the barb of the arrow in the last figure are _dotted_. Observe that the object is necessarily _inverted_, and also that those rays which traverse the central point of the Lens, or the centre of the _axis_, as it is termed, are not bent away, but pursue a course either coincident with, or parallel to, the original, as in the case of refracting media with parallel surfaces. SECTION III. The Photographic Camera. The Photographic Camera is in its essential nature an extremely simple instrument. It consists merely of a _dark chamber_, having an aperture in front in which a Lens is inserted. The accompanying figure shows the simplest form of Camera. [Illustration] The body is represented as consisting of two portions which slide within each other; but the same object of lengthening or shortening the focal distance may be attained by making the Lens itself movable. A luminous image of any object placed in front of the Camera is formed by means of the Lens, and received upon a surface of ground glass at the back part of the instrument. When the Camera is required for use, the object is _focussed_ upon the ground glass, which is then removed, and a slide containing the sensitive layer inserted in its place. The luminous image, as formed upon the ground glass, is termed the "Field" of the Camera; it is spoken of as being flat or curved, sharp or indistinct, etc. These and other peculiarities which depend upon the construction of the Lens will now be explained. _Chromatic Aberration of Lenses._--The outside of a biconvex lens is strictly comparable with the sharp edge of a _prism_, and therefore necessarily produces decomposition in the white light which passes through it. The action of a prism in separating white light into its constituent rays may be simply explained;--all the coloured rays are refrangible, but not to the same extent. The indigo and violet are more so than the yellow and red, and consequently they are separated from them, and occupy a higher position in the Spectrum. (See the diagram at p. 47.)