An old device for this purpose (W. J. Read, 1858) is an oblique diaphragm (Fig. 92), which was placed approximately at right angles to rays coming from the foreground, but which progressively reduced the light coming from the horizon or sky.
A device which allows of many variations consists in fixing an opaque screen with a serrated lower edge at some distance in front of the lens (Woodbury, 1885). The shape of the notched edge can be easily modified as required by making use of a screen consisting of a number of adjacent strips sliding in a suitable mount. A screen which is easily made is that shown in Fig. 93 (Busch, 1908). By rotation of the fixing ring on the lens mount and of the blade on the pivot, the serrated edge can be fixed at various distances from the optical axis or inclined at any angle, the effect produced being controlled by examination of the picture on the focussing screen.
The most usual form of commercial sky shade is that suggested by E. joly in 1892, and con sists of a uniformly graduated filter of gelatine or glass, usually of a yellow colour, which can be used as an orthochromatic filter when occa sion arises. Such sky filters are made in the shape of a long rectangle, which is carried in a mount, allowing it to be raised or lowered according to the effect desired. The shorter side of the rectangle should be wide enough to cover the lens when placed at a short distance from it, even when the longer side of the picture is horizontal. The farther these screens are placed from the lens, the greater is their effect. In the extreme position, i.e. if they could be placed in the plane of the diaphragm, their only effect would be a uniform absorption of a certain proportion of the light without any difference between the sky and the rest of the picture. The effect of a filter such as AB depends upon its differential absorbing capacity, which be comes greater as the aperture of the lens is reduced and the distance between the lens and the screen is increased (Fig. 94). The same results would obviously be obtained by placing a screen inside the camera (in the position shown by A'.81, but it is then difficult to adjust to
its best position. In practice, equivalent results can be obtained by the use of a sky filter, one half of which is uniformly coloured and the rest plain, the only condition being that it must be used slightly nearer the lens, such as the position CD. Such an arrangement even allows the exposure to be shortened, especially when using a non-orthochromatic plate.' It should be pointed out that in many cases the moderate use of these accessories distinctly improves the rendering of skies ; any exaggera tion of the effect should be avoided, since the sky takes on an unnatural aspect, suggesting that its colour near the zenith is a deep indigo.
Such photographs of clouds can have no value whatever as meteorological records.
126. Soft-focus Attachments. In passing, mention may be made of the relatively recent appearance of devices, which, when used with a perfectly corrected lens, introduce slight softening of the definition without altering the focal length or the position of the image.
In particular, the use of a piece of glass with plane and parallel faces, one of which is slightly grooved, has been suggested (Lenhard, 1890). These grooves superimpose a blurred image on to the sharp image transmitted through their interstices. If the grooves occur only on the extreme margins of the glass, the closing of the diaphragm will sharpen the definition, since only the central plane-parallel portion of the glass is used. :::, .
The use has also been suggested of a kind of comb with long triangular teeth cut from colour less embossed celluloid (Misonne, 1932) arranged to slide so as to cover at will a more or less extensive portion of each bundle of rays, or of a plate with plane and parallel faces which is constructed by assembling two lenses, a piano convex and a piano-concave, of glasses of about the same refractivity but widely different dis persive powers, thus introducing chromatic aberrations into the picture.