57. Effect of Internal Reflection. The light reflected at each free surface is, unfortunately, not lost ; a part of the beam which has suffered several internal reflections is sent back to the object, but another part passes on to the plate. In Fig. 39 it is seen that the beam which forms the image P also gives an image P, on the side of the object, after one internal reflection, and an image on the side of the image P after two internal reflections. If the incident beam is sufficiently intense, and if the exposure is sufficiently long, these images will be registered on the photographic plate as circular or elliptical areas of relatively large dimensions. The num ber of parasite (ghost) images reflected to the plate will be greater the greater the number of glass-air surfaces. The intensity of the images diminishes according as the number of reflections the beam has undergone is greater.
Number of glass-air surfaces . . 2 4 6 8 Number of ghost images . . . x 6I 5 28 These ghost images appear frequently in photographs taken at night which have had a long exposure and where the view contains light sources of great intensity towards the edge of the field. Owing to the symmetry of the lens round its axis, the secondary axis of the different beams arising from the same original beam are contained in a meridian plane. The centres of the areas corresponding with a single point source are thus all situated on the straight line which joins the image of the point and the point where the optical axis cuts the sensitive surface. Fig. 40 (from R. Sch:Ittauf) shows the limits of the six ghost images given by a rectilinear lens (symmetrical lens of two groups each consisting of two cemented lenses, so that there are in all four glass-air surfaces) where the object is a bright point on a black background.' In some of the old lenses one of the internally reflected beams gave almost a sharp image of the stop in the image plane, somewhat enlarged, centred on the optical axis, and superposing a bright patch (called the central flare spot) on the image.
In regular photographic work these ghost images are not seen individually, but the light directed towards the plate after internal reflec tion forms a slight fog over the whole image, reducing contrast. Fig. 41 (from measurements made by E. Goldberg) shows the effect of these internal reflections for an objective containing four independent lenses (eight glass-air surfaces) photographing a landscape of which HH is the horizon. Successive reflections of the light from
the sky produce on that part of the plate on which the landscape is recorded an amount of light which decreases as the distance from the horizon line becomes greater. The circles in dotted lines correspond with different obliquities of the beam ; the curves in full line join points of the image in which the parasite light has an intensity equal to 6 per cent, 5 per cent, . . . 2 per cent of that in the image of the sky. The intensity of this parasitical light is reduced appreciably when the lens is stopped down.2 The same author has been able to establish the fact that from these reflections and from the diffusion that is unavoidable at surfaces, even if perfectly polished and kept perfectly clean, the extreme contrast in the image yielded by a lens is always less than in the subject itself examined from the same viewpoint. A subject having a range of contrasts infinitely great is reduced to a contrast about 200 :1 with a single lens and to about 6o : with an anastig mat giving a sharp image over a relatively large field.
These measurements confirm the experience of the old photographers, who used for landscape work a single objective consisting of a number of lenses cemented together, considering that this type gave more brilliant images.' 58. Stereoscopic Effects. A lens of very large diameter, such as some at one time used as portrait lenses, gives an image of a near object in which appear certain parts of the subject which an eye (placed as close as possible to the lens) would see only if moved from right to left (Brewster, 186o) and up and down. The image of a small cube isolated in space, e.g. a die suspended by a thread in the optical axis of such a lens, would show five faces (Fig. 42), presenting thus the appearance of a truncated pyramid seen from the direction of the small end.
It has long been recognized that it is possible to obtain with such a lens, fixed with respect to the object photographed, two stereoscopic images by using an eccentric stop, rotated. through i8o° between the exposures, the aper ture being I/ in. in a horizontal direction from the centre, so that two successive positions of the aperture are at a distance apart equal to the mean separation of the eyes. The diaphragm merely extracts from the complete image certain details by isolating certain light-rays.