1911] ON THE GENERAL PROBLEM OF PHOTOGRAPHIC REPRODUCTION 69
should begin. When the printing process is laid down and the character of the results yielded thereby is determined, it becomes possible to say what is required in the negative; but it is not possible before.
In many photographs it would appear that gradation tends to be lost at the ends of the scale, that is in the high lights and deep shadow's, and (as a necessary consequence, if the full range is preserved) to be exaggerated in the half-tones. For some purposes, where precise reproduction is not desired, this feature may be of advantage. Consider, for example, the experimental problem, discussed by Huggins, of photographing the solar corona without an eclipse. The corona is always present, but is overpowered by atmospheric glare. The problem is to render evident a very small relative difference of luminous intensity. If the difference is exaggerated in a suitably exposed and developed photograph, so much the better. A repetition of successive copyings might render conspicuous a difference originally invisible. At each operation we may suppose a factor a to be introduced, a being greater than unity. After n copyings dt/t becomes andt/t. Unless the gain each time were very decided, this would be a slow process, and it would be liable to fail in practice owing to multiplication of slight irregular photographic markings. But a method proposed by Mach* and the present writer t should be of service here. By the aid of reflexion light at each stage is transmitted twice through the picture. By this means alone a is raised to equality with 2, and upon it any purely photographic exaggeration of gradation is superposed. Three successive copyings on this plan should ensure at least a ten-fold exaltation of contrast.
Another method, simpler in execution, consists in superposing a considerable number (n) of similar pictures. In this way the contrast is multiplied n times. Rays from a small, but powerful, source of light fall first upon a collimating lens, so as to traverse the pile of pictures as a parallel beam. Another condensing lens brings the rays to a focus, at which point the eye is placed. Some trials on this plan made a year ago gave promising results. Ten lantern-slides were prepared from a portrait negative. The exposure (to gas-light) was for about 3 seconds through the negative and for 30 seconds bare, i.e. with negative removed, and the development was rather light. On single plates the picture was but just visible. Some rough photometry indicated that each plate transmitted about one-third of the incident light. In carrying out the exposures suitable stops, cemented to the negative, must be provided to guide the lantern-plates into position, and thus to ensure their subsequent exact superposition by simple mechanical means.
When only a few plates are combined, the light of a Welsbach mantle suffices; but, as was to be expected, the utilization of the whole number (ten)
* Eder's Jahrbuchf. Photographie.
t Phil. Mag. Vol. XLIV. p. 282 (1897); Scientific Papers, Vol. iv. p. 333.