The smaller and more intense the light, the nearer we approach to the ideal projection illuminant, and the better will be the definition of our enlargements. The arc light most nearly fulfils the desired conditions, and if it be available it should certainly be employed. Next in point of utility comes the limelight, preferably in the form of the mixed jet, and those who understand its manipulation are recommended to adopt it, but the majority of amateurs will probably find it more convenient to use either incandescent gaslight or an oil-lamp. Parallel wick-lamps should be avoided on account of the unequal illumination they produce, and if oil must be used a good circular wick burner will be found more suitable. Where house-gas is available the incandescent gaslight is however much to be preferred. The light is perhaps not so powerful as that given by a really good parallel wick-lamp, but it is far more actinic and penetrating. The writer has used this light with great satisfaction, and therefore has no hesitation in recommending it. Some workers have been troubled by the appearance of an image of the mantle on the screen, but this can usually be got rid of by a suitable adjustment of the lenses and the light, and in any case by the interposition of a piece of ground glass between condenser and negative.
THE CHOICE OF THE LENS.
It is commonly stated that the lens with which the original negative was taken will serve equally well to enlarge it, and in the abstract the statement is perhaps not inaccurate. But assuming that a lens of a focus equal to about 1¼ times the base of the plate has been used, it will be found that better results, both in regard to definition and equality of illumination, will be obtained by substituting a lens of rather longer focus, for example a half-plate lens for enlarging from quarter-plate negatives. This, although applying to both methods of working is particularly desirable when enlarging by artificial light, for an objective of small diameter and short focus cannot possibly pick up or receive the whole of the cone of rays proceeding from the condenser. A reference to Figs. 7 and 8 will explain why this is so. In Fig. 7 we see what happens when a lens of too short a focus is used, but when one of longer focus is substituted, the whole of the cone of rays passes through and is utilized (Fig. 8). In selecting a lens one should be chosen which has a sufficiently large diameter to permit the apex of the cone of rays from the condenser to pass through. This point will, of course, vary with the degree of amplification, and in order to obtain the best results optically, the distance of the light from the condenser must be carefully adjusted in every case, and a clear, evenly lighted disc obtained before inserting the negative to be enlarged.
Fig. 7.
Fig. 8.
With regard to the type of lens, one of the rapid rectilinear form will answer well. A portrait lens is often used on account of the brilliancy of image, but although it answers well for enlarging portraits, the roundness of its field makes it less suitable for landscapes unless it is considerably stopped down. If the very finest results as regards definition are required, then one of the now numerous flat-field lenses should be used. The writer can from practical experience speak well of the Ross-Goerz and the Cooke lens. The latter is perhaps preferable for working with artificial light on account of the larger diameter of the back lens. Wide-angle lenses, on account of their small aperture and short focus, are not suitable for use with a condenser, both for the reasons given, and on account of the difficulty in focussing owing to the small amount of light transmitted. When daylight is used, however, there is less objection to their employment.