THE SOURCE OF LIGHT.
The initial points to consider are (it) con mience. (b) cost, (r) size limit, (1) class tegr s, (r) class of sensitive material.
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regard to (a) this is a matter best decided by the student himself, and tie pendent upon local circumstances ; whilst (b) so dependent upon special conditions that it need not be discussed. With reference to (e), the largest dimensions to which it is proposed to enlarge ex ercises some influence upon the choice of light, inasmuch as a powerful illuminant is imperative for enlargements of liameters ; whilst, for smaller sizes, al most any illuminant may be chosen. }or it must he remembered that the light in 1...usity falls off very rapidly as the dimensions of the enlargement are in creased. For example, an enlargement of four diameters will possess only one fourth the light intensity of one of two This shows how greatly the ifliculties are multiplied as the size of the enlargement is increased, and how essential it is that perfect negatives with clear shadows should be used, tigettner ith a brillia i hninant. This latter point shows t e temp of (1), which de pends not only on the matter just stated, but also upon the size of the negative. The larger the negative the
more chance of uneven illumination, es pecially when either of the methods of reflecting the light is employed. The weaker the light, the more chance of un evenness and of falling off towards the margins of picture. To sum up, there fore, the larger the negative it is intended to use, the more brilliant and actinic should be the illuminant (e). Obviously the same illuminant cannot be used with equal success upon all classes of sensitive material. For example, when enlarging upon bromide paper a different strength of light will be required to that used when making enlarged negatives, on dry plates, or even by the wet collodion pro cess. If, however, a sufficiently strong light is chosen, it can usually be toned down by one or more thicknesses of ground glass or opal, which serves further to pro duce more even illumination.
may be clone by means of a parabolic re flector constructed for burning magnesium ribbon at one side (see Fig. 437). The magnesium is ignited by means of a match through the hole shown.