ENLARGING WITH DIFFUSED LIGHT 761. Uniform. Illumination. with Diffused Light. By placing a light-source near the focus of a condenser we obtain uniform illumination of a diffuser placed on the other side of the condenser. While this arrangement has been actually used in some enlargers working with diffused light, it is usual to avoid in such cases the expense of a condenser, which can easily be dispensed with.
In the absence of a condenser, means must be taken to ensure the fairly even illumination of the diffuser employed.' While, in the case of negatives of very small size, the light afforded by an electric lamp with an opal bulb may be considered as uniform, it is necessary in all other cases to use simul taneously several lamps suitably or to reduce the excessive transmission in the central part of the diffuser. 3 To reduce the excess of light in the part of the diffuser directly facing the lamp, it is possible to use a lamp of which the bulb is ground or silvered at the tip. The central area of the diffuser is then lighted only by diffused light. When two diffusers are used, one behind the other, the absorption of light by the centre of the diffuser nearest the lamp may be consider ably increased. For instance, in an automatic enlarger (one in which the various movements are linked together) made in the United States to the specification of R. S. Hopkins (1918), the first diffuser consists of a sheet of glass of which only the centre is ground, with vignetted edges, while the second diffuser is a glass ground uniformly of fine grain. The same result can be obtained by varnishing or oiling the marginal areas of an ordinary ground glass and by in creasing the central absorption by pencilling, or again by placing on a sheet of clear glass cut-outs of tracing paper, or, finally, by using as a " vignette " a negative obtained by exposing a slow plate behind a yellow filter to the light of the lamp, the plate during this exposure being placed in exactly the same position that it will occupy when enlargements are being made. By suitable exposure and development
this negative will show a graduation between the centre and edges compensating satisfactorily for the inequalities in illumination.
Whatever the method of illumination em ployed, it is necessary to see that the diffusers used are of size distinctly greater than that of the largest negative to be enlarged if the marginal areas of the image are to be satisfactorily illum inated, even when enlarging on a relatively large scale, since in this case the extreme beams include the maximum angle of field.
Instead of securing uniform illumination of the diffuser, it is often advantageous to increase slightly the illumination of the marginal por tions, so as to compensate for the effect of the obliquity of the extreme beams (§ 54).
762. Vertical Enlargers. The very numerous patterns of vertical enlargers range from com mercial models for exact work, built with a rigid frame of cast steel resting on the floor, to the light-weight models for amateurs. These have a framework of sliding steel tubes for placing on a table. There are intermediate types of a semi-professional class with a wooden framework for fixing to the wall of the dark-room.
As a rule, graduations are provided indicating, for various degrees of enlargement, the respec tive positions of the lens front and of the nega tive carrier above the board or horizontal table on which the sensitive paper is placed. In many of the present-day instruments, there is an automatic linkage (§ 69) to keep the negative and the lens in conjugation with the easel. The operator is thus relieved of all attention to focussing, the movement of one part ensuring the correlated movement of the other, so that the image is caused to pass successively through all the possible degrees of An additional hand-focussing adjustment (heli coidal lens mount) should be provided. In spite of their obvious advantages the high cost of these fitments prevents their general use.