In a beam of directed light, the most trans parent parts of the negative are non-scattering, or scatter only slightly, and thus deviate from its normal path only a negligible fraction of the light transmitted. On the other hand, the densest parts are highly diffusing, and scatter in all directions an appreciable part of the light which has penetrated them. Nearly all this scattered light fails to reach the lens, and conse quently the dense parts have an apparent opacity greater than their actual opacity, with a resultant increase of contrasts.
In a negative illuminated by completely diffused light, that is a negative of which each point receives light at all possible angles of incidence, there is a balance between the light which is travelling towards the lens before reaching the negative and is scattered by the latter, and the light which has reached the negative at other angles and is likewise scattered in the direction of the lens.
The first study of this phenomenon was made by A. Callier in 1909, hence the name of Callier effect sometimes given to it. This investigator had admitted that the densities in condensed light for a given negative are equal to those in diffused light multiplied by a constant factor. Since then it has been recognized that the relation between these two values of density is considerably more complicated.' The various factors that reduce the scatter of light by the surface of the negative to be en larged, e.g. varnishing or immersion in a liquid, will also reduce the effect of superficial defects (scratches, retouching, etc.) but will increase the contrasts of the image (C. Winther, 1922). It is easy to understand that varnishing confined to certain parts of the image (parts treated with retouching medium for subsequent retouching) leads to markings with well-defined outlines.
All factors increasing the scatter of light by the silver deposit will obviously exaggerate the differences in rendering due to the two methods of illumination. These differences are more particularly marked in the case of a negative of coarse grain than with a negative of fine grain emulsion.
Enlargement with a condenser requires a new adjustment of the light-source for each variation in the scale of enlargement. To avoid these many adjustments, a sheet of ground glass is sometimes fitted between the condenser and the negative to be enlarged. The ground glass is thus uniformly illuminated, and its scatter is predominantly towards the lens, this action being then the sole advantage of using the condenser. The results thus obtained differ little from those with uniformly diffused light.
Finally, the graininess of the negative image is very much less apparent in prints enlarged by diffused light than in prints enlarged with a condenser. It is often found that an enlai ge ment of 10 times with diffused light shows less granulation than one of 3 times with directed light. 2 753. Negatives Best Suited for Enlarging.
The graininess of a negative being always greater as the densities are higher (L. A. Jones and N. Deisch, 1920), it is necessary to avoid all circum stances likely to increase the density of the image uselessly, e.g. over-exposure, fogging of various kinds, over-development (particularly as with nearly all developers graininess in creases much more rapidly than density) and all factors which may cause incipient reticula tion, such as a too warm developer, a warm fixing bath not containing a hardener, and prolonged washing in hot weather. If the water available is highly charged with minerals, pre cautions must be taken to avoid any deposit of lime on the negatives. Care must also be taken to avoid dust settling on the negatives while they arc drying.
The increase of contrast when enlarging with a condenser is of value in enabling the best use to be made of correctly-exposed and somewhat under-developed plates. Such nega tives are also suited for enlargement Fro. 192.
by diffused light on contrasty papers.
Negatives which have been retouched or negatives with small defects may usefully be varnished, or at least rubbed with an encaustic paste, being then best enlarged by diffused light.
754. Art and Enlargements. It is often said that a photograph becomes artistic by the sole fact of its enlargement. A badly-composed photograph gains nothing by enlargement, which can neither rectify the sorry effects due to a bad choice of viewpoint nor remedy the false values due to unskilled technique.
On the other hand, it cannot be denied that a satisfactory photograph, or a satisfactory portion of a photograph including too wide a field, can gain enormously by enlargement, which by lengthening proportionally the prin cipal distance (§ 23) enables the picture to be viewed under more favourable conditions (§ 27) and, by decreasing the often excessive sharp ness of very small negatives, can bring about the simplification necessary in any picture with aesthetic claims (§ 309).
755. Apparatus for Enlarging. For many years the only apparatus available for enlarging were, for use with daylight (diffused light), the triple-bodied camera (§ 149) and the fixed focus automatic enlargers or enlarging boxes used al most exclusively by amateurs, or, for use with " directed " enlarging lanterns comprising a lamp house, condenser, negative carrier, and, joined to the latter by a bellows, the lens board ; the image was projected on a vertical panel fixed to a wall or mounted on an A new type of enlarger, employing diffused artificial light, was introduced commercially in 1918. In this the optical axis is vertical, so as to save space in the dark-room and facilitate placing the paper in position. These vertical enlargers have become very popular, and many makers have manufactured very different patterns, some of which may be used as copying cameras.