The evidences of a too rapid immersion in the Bath are sought for on the side of the plate from which the Collodion was poured off. This part remains wet longer than the other, and always suffers the most; horizontal cracks or marks resembling vegetation are seen, each of which would cause an irregular action of the developing fluid. On the other hand, the upper part of the plate must be examined for the pale colour characteristic of a film which had become too dry before immersion, since the Collodion is thinner at that point than at any other. EXPOSURE OF THE PLATE IN THE CAMERA. After the plate has been rendered sensitive, it should be exposed and developed with all convenient despatch; the intensity of the Negatives being, with some Collodion, materially lessened by neglecting this point (see p. 100). Ascertain that the joints of the Camera are tight in every part--that the sensitive plate, when placed in the slide, falls precisely in the same plane as that occupied by the ground glass--and that the chemical and visual foci of the Lens accurately correspond.[37] [37] See the Second Section of this Chapter. Supposing the case of a portrait, next proceed to arrange the sitter as nearly as possible in a vertical position, that every part may be equidistant from the lens. Then, an imaginary line being drawn from the head to the knee, point the Camera slightly downwards, so that it may stand at right angles to the line. If this point be neglected the figure will be liable to be distorted in a manner presently to be shown (p. 228). In order to succeed well with portraits, the sitter should be illuminated by an even, diffused light falling horizontally. A vertical light causes a deep shadow on the eyes and makes the hair appear grey: it must therefore be cut off by a curtain of blue or white calico suspended over the head. The direct rays of the sun are generally to be avoided, as causing too great a contrast of light and shade. This is a point on which the operator must exercise his judgment. With a feeble Collodion, a better Negative picture may often be obtained by placing the sitter quite in the open air, but when the Collodion and Bath are in the condition for giving great intensity of image, the gradation of tone will be inferior unless the light be prevented from falling too strongly upon the face and hands. In focussing the object, cover the head and the back part of the Camera with a black cloth, and shift the Lens gently until the greatest possible amount of distinctness is obtained. Then insert the sensitive plate, and having raised the door of the slide, cover all with a black cloth during the exposure, as a security against white light finding entrance at any part excepting through the Lens. With regard to the proper time for the exposure, so much depends upon the brightness of the light and the nature of the Collodion, that it must be left almost entirely to experience. The following general rules however may be of use:--