Flare Spot.
Sometimes it will be noticed in the case of a landscape negative that the central portion of the plate for a space of possibly a third of its length is fogged, whilst the rest remains clear. This defect is called "flare spot." It h due to a defect in the lens, and makes itself evident it the case of certain subjects. If there be a very bright light ahead, as, for example, may be reflected from the sky, and at the same time there be very deep shadows, so that a comparatively long exposure is necessary, the defect may be seen. It scarcely ever occurs except when a lens of the kind known as the single achromatic is used. The. lens may be so altered as to make it cease giving a flare spot by slightly changing the distance between the stop and the glass.
Ghost Images.
This is a defect somewhat similar to the last. If a subject consist almost entirely of dark shadow, but with a few points of very bright light, as, for example, would be the case with a cathedral interior in which there were small windows through which the sky could be seen, a second faint image of the bright spots may be impressed on the films. This second image would appear to be clue to reflection from some part of the glass of the lens. When a subject such as the one we have described is attempted, we should avoid the use of a lens which we find produces the defect.
Browim Colour of the Negative.
Sometimes a negative, instead of showing a black or olive-green colour in the densest parts, shows a brown ness in these, and a yellowness in the parts which ought to be transparent. This defect is to be distinguished from the yellow variety of colour fog by the fact that it is visible only by transmitted light and not by reflected light at all. It does not in any way affect the quality of the print which is got from the negative, but it greatly protracts the time taken in printing. It occurs if alkaline development have been continued for a very long time, and if the alum bath have not been used between development and fixing. To get rid of it we take half a pint of the alum solution, awl pour half an ounce of hydrochloric acid into it. If the plate be dipped in this solution for a few minutes the brown or yellow colour will disappear. The plate must be after
wards very carefully washed.
Lines across the Negative.
If the developing solution has not been caused to flow in one wave across the plate, hut has left one portion of the film dry whilst another has been wetted with the developer, the result is that there is a line which distinctly marks the outline of the wetted portion. A camel's-hair brush, used as we directed to prevent air bubbles, will also prevent this defect if the operator has not enough skill to avoid it otherwise. A better plan is, however, to make use of the rocking developing dish, which we here illustrate.
In this not only is there the advantage that there can be no difficulty in flowing the developer, but besides this the negative may be examined in regard to density without removing it from the dish.
It will be seen that the apparatus consists in a dish suspended on " trunnions," so that it may rock, and having a trough at one end to contain developing solu tion. The bottom of the dish is of glass, so that all that is necessary to watch the course of development is to raise the dish till it is nearly vertical, when we may look through the plate at the red light.
Florescent Appearance of Films.
It will sometimes be found after a negative is finished that there is upon it, and particularly towards the edge, a peculiar florescent or metallic lustre ; the negative appears at these places somewhat fogged when examined by transmitted light.
This occurs if the plates have been kept for a long time—say, many months—in an impure atmosphere ; for example, if they have been kept on a shelf near the roof of a room in which much gas is burned. The defect is, so far as we know, incurable. To prevent it, the plates, if they are to be kept for long, should he stored in an attic or some such place where the air is pure and dry. They will there keep for a very long time; we do not know for how long, but certainly for a time measured by years. Dampness is to be par ticularly avoided. heat to any moderate extent appears to he harmless.