REVERSED with the image reversed in position as regards right and left. In photo-mechanical printing this is a necessity, otherwise the prints finally obtained would be incorrect, the right side being to the left, and vice versa. Reversed negatives can be made in several different ways. These can be divided into three different classes: First, reverse negatives obtained direct in the camera; second, negatives reversed by removing and reversing the films of the negative; third by reproduction from other negatives.
The simplest method under the first class is to photograph the image upon the plate reversed in the camera. To do this the back of the dry plate must be thoroughly clean, and also one should be selected having a perfect glass support, free from bubbles or other defects. The dry plate is then inserted in the dark slide, film side down, or the opposite to the usual way. A little care must be taken that the spring used for keeping the plate into position has no injurious effect upon the film. After focusing, the back or front of the camera is moved so that the distance between the lens and the screen is decreased exactly the thickness of the dry plate.
The next method of making reversed negatives direct in the camera is by means of a sil vered reflector. Metal or glass silvered on the external surface should be used. The accompanying sketch, Fig.
389, will show how it is arranged: E is the camera, L the lens, A C B D is the section of a hood fitted round the lens and attached to the camera, A B is a mirror, as described, fitted at an angle of 45 deg. with the axis of the lens, and so arranged that the center of the mirror is its continuation, D is a small door which can be opened and shut at will. The camera is turned sideways towards the object, and a little consideration will serve to show that the image obtained on the focusing screen and upon the plate will be reversed.
In the third method of the first class a right-angle prism is used fitted on the lens. See Fig. 39o.
On reference to Fig. 390 the system will be seen at a glance.
The prisms are made of one solid homogeneous mass of glass with silvered hypothenuse.
Of the second class, i.e., negatives reversed by reversing the film, the most convenient method is the following: The negative is made in the ordinary way upon a gelatino-bromide plate. It is then washed and dried and coated with a solution of indiarubber dissolved in benzole, and when this coating is dry it is flowed over with transfer collodion, composed of— Methylated spirit (.805) 2 ounces Methylated ether (.73o) i ounce Pyroxylin 24 grains Castor oil 24 minims When this is dry the negative treated in this manner is immersed in— Water so ounces Hydrofluoric acid ounce The film should next be cut round the edges with a penknife and stripped away gently, and floated off into a bath of clean water to wash. A glass plate is then coated with a warm solution of gelatine (about five grains to the ounce), and the film floated on to it in the reverse position—that is to say, with the collodionized side downwards—with a soft squeegee it is pressed into contact with the glass, and then set aside to dry.
The best class of processes for producing reversed negatives is by reproduction. There are several methods. The powder process is very often employed ; this will be found described under that heading. Bolas's method is as follows : A dry plate is soaked for five minutes in a four per cent. solution of potassium dichromate, rinsed once or twice with water, and then dipped into a bath of equal parts of methylated spirit and water, the superfluous mixture blotted off and dried. All these operations must be conducted in the dark-room. It is then exposed, under the negative to be reversed, to sunlight for three or four minutes, when a faint delicate image will be seen impressed ; it is then washed and developed with ferrous oxalate or alkaline pyro, and fixed and washed as usual. The result is a reversed negative image.
Now that sensitive bromide negative films are made so perfect, there is no doubt that they will be largely used in obtaining negatives required to be reversed, as they can be printed from either side with the same effect.