MASKING.
By the term masking is meant the taking of a print from a negative through an opaque mask, with openings shaped according to fancy and suitability to the subject in hand. This opaque medium, termed a mask, is generally inserted between the film of the negative and the front side of the paper or sensitised sur face upon which the positive is to be received. The dimensions of the openings must be within the limits already stated for trimming prints. It will sometimes be necessary to reduce the width of the two parts forming the stereoscopic nega tive. This should be done at the stage of piece of paper and in their proper order for pasting upon the card. When, how ever, they have not been treated in this manner, care will have to be exercised in that the right-hand picture is placed on the right-hand end of the stereoscopic mount, and the left-hand pic ture on the left-hand end of the mount.
Failing this, what is known as a " pseudo scopic " effect will be produced when the pictures are examined in the stereoscope, i.e. objects in the composition that should occupy a remote distance appear in the foreground, while objects that should stand in the foreground recede. Under these conditions, concave becomes con vex, and vice versa. To prevent error in mounting, it is therefore necessary to mark the pair of prints in some way. This should he done before the double photograph is separated. Place it on the table face uppermost, with the subject erect, and then turn it over without inverting it. On the left-hand end and back of the print mark L, and on the right-hand end mark R. • They may now be numbered in pairs and cut apart, when it will only be necessary to refer to the letters on the back when mounting.