The 3 1/4 in. plate is now clipped in the block, and dropped carefully into the opening over the negative, and after exposure is lifted out in the same way, so that any number of exposures may be made exactly registered in the same position, without the chances of injury to the negative which often occurs unless some such arrangement is adopted. (A. Cowan.)
The size generally adopted for a lantern slide is 3 1/4 in. square, with 1/4 in. margin, which gives the sight of the picture 2f in. square. By "square" 1 mean the so-called cushion shape - that is, square, with the four corners rounded. All lanterns of any pretensions are fitted with condensers of 4 in. diameter, and this will take nicely the shape above-named without any falling off at the corners, the diagonal of the opening of the masks for the purpose being 3 3/4 in. If the lantern is fitted with condensers of 3 1/2 in. diameter only, masks with round openings can only be used for all pictures, unless one is satisfied with only small-sized pictures. As regards condensers, many of the modern lanterns are fitted with condensers made with two plano-convex lenses, mounted with their convex sides to each other, and the two piano surfaces outwards. For limelight, a condenser of this kind is the worst form, as it is almost impossible, when centering the light (especially when the mixed jet is used, where the light emanates from a very small spot only), to get a perfectly flat field, or "depth of focus "; and the adjustment of the light requires to be very accurate or the disc is not perfectly white.
Not so if the other form of compound condenser be used, made up of a bi-convex lens, and the other lens of a meniscus form; and when mounted, the side which is concave being towards the light, and the convex side towards the screen. A condenser of this kind always gives the best results with the mixed jet. The defect in the first-named condenser is not so great if the safety or blow-through jet is used, as the spot of light,on the lime is much larger; and the defect is still less in the oil lantern, as the size of the flame is still larger. The piano-condensers are used as being much cheaper.
To avoid mistakes, the masks for lantern slides are best made of what is known in the paper trade as surface paper - that is, paper black one side and white on the other, the black side being placed towards the film, and the white side towards the cap glass, and on this white side of the paper the name of the subject can be written with pen and ink. The white side is also very convenient, as it can be easily seen in a very dull light by the operator. In making slides for amateurs, I generally mount them in this way; but I think a more elegant way is to use masks black on both sides, and write the title in white with a pen. To do this, get a bottle of Chinese or permanent white, empty the contents into a much larger bottle, and thin down with water, stirring well with a piece of stick kept for that purpose, to a consistency that will flow in an ordinary fine-pointed pen. Or, if the title is long, it is best to use a' steel pen called crow-quill size, or a lithographic pen; the writing is then quite as easily done as with an ordinary pen and ink.
The white must be occasionally stirred or shaken up, as the pigment being heavy soon settles.
Whichever system of mounting is used, the white side, or where the title is written, this side always goes to the light, unless the pictures are to be seen by the audience on the other side of the sheet, then matters are reversed, and the white or written side of the slide must go towards the sheet.
The shape of opening of the mask is a matter of taste and judgment according to the subject; for ordinary landscape or interiors, [the cushion shape will generally be found the best, except in some instances where there is something objectionable in the corners, then a circular opening may be an improvement. Or take the subject of a distant landscape with too much of a grassy field in the foreground, the circular shape decidedly will be a very great improvement; in few instances an oval will be a greater improvement still. For portraits, the oval form is far superior to any other.
In many instances, the square or cushion shape is not suitable to the subject, especially where slides have to be made from negatives not taken for that purpose. The 71/2 in. by 5 in. and the 7 1/4 in. by 4 1/2 in. size negatives, are mostly very unsuitable to contract into a square shape, unless the sides contain unimportant matter and can be cut off; if the whole of the subject has to be included, it is better to make a mask of the same proportions. In some instances, the sky can be made higher; if the subject be a flat country scene, then the sky being made higher will give a better representation of the flat or marshy country; but if the subject be mountain scenery, making the sky higher would have the effect of dwarfing the mountains, which would not be truthful; just the same as in mounting an ordinary portrait, the higher the subject is mounted the taller he or she looks, and the lower the shorter, and it is just the same with the landscape subjects before alluded to. Many pictures are entirely spoiled by injudicious mounting, whereas, with a little care and taste, their value would be greatly enhanced.
The masks I use I generally purchase by the gross, of the standard sizes, taking care that they are accurately cut and not out of the centre; and for odd sizes I have a lot of shapes made of hard sheet brass, 3 1/4 in. square outside, with the various sized openings, using the Woodbury cutter, which is very easy, cutting on a piece of plate-glass.
As to the binding of slides, nothing is better than thin black paper - not the ordinary so-called needle-paper that is generally used. The thicker the paper the more easily it is pushed off. Many use gum to stick it on with. Paste with the thin paper sticks far better than gum. The heat of the lantern makes the gummed paper tumble off the glass, whereas it has no effect if paste is used. To make the binding more durable, if a little trouble is not an object, and the thin paper is used, just pass a camel-hair brush, charged with ordinary negative varnish, over the paper, which makes it very hard, and stand a lot of rubbing without getting damaged. (W. Brooks.)