Appendix I 163 



sodium citrate is added to the developer. The plate should be 

 developed until the image appears clearly upon the back, when 

 the lines will look a little furry from the front, but this dis- 

 appears on fixing. The positive should be fixed as before, washed 

 for an hour in running water, drained and air-dried. It is then 

 ready for masking and binding. 



Masking. Bought masks are frequently of unsuitable sizes for 

 the masking of slides prepared from book illustrations, but it is a 

 simple matter to cut a mask of any desired size or shape by placing 

 the slide film downwards upon some black (or dark) " cover " 

 paper, marking the outline of the square slide, and by means of 

 a straight-edge and pencil, indicating at the edges of the square the 

 position of the required opening. Four such lines being indicated, 

 the plate is lifted, the opening cut out with a very sharp knife, and 

 the mask cut out similarly. If a bought mask of standard size cuts 

 out all the text not required on the slide, so much the better. 



Binding. Gummed binding-strips are sold for this purpose, 

 the most satisfactory being the " Specialist " strip, which adheres 

 particularly well. They should be new, however, as the compound 

 used as adhesive deteriorates in time. " Specialist " strips are 

 cut 3^" long, consequently corners are no trouble. The strip is 

 moistened, laid gum upwards upon the bench, and the positive, 

 mask, and cover glass (clear 8 or 10 ounce glass 3}" square), 

 pressed in the centre of the strip, the edges of which are then 

 folded upwards and pressed until they adhere to the glass. The 

 three other sides are similarly dealt with. 



2. Slides prepared by Tracing upon Ground. Glass. 

 A 3}" x 3^" piece of finely ground glass is placed above the 

 picture to be copied, and firmly traced with a hard, well-sharpened 

 pencil (" Koh-i-noor" HHH is perhaps the most suitable). The 

 plate is then removed to a 9" square of brass or aluminium and 

 heated with a Bunsen until just too hot to handle comfortably. 

 A second square of plain glass is placed by the side of the plate 

 and similarly heated. From a wide-mouthed bottle a quantity of 

 filtered and partly evaporated l Canada balsam is poured on the 

 centre of the plate, care being taken that in the pouring no air- 

 bubbles are drawn into the balsam. 



Some little skill is required in doing this the object being to 

 pour the balsam out in sufficient quantity, and to leave no 

 stringy tails of balsam adhering to the bottle. The best way is to 



1 Prepared by evaporating on a water-bath a quantity of ordinary 

 filtered Canada balsam, until its volume has decreased 25 per cent. 



