4i6 OPTICAL PROJECTION 



inches square) should have a mask laid on the face, and then 

 another covering-glass, cleaned and dried, which will be kept 

 from contact over the visible portion by the mask. The 

 double plate is then secured by an edging of thick dark paper 

 twice coated with gum-arabic. The way to do this is to cut 

 the gummed paper into strips 6 \ inches long and \ inch wide, 

 cutting off the corners, and corresponding notches out of each 

 side at the centre. A strip is first licked on the ungummed 

 side, and then on the gummed, and laid with gum uppermost 

 on the table. One corner of the double glass is then placed 

 on the strip at the middle notch, the glasses inclined down 

 each way, so that the gum may stick all along two edges, 

 and the strips are then bent over and worked down with 

 the finger and thumb. The other two sides are treated in 

 the same manner with another strip, and the slide is com- 

 pleted, any smears of gum being cleaned off when all is dry. 

 A proof of an engraving on a sheet of mica or gelatine is 

 mounted between two glass plates in a similar way, the mask 

 being laid on the printed side. The card masks may be used 

 for mica films ; but the crinkling of gelatines is less apparent 

 if the mask be cut out of plain dark paper, which is thinner. 

 This is, however, only for appearance, the most conspicuous 

 crinkles having no effect on the screen. 



