92 
SCJMMARY OF CURRENT RESEARCHES RELATING TO 
It consists of a lacquered aluminium plate (fig. 13 ), with two circular 
perforations of 13 cm. diameter. Two aluminium discs rotate in the 
Fig. 13. 
a. 
brass grooves b b by pressing the knob d. These discs are perforated 
rectangularly, and h li are tires for the slides. The left-hand one can 
receive an upright or a lateral slide. To adapt it for the former the 
knob d is pushed down, thus releasing the catch /, and the disc rotates 
until f catches in e. If smaller diapositives are to be used, a suitably 
sized frame m is slipped into h li , as shown in the right of the figure. 
