380 



SUMMARY OF CURRKNT RESEARCHES RELATING TO 



for use with plates of various sizes, and the largest plate which it 

 projects is 12 by 9 cm., casting an image 230 by 172 cm. (90 by 68 inches) 

 on a screen at 4 metres distance with a lens of 200 mm. focus. 



8. Episcopic Projection {from above). — For this purpose the optical 

 bench with its fittings is swung aside, the stop-pin on the radial arm 

 released, and the objective Q (figs. 102) of 400 mm. focused in the optic 

 axis. The mirror G is then inclined at an angle of 45° to the axis 



Fig. 105. 



of the lens, this being greatly facilitated by the pressure of a spring 

 catch. The lamp is tilted at 45 :> by the handle fitted at the back until 

 the quadrant fixes its position. The mirror G, being silvered on its 

 outer worked surface, should not be touched, and should not be cleaned 

 otherwise than with a soft camel-hair brush ; after use the mirror should 

 always be covered with its protecting cap. The illuminated area is of 

 the form of an ellipse, measuring 28 by 20 cm. in the extreme case. 

 By withdrawing the lamp from the lens the light may be concentrated 



