upon Surface-Films. 373 



The projection of the figures, developed upon the glass by 

 breathing, requires a special arrangement, which it is the 

 principal object of this note to describe. For this purpose 

 the light simply transmitted by the undimmed parts of the 

 plate must be intercepted, leaving the image to be formed by 

 the light diverted from its path by the condensed breath. The 

 arrangement was as follows : — 



The ordinary condenser B (fig. 3) of the electric lantern 

 was stopped down to an aperture of j inch, and provided a 



Fig. 3. 



SCALE 40 



somewhat divergent beam of light of corresponding diameter. 

 At a distance of 15J inches from the condenser was placed 

 the slide C upon which a figure had been impressed. The 

 focusing lens D was of plate-glass, 6 inches in diameter 

 and 25 inches focus, and was of course distant from the breath 

 figure by an amount slightly exceeding its own focal length. 

 Any light that might pass outside was intercepted by a suit- 

 able mounting. So far there was nothing peculiar, except in 

 respect to the dimensions of the focusing lens. But now 

 between the latter and the screen was inserted a disk E of 

 black card 2 inches in diameter, at such a distance (40 inches) 

 from the lens as to receive a well-defined image of the hot 

 carbons A. By this disk all regularly refracted light would 

 be stopped, so that the screen would appear dark. If, 

 however, any part of the prepared glass be dimmed by the 

 breath, light is there diverted from its path, and thus escaping 

 the stop proceeds to form an image of the part in question 

 upon the screen. The dewed parts of the breath-figure are 

 accordingly seen bright upon a dark ground ; and with the 

 arrangement described, in which the large diameter of the 

 focusing lens is a leading feature, the projected images are 

 very beautiful. A similar method would probably be adequate 

 to the projection of smoke-jets. 



In conclusion I may mention that the latent images can 

 be developed in a more durable manner by a deposit of 

 silver, the arrangements being such as are adopted for the 

 silvering of mirrors, except that the action is stopped at an 

 earlier stage. The washed and dried deposit may then be 

 protected from mechanical injury by a coat of varnish. 



March 4, 1892. 



