SCIOPTICON MANUAL. 171 



An exhibition of a mixed lot of slides may be made 

 continuous and uniform by holding the wood-mounted slides 

 upon the ledge in front of H, by pressing the draw-front back 

 against them, which does not interfere with alternating 

 them with glass slides in the grooves ii. 



THE DISSOLY1NG COCK. 



The oil lamps in a pair of dissolving lanterns are kept 

 steadily burning, while the light of each is cut off from the 

 screen alternately by the crescent-shaped dissolver, as shown 

 Fig. 16, p. 40. 



With the blow-pipe the lights themselves are made bright 

 alternately. The expense of keeping two under full head 

 when only one shines upon the screen at a time, becomes 

 worth considering; besides the external cut-off does not 

 produce so soft and pleasing an effect with a concentrated 

 light. 



With alcohol burners the oxygen is switched off, so to 

 speak, from one to the other alternately ; the deserted lime 

 becoming dim at the same rate its alternate becomes 

 incandescent. The dissolving cock, held to the stand by 

 screw-heads, has a nipple to connect with the oxygen 

 reservoir, and one for each blow-pipe. 



With the mixed jet, the flame, when deserted by oxygen, 

 spurts far out, making it necessary to cut off a portion of 

 the hydrogen also. 



The mixed jet stop-cock, therefore, has three additional 

 nipples (the three next to the lever) with the stopper so filed 

 out as to only partially cut off the hydrogen by the same 

 turn of the lever which completely cuts off the oxygen. 



The play of the lever in the Marcy dissolver is lessened 

 by side screws when the concentric jet (No. 2) is used, or 

 when the gas cylinders are used, because a fuller flow of 

 hydrogen is less liable to be extinguished by an over 

 balancing and sudden admission of oxygen. There is con- 

 siderable difficulty, at best, in dissolving with the cylinders. 



