106 OPTICAL PROJECTION 



this, a programme should be written if this method is adopted; 

 but as the sudden splitting of a lime might possibly upset 

 the order, it is better to carry out the system thoroughly, 

 and register both lantern and slides ; moreover, the lantern 

 registration is done once for all. 



For dissolving views with oil lanterns, these must be used 

 side by side, and both lamps must be kept full on, as it would 

 be impossible to turn the lights up and down. Generally a 

 shade, going off at one side into taper teeth like a comb, 

 travels across each lantern nozzle, so as to cover and uncover 

 each in turn ; the two shades being connected so that as one 

 covers, the other uncovers. Plans have been published for 

 using one oil lantern over the other, but none such are really 

 safe, or tolerable in use. 



58. Dissolving Taps. Two lanterns with Argand gas- 

 burners will work dissolving views very well on a small scale. 

 The lanterns must still be side by side ; but the dissolving can 

 in this case be done by a tap which turns one light down 

 whilst the other is turned up, both lamps being ' on ' whilst 

 the tap is half-way. Such a tap is additional to the tap on 

 the burner itself, and is known as the ' dissolving- tap.' In 



this case it is what is called 

 a * three-way tap,' shown in 

 fig. 59, there being three 

 branches. The centre one 

 comes from the gas supply, 

 and the lever of the tap directs 

 this into one or the other of 

 the two channels, the amount 

 . 59.-rLe.way Tap bein g regulated by their own 



taps. 



A similar tap is used for dissolving the oxy- spirit jet 

 described on p. 46, but in this case the tap governs the oxygen 

 supply, the spirit-flame being left on. This flame is too weak 

 to give any perceptible image of the ' dead ' slide upon the 



