142 OPTICAL PROJECTION 



open space of a window or verandah, which is kept on the 

 screen from the other nozzle of a bi-unial lantern. The effect 

 is as if the panorama were passing the window; but of course 

 this manner of exhibiting uses a great deal of gas, both 

 lanterns being on all the time. 



The drawback to panoramas is, that having to be specially 

 prepared they are expensive ; and the long unprotected slips 

 are rather liable to breakage. 



Lever slides have the movable portion painted on a 

 circular glass mounted in a turned brass rim which rotates in 

 a circular socket, and which can be thus rotated through a 

 certain arc only, backwards and forwards, by a lever projecting 

 at one side. This kind of movement is used to represent an 

 animal stooping its head to drink and lifting it again, a child 

 swinging or see-sawing, a man breaking stones with a ham- 

 mer, and similar alternating movements. 



Backivork slides are essentially of the well-known chroma- 

 trope kind, wherein two symmetrical geometrical designs are 

 rotated in opposite directions, each being mounted in a cir- 

 cular brass rim rotated by a toothed pinion. Straightforward 

 chromatropes, as they might be called, are easy to understand; 

 but few would imagine the startling change of effects in some 

 skilful designs, which may be found in the stock of any 

 London optician noted for this class of work. 



The same movement is applied to many other slides. 

 Single racks represent mills, swarms of bees, sleepers swal- 

 lowing rats, &c. Double racks represent fish in an aquarium, 

 and the magnificent effect (when well executed) of a fountain 

 with moving water. Single or double racks are used to 

 produce the ascending smoke and flame of conflagrations or 

 volcanic eruptions. 



A combination of rack and levers makes an exquisite 

 representation of waves in motion, with the riding of vessels 

 upon the heaving water. 



The highest type of rackwork slide is one constructed 



