146 OPTICAL PROJECTION 



round the circle as at B. For the lantern, the tube encasing 

 the mirrors has a lens at each end, as in c, the tube with the 

 pair forming an objective, which takes the place of the 

 ordinary one, and focusses in a sliding jacket any object in 

 the lantern stage. A rackwork slide containing some loose 

 pieces of coloured glass, beads, and other small objects, 

 between two glasses, is usually employed; but a revolving 

 chromatrope also gives magnificent effects, and ears of bearded 

 wheat or barley or oats, a loosely webbed feather or two, a 

 key, bits of lace, &c., will give interesting patterns. 



The kaleidoscope requires very special management. It 

 is placed in the lantern so that the mirrors stand with the 



FIG. 76. Kaleidoscope 



edges upright like a V, and is focussed on the screen to its 

 proper slide, or whatever is to be used. The effect, however, 

 is at first a mere nothing scarcely visible. The lime-jet 

 has now to be raised on the rod for a distance found by ex- 

 periment, but which usually lies between f inch and 1J inch. 

 The disc on the screen brightens up at once ; and the light 

 has finally to be carefully adjusted, both as to position and 

 distance, so that all the segments on the screen are illu- 

 minated as equally as possible. Generally the instrument 

 has also to be turned or adjusted a very little to the right or 

 left, to avoid patches of darkness and get the best effect. 

 The lantern kaleidoscope entirely depends for success upon 



