PHANTASMAGORIC EXHIBITIONS. 161 



P Q, by the object lens D. When the car H is drawn 

 back on its wheels, the rod I K brings down the point K, 

 and, by means of the rod K L, pushes the lens D nearer 

 to the sliders in E F, and when the car advances to P Q, 

 the point K is raised, and the rod K L draws out the lens 

 D from the slider, so that the image is always in the 

 conjugate focus of D, and therefore distinctly painted on 

 the screen. The rod K N must be equal in length to I K , 

 and the point I must be twice the focal length of the lens 

 D before the object, L being immediately under the focus 

 of the lens. In order to diminish the brightness of the 

 image when it grows small and appears remote, Dr. Young 

 contrived that the support of the lens D should suffer a 

 screen S to fall and intercept a part of the light. This 

 method, however, has many disadvantages ; and we are 

 satisfied that the only way of producing a variation in the 

 light corresponding to the variation in the size of the 

 image, is to use a single illuminating lens C, and to cause 

 it to approach E F, and throw less light upon the figures 

 when D is removed from E F, and to make C recede 

 from E F when D approaches to it. The lens C should 

 therefore be placed in a mean position corresponding to a 

 mean .distance of the screen and to the ordinary size of 

 the figures, and should have the power of being removed 

 from the slider E F, when a greater intensity of light is 

 required for the images when they are rendered gigantic, 

 and of being brought close to E F when the images are 

 made small. The size of the lens C ought of course to be 

 such that the section of its cone of rays at E F is equal to 

 the size of the figure on the slider when C is at its greatest 

 distance from the slider. 



The method recommended by Dr. Young for pulling 

 out and pushing in the object lens D, according as the 

 lantern approaches to or recedes from the screen, is very 

 ingenious and effective. It is, however, clumsy in itself, 



