84 LETTERS ON NATURAL MAGIC. 



car H is drawn back on its wheels, the rod IK 

 brings down the point K, and by means of the 

 rod K L, pushes the lens D nearer to the sliders 

 in EF, and when the car advances to PQ, 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 there- 

 fore distinctly painted on the screen. The rod 

 KN must be equal in length to IK, 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 varia- 

 tion in the light corresponding to the variation in 

 the size of the image, is to use a single illumi- 

 nating lens C, and to cause it to approach EF, 

 and throw less light upon the figures when D is 

 removed from EF, and to make C recede from 

 EF 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 EF is equal to the size of the 

 figure on the slider whe* C is at its greatest dis- 

 tance from the slider. 



