DEMONSTRATIONS OF APPARATUS 217 



trating the equilibrium and parallelogram of forces. I pur- 

 posely select this, as probably almost the last experiment that 

 it would occur to anyone to demonstrate by this method. As 

 is well known, we arrange two pulleys H and K, over which 

 hang weights p and Q, drawing two flexible cords from the 

 point A, from which depends another weight E by the line A L, 

 the lines A B and AC measured from any point D on the per- 

 pendicular by lines D B and D c parallel to the cords, giving 

 the ratio of the weights P and Q, and A D that to both of the 

 weight E. Simply using rather thicker flexible cords than 

 usual, all this will be projected on the screen with the greatest 

 facility, and on a large scale ; while by supporting in a clamp 

 a plate of glass just in front, and in the plane, of the cords, 

 the parallelogram may be actually 

 traced out with a black brush, appear- 

 ing on the screen as drawn, and the 

 law verified by measurements. 



It has been said that some little 

 modification is required to use this 

 method with the lantern ; but it is 

 very slight. We merely want to pro- Fm 



vide as small and intense a radiant 



point as possible. When the radiant in the lantern is an arc- 

 lamp of moderate power, sufficiently sharp projections will be 

 obtained from the arc direct, removing all lenses from the 

 lantern front. An incandescent lime is too large for sharp- 

 ness, however, and the best method is to place on the front 

 the little ' pencil ' attachment shown in fig. 95, p. 173, removing 

 the parallelising concave lens. The chief part of the rays 

 will then be passed as a conical pencil through one of the 

 holes on the front, the hole being reduced till the desired 

 sharpness is obtained. 



The range of the lantern is immensely extended by thia 

 method of projection, which has two advantages. The image 

 on the screen is not inverted ; and it can be made of any size 



