ELECTRIC LIGHTS. 171 



ignite a piece of sodium as large as half a pea, and 

 hold it while burning so that the light from the sun must 

 pass through the flaming sodium that is immediately 

 in front of the slit. The yellow of the sunlight will be 

 stopped, and a large and densely black line will be 

 seen in the place of the yellow in the spectrum. A very 

 good way to ignite the sodium is to provide a soft pine 

 stick, six or eight inches long and half an inch or more 

 thick. Close to one end cut out a hole large enough to 

 hold the bit of sodium to be used, and crowd this into 

 it. The end of the stick can be lighted in a gas or 

 alcohol flame, and then hastily moved to the position 

 where it is needed. The inflamed wood will set fire 

 to the sodium in a few seconds, when it will burn with 

 a great flame and dense fumes, yet without endangering 

 the hand. The yellow flame and the light from it 

 will not seriously impair the appearance of the spectrum 

 upon the screen. 



RELATION BETWEEN SIZE OF OBJECT, SIZE OF IMAGE, 

 AND FOCAL LENGTH OF OBJECTIVE. 



It is often convenient to know how large a given 

 picture will be upon the screen when projected, what 

 kind of an objective to use to obtain a picture of a 

 definite size, and so on. The following rule will enable 

 one to know and provide such conditions. 



Let A represent the focal length of the objective ; let 

 B represent the distance from the objective to the 

 screen ; let C represent the diameter of the space to 

 be projected ; let D represent the diameter of the 

 lighted space upon the screen. Then, as A : B : : C : D. 

 Three of these will nearly always be known. Suppose 

 the transparency to be projected be 3 inches in 



