138 Transactions of the American Institute. 



the chemical beams of the sun in their photogi'aphic power. Many 

 years ago, Prof. J. W. Draper suggested a simple instrument con- 

 taining a mixture of hydrogen and chlorine gases, which by their 

 gradual combination should test the actinic force of a beam of light. 

 Another method is the direct measure of the illuminating power. 

 The most simple way to test this is by comparing shadows cast upon 

 a screen ; and for this purpose we will now darken the room, and 

 you will observe, first, the shadow cast by a metallic rod upon this 

 screen, when we have only the illumination of a candle flame. Now, 

 lighting this gas-burner, giving a flame from the ordinary street gas, 

 you observe that the shadow of the bar from the candle has become 

 invisible, or can hardly be seen, and the shadow from the gas alone 

 appears upon the screen. Let us now allow the oxygen jet play upon 

 a piece of compressed magnesia. We have a still more brilliant 

 light, which grows l)righter until not only the shadow of the brazen 

 bar from the candle but that from the gas disappear. I now burn a 

 strip of metallic magnesium which overpowers by its brightness the 

 other lights. jSText we come to the galvanic light ; and now you see 

 that the magnesia shadow, as I move it, is itself very faint, although 

 the galvanic light is at a greater distance. You see how the galvanic 

 light overshadows all the others. It is the brightest of all known lights. 

 ]N^ow, lowering these lights, I will introduce, while the room is still 

 darkened, another experiment — the production of the electric light. 

 All are familiar with the fact that the electric spark can be excited 

 by friction upon dry, warm, brown^ paper, glass, vulcanized india- 

 rubber, or gutta-percha. 



"When we pass this current through a vacuum, we have not pm*e 

 white light, but a series of colored lights, very much such as we 

 witness in the higher part of our atmosphere in the winter season, 

 especially in the arctic regions, and which we know under the title 

 of the aurora borealis. We can imitate this by a small electric 

 machine. To o1)tain an imitation of the peculiar light, we will pass 

 the current through a series of tubes, about twelve feet in length, and 

 filled with different gasses. When these tubes are revolved with 

 sufficient rapidity, it produces the effect the pyrotechnist sometimes 

 exhibits, of wheels apparently rotating in the air. Before lighting 

 up the room, I will show you an experiment to demonstrate how 

 harmoniously colors are blended. In these plates before me [extend- 

 ing entirely across the platform in large numbers] we liave prepared 

 a light diet of salt moistened with alcohols. Setting fire to this it 



