268 THE POPULAR SCIENCE MONTHLY. 



to the spectrum diagram (Fig. 5) will show that at each end of the 

 colored rays there is a large space inactive, as far as the eye is con- 

 cerned, but active in respect to the production of motion strongly so 

 at the red end, less strong at the violet end. Before the instrument 

 can be used to measure luminosity, these rays must be cut off. We 

 buy gas for the light that it gives, not for the heat that it evolves on 

 burning, and it would therefore never do to measure the heat and pay 

 for it as light. 



It has been found that a clear plate of alum, while letting all the 

 light through, is almost if not quite opaque to the heating rays below 

 the red. A solution of alum in water is almost as effective as a crys- 

 tal of alum ; if, therefore, I place in front of the instrument glass 

 cells containing an aqueous solution of alum, the dark heat-rays are 

 filtered off. 



But the ultra-violet rays still pass through, and to cut these off I 

 dissolve in the alum-solution a quantity of sulphate of quinine. This 

 body has the property of cutting off the ultra-violet rays from a point 

 between the lines G and H. A combination of alum and sulphate of 

 quinine, therefore, limits the action to those rays which affect the hu- 

 man eye, and the instrument, such as you see it before you, becomes 

 a true jjhotometer. 



This instrument, when its sensitiveness is not deadened bv the 

 powerful control magnet I am obliged to keep near it for these ex- 

 periments, is wonderfully sensible to light. In my own laboratory, a 

 candle thirty-six feet off produces a decided movement, and the mo- 

 tion of the index increases inversely with the square of the distance, 

 thus answerirfg the third question, " Is the amount of action in direct 

 proportion to the amount of radiation ? " 



The experimental observations and the numbers which are required 

 by the theoretical diminution of light with the square of the distance 

 are sufficiently close, as the following figures show : 



Candle 6 feet off gives a deflection of 218-0 



The effect of two candles side by side is practically double, and of 

 three candles three times that of one candle. 



In the instrument just described, the candle acts on a pith-bar, 

 one end of which is blacked on each side. But suppose I black the 

 bar on alternate halves, and place a light near it sufficiently strong to 

 drive the bar half round. The light will now have presented to it 

 another black surface in the same position as the first, and the bar 

 will be again driven in the same direction half round. This action will 



