142 Tbaxsactioxs of the American Institute. 



a parallel line so as to produce a more brilliant light. Observe the 

 tints of the silk of these flags as I hold them before this reflector. 

 Let me now ask your attention for a moment to the instrument com- 

 monly employed in photoinetric investigations. It was invented by 

 Prof. Bunsen of Heidelberg, to whom we are indebted for other most 

 beautiful physical and chemical discoveries. This is a beautiful 

 application of the law that light is in inverse proportion of the square 

 of the distance. Here we have a small bit of paper, which, whe.n 

 you view it by reflected light, shows a central disk which is white. 

 Viewing it by transmitted light, the central disk is black, the paper 

 being greased upon the circumferences, rendering it semi-transparent, 

 excepting this central spot. This instrument is placed in a room 

 perfectly dark, the walls, ceiling, and floor being blackened. There 

 is a graduated scale, on which this disk is placed. At one extremity 

 of the scale is placed a standard candle; and at the other end the 

 candle or other light which we M'ish to measure. This disk slides 

 easily back and forth, and is so adjusted that the light upon the two 

 sides shall be of equal intensity, so that the central spot is almost 

 invisible. If either appears brighter, we slide the disk toward the 

 feebler light until they agree. Then if we find that the distance of 

 the candle from the disk is five times as great as of a gas-light, we 

 must square this number, producing twenty-five, and it will show the 

 brilliancy of the gas-light to be twenty-five times as great when 

 compared with this standard candle. If the distance is only four 

 times as great, the square of four being sixteen, we know that the 

 gas-light is sixteen times as brilliant. Ordinarily, if gas be burned in 

 a circular Argand burner, possessing fifteen apertures, and having a 

 glass seven inches in height, burning five cubic feet per hour, under 

 a pressure of half an inch, its light will be equal to sixteen caudles. 

 In some cities, as in Baltimore, and I was told to-day, by Prof. 

 Silliman, in Chicago, they publish every week the illuminating 

 power of the gas produced in those cities, so that the public 

 have a check upon the gas companies, I ought to say of the Argand 

 burner, that the amount of light depends partly upon its form, 

 whether the orifices are larger or smaller, whether the chimney is 

 higher or lower, &c. ; and all these circumstances must be taken into 

 consideration. But we know that candles differ. The standard is 

 that it must burii two grains per minute. The candle is lighted, and 

 when it is fairly burning, we counterbalance it with little weights; 

 and thus we can easily ascertain the amount of material consumed in 



