OF AETS AND SCIENCES. 421 



XVII. 



CONTRIBUTIONS FRO:sr THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



E. C. PICKERING, Pkofessor of Physics. 



IIL — INTENSITY OF TWILIGHT. 



Br Charles H. Williams. 



Read, Blay 11, 1875. 



During the fall and winter of 1874 an attempt was made to measure 

 the amount of light given by the sun when at different distances below 

 the horizon. Days were chosen when the sky was perfectly clear at 

 sunset, though a few observations were made when it was snowy or 

 cloudy. 



The instrument used was the photometer first described in the report 

 of the Total Eclipse Expedition for 1870. It consisted of a box about 

 five feet long, eighteen inches high, and twelve inches wide ; over the 

 top and sides, which were of light framework, black cloth was stretched ; 

 a circular hole, about five inches in diameter, was cut in one end and 

 covered by a Bunsen disk, and a standard candle, in a spring candle- 

 stick, was moved along the centre of the box by means of a rod 

 attached to it ; the distance from candle to disk being varied at pleas- 

 ure, and measured by a mms. scale attached to the rod. 



It was found inconvenient in practice to be obliged to read the 

 scale at every observation, and the disappearance of the spot could be 

 better watched if the eyes were kept fixed on the disk. An arrange- 

 ment for automatic registering was therefore added. A piece of 

 sheet-iron connected the candlestick with a rod moving outside the box 

 along its whole length, the iron clasped the rod and was held in place 

 by friction ; to the iron was fixed a movable point, which could be 

 pressed into a fillet of paper by means of a string passing from the 

 iron round a pulley at each end of the box. The position of the 

 candle was varied by moving the rod ; the point where the observation 

 was taken was marked on the paper, and the distance of the candle 

 from the disk in rams, was read off afterward from a scale. 



