OF ARTS AND SCIENCES. 423 



assumed the neutral tint, and the mean of ten readings taken. The 

 amount of liglit admitted to the room was then increased, and eight 

 sets of readings thus taken. The preceding table gives, in tlie first 

 column, the mean distance of the candle from the disk for each series 

 of ten settings ; in the second, the probable error for each reading ; 

 and in the third, the percentage of error. 



It will be seen that the i:)robable error is not large enough to 

 seriously invalidate the results of the observations, as the readings 

 taken by the photometer denoted the distance at which a standard 

 candle should be held from the disk to give a light equal to that from 

 the sun at a given time ; it was thought best to reduce those readings 

 to some standard, and compare them with the light given by a standard 

 candle burning at a distance of one metre from the disk. Suppose we 

 wish to reduce a reading of 200 mms. to this standard, G, or 1,000 

 rams. Let /= actual intensity : — 



(7:7=200-1000—^:^-1,; 

 whence 1= C (^) 2 = 25 C. 



In this way Table II. was constructed, giving the actual amount 

 of light, the readings being taken every minute. 



Ou the days represented in the first six columns, the observations 

 were all taken with an unobstructed horizon. On Nov. G and 7, part 

 of the sky was shut off by surrounding buildings. Jan. 15, there 

 were a few clouds ; and Jan. 3, the whole sky was overcast. Nov. 13, 

 a cell with sulphate of copper solution was placed behind the disk ; 

 and on Dec. 31, a cell with solution of indigo was used. 



To see whether the light decreased according to any function of the 

 time, curves were constructed, taking for vertical distances the loga- 

 rithm of the observed reading, and for horizontal distances the minutes 

 after sunset at which the observations were made. The result gave a 

 series of nearly straight lines all running in the same direction. In 

 some of the lines, there was a decided bend in the middle, and traces 

 of this were found in almost all. To make this bending more apparent, 

 a residual curve was constructed ; this was obtained by comparing 

 each of the curves with a straight line drawn in their mean direction, 

 and makinir the ordinates of the desired curve the mean of their dif- 

 ferences from this straight line. In this way the deviation of the 

 original curves from the straight line was made quite apparent, though 

 the difference was not originally very great. To find the curve which 

 should represent the diminution of the light for each minute after sun- 



