422 



PROCEEDINGS OF THE AMERICAN ACADEMY 



To use the instrument, a suitable day being taken, the disk was ex- 

 posed to the horizon a few minutes before sunset, the candle lighted, 

 and placed at about fifty rams, from it. The disk was then watched, 

 until it became dark-centred ; the distance of the candle from the disk 

 was now adjusted, so that the centre of the disk should nearly dis- 

 appear, when the time was noted, and observations were then taken 

 every minute till the light became very feeble. It was found im- 

 possible to get a perfect disaj^pearance of the spot, owing principally 

 to the difference in color of the two lights, the candle being mush 

 more yellow than the sun ; a certain neutral shade between the dark 

 and light centre was therefore taken as the point for making the ob- 

 servations. Various attempts were made to get rid of this difference 

 of color, but without success. A cell filled with a solution of sulphate 

 of copper of different strengths was placed on the candle side of the 

 disk, also indigo and other blue solutions ; the only effect of these was 

 to give the whole surface of the paper a greenish tint when the candle 

 was brought near, without making the disappearance of the spot more 

 perfect. Disks made of pajjer of different colors, and sheets of plaster 

 of Paris, made extremely thin by pressing the fluid plaster between 

 sheets of plate glass, were tried with the same results. The best 

 material seemed to be fine white paper painted with s]3ermaceti, ex- 

 cept at the centre. 



It seemed to make no difference in the relative diminution of the 

 light, whether the observations were taken with a clear horizon or 

 with part cut off by some adjoining building ; the readings from the 

 upper part of the building looking over the roofs agreeing very well 

 with those taken below. Having made a number of observations on 

 different days, the instrument was tested to get the probable error of 

 any reading. The photometer was placed in a dark room and a fixed 

 amount of daylight admitted, the candle was moved till the disk 



TABLE I. 



