TKAll'KRATUltE OF THE SURFAC"!-] OF TJIF MOON. 



35 



is the same by uight as by day, but that other circuuistauces add to the difflculty iiufoiniiing exact 

 couclusious. 



The api)aiatus eiuployed in the following obseivatious for lunar rays consisted of the concave 

 mirror and bolometer shown in Plate 1. These were used at night, while in the day the sun's rays 

 passed through a narrow aperture and fell on the bolometer placed at a considerable distance in the 

 divergent beam. 



The following are the principal constants of the apparatus : 



Let S=lunar apparent semi-diameter at the time of observation, obtained from the geocentric 

 semi-diameter corrected for augmentation. 

 /=the focal length of concave mirror=73.4 centimeters. 



A=the radius of lunar beam falling upon the concave mirror=10.2 centimeters. This 

 radius i.s limited by that of the hole in the north wall by which the beam from the 

 siderostat enters. 

 Let s be the semi-diameter of the lunar image in the focus of the concave uuri'or. Then 



A 2 



/sin S=s, and -, =concentration of lunar beam. 



The absorption by the silver of the mirror aud the loss by non-perpendicular incidence of the 

 rays on the bolometer strips are here neglected. 



Again, let a be the semi-diameter of the aperture used for transmitting the solar rays, S' the 

 solar semi-diameter, and d the distance of the bolometer strii)s from the diaphragui. Then 

 TT {d sin S' + a) ^ttP, where I is the radius of the circle formed by the divergent solar beam at the 

 distance of the bolometer strips, and, neglecting the effect of diffraction at the diaphragm, the 



diminution of the solar beam = ,,. The ratio ^ to ,2 is that of sun heat to moon heat, or rather 



that from an element of the center of the sun's surface to the mean value of the heat from the full 

 moon. Observations of this kind were made on the following evenings, November 29, December 

 2, and December 3, 1884. 



As an example we give that of December 2, 1884 : 



The sky during these observations was quite good and cloudless. A light haze having gath- 

 ered shortly after midnight, observations were discontinued before the moon Iiad quite reached the 

 meridian. 



On the following day (December 3) observations were made on the sun at noon. The state of 

 the sky appeared to be about the same as during the preceding lunar observations, and the battery 

 current employed was adjusted to the same strength. 



