TEMPERATURE OF THE SURFACE OF THE MOON. 



31 



similar oirciimstances it would be convex. It is eviileiit that the siiiiliglit in these measures 

 is at a great disadvantage in respect to the moonlight, especially in the upper regions of the 

 spectrum, since the violet light from the sun, which was observed at a much lower altitude, 

 had been more powerfully absorbed by the atmosphere. This absorption was even greater than 

 could be exi)ected from the mere dilference in altitude, for the sUy at night was almost invariably 

 better than in the daytime, and, moreover, the cloud of smoke, which always hangs over the city 

 of Pittsburgh towards the south, gives an absorption for large zenith distances much greater than 

 the mass of air traversed would iiroduce alone. 



If the observations had been made under precisely similar circumstances, the preponderance 

 of violet in the solar spectrum would be far more pronounced. 



In addition to this table we give another, containing the results obtained by different observ- 

 ers, reduced by intepolation from smooth curves to the same points measured on in 1884. As 

 some of these measurements were made under circumstances exactly opposite, as regards the rela- 

 tive heights of the sun and moon, to those we have described, we may expect from a combination 

 of them all to obtain a result more nearly free from the effects of unequal absorption of the light 

 from the two bodies by the atmosphere. 



Relative intensities of sunlight and moonlight. 



Wave- 

 length. 



Pickering. 



/' 

 0. 687 

 0.649 

 0.599 

 0.586 

 0..518 

 0.486 

 0.470 

 0.415 



.48 

 .64 

 .89 

 l.O 

 2.2 

 4.6 

 6.3 

 13. 



Preliminary 



observations 



of 1883. 



Observations 

 of 1884. 



Vogel. 



.7 

 1.0 

 6.5 

 9.5 



13. 



20. 



.68 

 .81 

 .96 

 1.00 

 1.18 

 1.26 

 1.31 

 1.43 



.90 

 .92 

 .98 

 1.00 

 1.26 

 1.40 

 1.54 

 2.10 



Mean* by 

 weights. 



Moonlight. 



Sunlight. 



70 

 77 

 92 

 00 

 68 

 37 

 72 

 22 



43 

 30 



08 

 00 

 60 

 42 

 37 

 24 



' The values in the sixth column have been used as ordinates for the curve. (Plate 5.) 



In obtaining the column headed "Mean" the weight 5 has been given to each of the two preceding columns 

 and the weight 1 to each of the others. The observations of Mr. Pickehixg on the moon were made under unfavorable 

 circumstances, and the light ratio in the violet depends upon a single series of three readings. Those made at this 

 observatory in 1883 were for the purpose of experimenting on the best arrangement of apparatus, and not made with 

 a view to obtain the best quantitative results, while the values given by Dr. Vogel and by the Allegheny ob.servations 

 of 1864 are the results of many and careful observations throughout the entire range of the spectrum. The weight 

 which we have assigned to them would not therefore appear to be too great. 



With the aid of this table we may make an efifbrt to draw the lunar energy curve. Within 

 the limits of our observations an increase in energy in a definite part of the spectrum is followed 

 by a proportional increase in brilliancy,* so that the figures in the last column, which represent 



* With iutenser lights than we employ certain physiological phenomena affect this proportionality, which is 

 here, however, sensibly exact. 



