40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 52 



intensity of the infra-red spectrum, ratios like these just given are 

 not to be regarded as constants. In order to avoid errors from this 

 cause, care was taken on eclipse day to determine the transmissibility 

 of sun-brightness immediately before and after totality, as will appear 

 in its place. 



On other days before the eclipse, values of the transmissibility 

 ratio for sun-brightness were obtained, ranging from 029 to 0.37. 



On the morning of December 27, at 3^ 30™ a. m., the transmissi- 

 bility of the moon-brightness was found to be 0.50.^ It is very sig- 

 nificant to note that the day sky and the moon, both reflecting sun 

 rays, alter the quality of sun rays in opposite directions and in such 

 marked degrees. The blue quality of the sky-brightness, as Lord 

 Rayleigh has shown, is probably due to the fact that the reflection 

 takes place from particles small compared with the wave-length of 

 light, and principally perhaps from the molecules of air themselves. 



In view of the data just given, we should suppose that the bright- 

 ness of the solar corona, if we imagine it to be caused merely by the 

 reflection of ordinary sun rays, would be more transmissible to the 

 asphaltum screen than sun-brightness, if the reflecting particles are 

 of gross magnitude, like those composing the surface of the moon ; 

 but less transmissible than sun-brightness, on the other hand, if the 

 reflecting particles are minute like the molecules of gases. 



Observations on Eclipse Day 



The approach of totality was uncommonly exciting on this oc- 

 casion. Early in the morning the sky was overcast with high clouds, 

 but these gradually grew thinner, so that after 9 a. m. the prospects 

 indicated a streak}^ sky containing something almost too thick for 

 haze, but almost too thin for cirrus clouds. These prospects were 

 fulfilled exactly during totality, but in the quarter of an hour next 

 preceding a thick cloud came up, rain fell fast from 11*^ 08™ to 

 II '^ 14°^, and the view of the sun became clear of the rain-cloud only 

 15 seconds before totality, at the Smithsonian station. The rapid 

 change from fair prospects to completely discouraging ones and the 

 return of good conditions just at the critical time will long be re- 

 membered- Our entire immunity from rain during totality was due 

 to the fact that our station was about 1,000 feet north of the one 

 occupied by the Lick Observatory. Second contact was observed by 

 the writer, and recorded by Yeoman Chase at 11^ 15"^ 7.^5, local civil 

 time. 



' See also Langley's comparison of the visible spectra of the sun and moon. 

 Memoirs National Academy of Sciences, vol. in, 1884, p. 21. 



