SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. 1 69 



TRANSPARENCY OF THE ATMOSPHERE. 



I have previously alluded to the dust-storms which sometimes 

 enter the San Gabriel Valley through the Cajon Pass from the 

 Mojave Desert, and those much rarer storms in which the dust 

 is carried by the wind completely over the Sierra Madre Mountains. 

 In the more common form of dust-storm (the so-called "Santa Ana") 

 the dust enters the valley in a fairly well-defined mass and proceeds 

 westward along the canyon of the Santa Ana River. In approach- 

 ing the coast it spreads over a large area and dififuses itself with 

 tolerable uniformity through the lower atmosphere. I have seen 

 from Mount Wilson a dust-storm in the region of Riverside, 

 which in twenty-four hours had spread itself over Los Angeles 

 and Pasadena. When it reached this part of the valley there was 

 almost no wind, and the dust seemed to diffuse itself through the 

 air. Such storms sometimes completely hide the Sierra Madre 

 Mountains from observers in Pasadena. Fortunately they are 

 almost always confined to the lower atmosphere, and do not appre- 

 ciably affect the transparency of the sky above Mount Wilson, 

 where daily observations show that the transparency of the day and 

 night sky are very satisfactory. 



SEEING. 



Systematic tests of the definition of the solar image have been 

 made on ]\Iount Wilson with a telescope of 3^4 inches aperture, with 

 an eyepiece giving a power of about 100 diameters. At first the 

 character of the seeing was rated on a scale of 5 ; but it soon ap- 

 peared that a scale of 10 would be preferable under the existing con- 

 ditions. Accordingly, the seeing as recorded in the following table 

 is given on a scale of 10. Seeing 8, which is so frequently obtained 

 during the early morning hours, represents a sharply defined image 

 of the sun. showing the granulation and the details of the spots with 

 great distinctness, and indicating practically no trembling at the 

 limb. Such seeing occurs at the Yerkes Observatory only occa- 

 sionally, although that observatory seems to be better situated than 

 many other institutions for work on the sun. 



An examination of the table will show that the seeing is best dur- 

 ing the early morning hours, although the image is frequently very 

 good in the late afternoon. Shortly after sunrise the sun's limb is 

 serrated, but this effect becomes less and less marked as the sun's 

 altitude increases. Usually, at this time in the morning, the atmos- 

 phere is almost perfectly calm and cloudless. The seeing usually 



