TRANS ACTIOXS OF SECTION A. 585 



4. Atmosphere in its Effects on Astronomical Research. 

 By Peucival Lowell. 



In everj' astronomical observation the rays of light which give us our know- 

 ledge of things beyond the earth have to traverse three media — the air, the lens, 

 and the eye. Though much attention has been given to the lens, and not a little 

 to the observer, almost none has been paid to the atmosphere, which on investiga- 

 tion turns out to be the most important factor of the three. 



For the purpose of applying and studying this neglected and practically 

 unknown factor, the first work in which we owe to Professor W. II. Pickering at 

 Arequipa, the Lowell Observatory was put up at Flagstaif, Arizona. The practical 

 results on Mars, Venus, Mercury, Jupiter's Satellites, and Uranus were both sur- 

 prising to observers generally and revolutionary of previous ideas of these bodies. 



Mr. Douglas then discovered that the cause of bad seeing could itself be seen,. 

 and that it was due to distorting refraction produced by waves of condensation and 

 rarefaction in the air currents, which waves could be rendered visible as shadow 

 bands crossing the field of view. He determined the size of these waves, their 

 respective refractive powers, and the kind, speed, direction, and height of tho 

 currents. He thus found (1) that the seeing depends upon the absence of certain 

 currents ; (2) that the effectiveness of the object glass depends upon the size of 

 the waves prevailing at any given time and place ; (3) that the visibilities of limb 

 and detail are different ; (4) that the noxious currents can already, more or less, 

 be predicted. 



To minimise the harmful currents is, therefore, the object from a practical 

 point of view. To do this the locality must be as free as possible from- 

 moisture, since water vapour is an unsettling element, and be as little as possible 

 subject to change of any kind. These conditions are best satisfied by a large 

 oasis in the midst of a desert, which is the case at Flagstaff. 



Lastly, there is an absolute test of seeing, due to the laws of light, which can 

 and should be generally applied — the condition of the spurious disk and rings of 

 a star seen through a telescope. The scale is as follows : — 



Seeing 10. — Disk perfectly defined, rings the same, both motionless in field. 



Seeing 9. — Disk perfectly defined, rings the same, both moving slightly together 

 in field. 



Seeing 8. — Disk well defined, rings complete but moving, no bodily motion. 



Seeing 7. — Disk well defined, rings complete but moving, slight bodily motion. 



Seeing 6. — Disk well defined, rings tolerably complete, some bodily motion. 



Seeing 5. — Disk well defined, rings tolerably complete, bodily motion. 



Seeing 4. Disk well defined, rings broken into lines and dots, more bodily- 

 motion. 



Seeing 3, — Disk well defined, rings broken into lines and dots, much bodily 

 motion. 



Seeing 2. — Disk tolerably defined, no evidence of rings. 



Seeing 1. — Disk and rings in one confused mass, motion, slight increase in size. 



Seeing 0. — Disk and rings in one confused mass, violent. motion, image greatly 

 enlarged, 



5. Automatic Operation of Eclipse Instruments. 

 By Professor David P. Todd. 



6. The Cause of the Semi-annual Inversions of the Type Solar Curve in 

 the Terrestrial Magnetic Field. By Professor Frank H. Bigelow, 

 U.S. Weather Bureau, Washington, B.C. 



This paper gives a brief outline of the computation leading to the type curve, 

 the phenomenon of semi-annual inversion, and the explanation of the same. Thia 

 conclusion is then used to criticise certain views of the origin of the diurnal and 



