M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 7 1 



observing. The smoke gradually filled the Sacramento 

 Valley to a height of 10,000 feet, and obscured the outline 

 of the Coast Range Mountains; but we observed the helio- 

 tropes through this irregularly heated atmosphere with little 

 difficulty, and at the close of the operations the probable 

 error of a direction was only seven hundredths of a second 

 of arc. 



Besides the disturbance of the outer atmosphere near the 

 observer, there is another source of disturbance in the air 

 within the tubes of the great telescopes, especially in sun- 

 light observations or at the sudden change of temperature 

 at sunset. This is visibly exhibited in the blurred, unsteady 

 and confused outline of the Sun and the Sun-spots projected 

 upon the ground-glass plate of the horizontal heliograph 

 tubeless telescope of forty-feet focus. In the transit of 

 Venus of 1882 the image of the Sun at Cerro Roblero was 

 projected through a forty-feet tin tube of large diameter, 

 and was much blurred ; but this was largely corrected when 

 the tube was covered with a wooden roof not in contact. 



Professor A. E. Douglass, of the Lowell Observatory, 

 has shown that the waves of the disturbed atmosphere out- 

 side and inside of the large telescope tubes are distinctly 

 visible to the naked eye when it is placed in the focus of 

 the objective, as in the Foucault test. And even more than 

 one series of such waves of disturbance at different dis- 

 tances and moving in different directions is not infrequent. 



It is well known that to obtain the best results with the 

 great telescopes it is imperative that the air inside the dome 

 and inside the telescope tube shall have the temperature of 

 the outside atmosphere. 



At Arequipa, Peru, the Harvard Observatory was situated 

 close to the valley of a mountain stream or arroyo, down 

 which, on clear nights, a swift stream of cold air descends. 

 Whenever this cold stream overflowed the banks and envel- 

 oped the observatory, and rose to the height of the objective, 

 the seeing was immediately ruined for the rest of the night. 

 In such circumstances, if the eye-piece of the telescope was 

 removed and the eye placed in the focus, fine parallel, dark 



