REPORT OF THE SECRETARY. 95 



and several (juite ileciKJve experiiiientH wi're made ))y Vdiir direi-tion to determine 

 their effectiveness. 



Expcrimenis on "boiling^^ r>f llic image. — 1 add a few words, partly in anticipation, 

 in reference to your newly-introduced device for preventing the well-known effect 

 of "boiling" the telescopic image, a ditficnlty due to the earth's atmosphere and 

 which has existed always and everywhere, and which has seemed until lately so 

 insurmountable that it has not even been thought of as subject to correction. Tlie 

 device that you have suggested consists essentially not in keeping the air still within 

 and without the telescope tube, \mi in violently agitating it over as great a range as 

 possible. For this inirpose, under your iiistructions and with the object of rehears- 

 ing on a small scale what, if the device is successful, will be later tried on a large one, 

 a silvered glass mirror of 5 inches diameter and 40 feet focus has been set uj) in a tube 

 with several concentric walls, so as to leave the interior air as still as possible. In 

 jireliminary experiments the air, in spite of these precautions in the installation, gave 

 such "l)oiling" of the image as to seriously i)rejudice the delinition, and this, 

 although the 40-foot tube had no less than three walls, being 7 inches in interior 

 diameter and 15 inches in exterior diameter, and inclosing the beam all thcAvay from 

 a second plane mirror near the coelostat to the concave mirror and thence to the focus. 

 Though this tube containing the stillest air of the most uniform temperature was 

 sheltered throughout l)y a canvas tent, the "boiling" was but a little diminished. 

 Nothing was gained by diminishing the a])erture of the several mirrors down to 1 

 inch in diametei', and all this was only what had l)een anticipated from the ordinary 

 experience of astronomers. A 12-inch blower run ])y an electric motor was now 

 caused to exhaust air from tlic inner tube at a half dozen [loints along the tube, and 

 to force air in at other points alternate with these, so as thus violently to disturb or 

 "churn" the air by forcing a vigorous circulation of it along the whole path of the 

 beam from the ccelostat to the solar image. 1;^\m unquestionably reduced the "boil- 

 ing." An artificial star was now provided at the focal plane, the plane mirror near 

 the C(X'lostat was placed at right angles to the tube, and thus the concave mirror, act- 

 ing as both collimator and oljjective, broug])t the star image to focus at the star itself. 

 Here it was exanuned with an eyepiece. With still air the definition was often very 

 poor. The image assumed very variable shapes, with wings or streamers; and being 

 also colored by diffraction effects, reminded one of a kaleidoscope field. On starting 

 the blower the definition immediately became sharp. Violently stirring the air in 

 the tube, therefore, eliminates the "boiling" within the tube and (i)aradoxically) 

 produces a still image. As before said, the solar image was clearly imjiroved liy the 

 stirring; but further improvement was still to be desired. Accordingly a tin tul)c 44 

 feet long, with two walls of 11 inches interior diameter and Ifi inches exterior 

 diameter, was provided and arranged to point toward the sun, as sliown in Plate VI, 

 so that the beam passed down through it before reaching the mirror system. This 

 tube was connected to the blower exactly like the horizontal one, and l)oth could 

 bestirred at once. There was very marked "Iwiling" before starting the blower. 

 This nearly disappeared while the blower was running. One observer estimated 

 the "boiling" as four-fifths overcome; another thought little more than a quarter 

 remained, and all were unanimous that what was left was very little prejudicial to 

 the definition. This last result is so surprising that I feel constrained to add that 

 the experiments so far are not exhaustive, having been carried on but a short time, 

 and without that solidity of piers which would allow of exact estimate or photo- 

 graphic determination of the " l)oiling" of the solar image before and during agita- 

 tion of the air in the tube, as distinguished from mechanical jarring. Further experi- 

 ments are in progress. That agitation is of very great advantage to diminish "boil 

 ing" there is no question, but the exact measure of the advantage for all circum- 

 stances of bad seeing requires further study. Incidentally, the air blast has the added . 

 advantage of keeping the mirrors at uniform temjxirature. This and the abolish- 



