abbot] new ccelostat and horizontal telescope 85 



4.70 meters focus respectively, in connection with a ccelostat and 

 second reflection from a plane mirror east of the polar axis of the 

 ccelostat, while doing away with rotation of the field, and with the 

 chief defects in following, still furnished a very poor image, subject 

 to changes of focus of ten feet between full and cloudy sunlight, and 

 attended with serious " boiling," which was not at all reduced by 

 providing a canvas tube for the beam, but which did diminish when 

 the sky was very thick with cirrus clouds or haze. 



It was thought best to arrange to cast the image to its full size of 

 forty centimeters by means of a single concave mirror, but before 

 ordering this, Mr. Langley, following his well-approved policy of 

 trying a new thing on a small scale, first procured a five-inch mirror 

 of forty feet focus, and directed that this should be set up for a pre- 

 liminary trial. Recognizing that the " boiling " (or fluttering con- 

 fusion of all parts of the image due to variability of the strata of air 

 traversed by the beam) would probably prove the main obstacle to 

 forming a well-defined solar image, he devised and directed a novel 

 experiment of " churning " the column of air traversed by the beam. 

 This experiment was made with a very satisfactory result as de- 

 scribed by Mr. Langley in his article on Good Seeing,'^ and with 

 more detail as regards apparatus in the appendix to the administra- 

 tive report of the Secretary of the Smithsonian Institution for the 

 year ending June 30, 1902. 



In brief it appeared that " boiling " caused within the telescope 

 tube itself could be entirely removed by churning the air, and that 

 for high sun a churned tube pointing toward the sun and reaching 

 forty feet above the surface of the ground sufficed to overcome the 

 main portion of the prejudicial disturbances of the air, so that the 

 remaining " boiling " of the solar image did comparatively little harm 

 to the definition. It was observed with the forty-foot focus telescope 

 that the vigorous churning of the air seemed to decrease those 

 changes of focus with varying cloudiness which had been noted in 

 the earlier work, and which were observed also with the forty-foot 

 focus instrument when the stirring apparatus was stopped. Among 

 the incidental advantages of the stirring may also be included the 

 more rapid convection of the heat of absorption of the solar beam 

 at the mirror surfaces, and consequent diminution of the alteration of 

 figure which is always caused by unequal heating of glass mirrors. 

 In the preliminary tests it was found that the stirring apparatus 

 communicated vibrations to the ground sufficiently serious to pro- 

 duce a prejudicial tremor of the image, but it was noted that the pass- 



^ American Journal of Science, February, 1903. 



