THE NEW CCELOSTAT AND HORIZONTAL TELE- 

 SCOPE OF THE ASTROPHYSICAL OBSERVATORY 

 OF THE SMITHSONIAN INSTITUTION 



By C. G. abbot 



INTRODUCTION 



About two years ago certain observations on the absorption of the 

 solar envelope were begun at the Astrophysical Observatory of 

 the Institution under Mr. Langley's direction. For these experi- 

 ments a solar image 40 centimeters in diameter was formed at 

 the slit of the spectro-bolometer, and energy spectra were obtained 

 of the radiation from the center of the disk and at points near the 

 limb. As it is clear that the radiations coming from points more 

 and more remote from the center must traverse greater and greater 

 thicknesses of the solar envelope, this method of investigation offers 

 a means of finding out something about the amount and quality of 

 its absorption. It was hoped to determine also the nature of the 

 energy spectrum of sun spots, but the arrangements then used proved 

 inadequate to give sufficient steadiness and distinctness of the solar 

 image even for satisfactory measures of the absorption of the solar 

 envelope, to say nothing of measures on sun-spot spectra. 



The apparatus used at that time for forming the solar image con- 

 sisted of a large Grubb siderostat of the Foucault type giving a hori- 

 zontal southerly directed beam, a nine-inch concave mirror of 2.30 

 meters focus, and a small convex mirror of about a meter radius of 

 curvature placed inside the focus of the concave mirror. These 

 latter two mirrors both had to be out of axis in order to form the 

 image at one side of the concave mirror, and both were within the 

 observatory, so that the beam from the siderostat passed through 

 an aperture in the wall to reach them. With these crude arrange- 

 ments it is not surprising that the solar image was in very bad and 

 changeable focus, and subject to continual and excessive disturbance 

 by " boiling," tremor, and bad following, besides being subject to 

 the rotation of the field necessarily accompanying the use of the 

 Foucault form of siderostat. 



A trial of a combination of two concave mirrors of 2.30 meters and 



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