ASTRONOMY — HALE. 



^7 



seemed sufficient to warrant the construction of electric heating coils, 

 which will be mounted behind the three mirrors of the Snow telescope. It 

 is obvious that the manipulation of such a device will not be easy, and it 

 has not seemed wise to attempt to use it during the fine summer weather, 

 since the necessary experiments might affect seriously the routine obser- 

 vations. The difficulties which are experienced from changes of focal 

 length do not interfere appreciably with work on spot spectra, the solar 

 rotation, or holographic investigations. Even in spectroheliographic 

 work, which is much more exacting, the difficulty is reduced to a mini- 

 mum by shortening the exposure time as much as possible. 



The important results achieved by Dr. A. L. Day, in the manufacture 

 of fused quartz, are reported by him elsewhere. It seems probable that if 

 suitable graphite slabs can be obtained, the manufacture of fused quartz 

 disks for telescope mirrors will become feasible. The question at present 

 is whether sufficiently homogeneous graphite can be secured. 



As it has been proven in our tests that the distortion of a mirror in 

 sunlight is due to an actual bending of the disk, the front surface becom- 

 ing convex and the rear surface concave, the idea of employing very thick 

 mirrors has naturally suggested itself. Accordingly, the mirrors of the 

 coelostat to be constructed for the new "tower telescope," though only i6 

 inches in diameter, will be made of glass 12 inches thick. It is hoped 

 that the resistance to bending of such thick disks will materially reduce 

 the difficulty. 



Our experiments with speculum metal have indicated, as suggested in 

 the last annual report, that its distortion when exposed to sunlight is 

 somewhat less than in the case of glass. However, the low coefficient of 

 reflection of speculum metal is a serious objection to its use in an instru- 

 ment like the Snow telescope. 



Direct Photography of the Sun. 



Daily photographs of the sun, on a scale of (i.y inches to the diameter, 

 are taken by Mr. Ellerman with the Snow telescope. These plates are 

 used mainly for the purpose of comparison with spectroheliograph plates, 

 in studies of the spots and faculse. On account of the great pressure of 

 work with the Snow telescope, it is hoped that the direct photographs 

 may ultimately be made with another instrument, such as the tower tele- 

 scope. 



Work with the Spectroheliograph. 



The 5-foot spectroheliograph, mentioned in the last annual report, was 

 erected in the Snow telescope house in October, 1905. A general descrip- 

 tion of this instrument has been pubHshed in Contribution No. 7 of the 

 Solar Observatory, and a more detailed account, with scale drawings, will 



