RECENT ADVANCES IN SCIENCE 191 



both tubes being perfectly alike must have the same flexure. 1 

 Control experiments were made to prove that the apparatus 

 actually behaved in accordance with this conclusion, which 

 was found to be closely justified. 



The flexure of the telescope at any zenith distance is deter- 

 mined by bringing the horizontal wire of the ocular micrometer 

 upon its image reflected from the mirror back through the 

 telescope. Great precision in the results can be obtained. 

 The method was seen in application by the writer at the 

 Poulkowa Observatory in 19 14. The mean error for one ob- 

 servation was ±o"*c»5 and the flexure in any position was found 

 to be very closely in accordance with the formula asinz, 

 z being the zenith distance and a a constant which had the 

 value o"*405 for the telescope in question. 



When star observations are required to be made the 

 aluminium plate is removed by unhooking the ends of the 

 four spiral springs, counterpoising weights being placed on 

 the ends of the steel tubes. As this operation only takes about 

 one minute, determinations of flexure may be made, if so 

 desired, at any time during an evening's observations. 



A detailed account of the apparatus and of results ob- 

 tained is given in a jubilee volume to Prof. Donner, " Festskrift 

 tillegnad Anders Donner pa haus Sextioarsdag den 5 Nov. 

 1 9 14 af forne Elever." 



The Magnetic Field of the Sun. — The deduction from the 

 electromagnetic theory of light made theoretically by H. A. 

 Lorentz and verified experimentally by P. Zeeman, that by 

 passing a beam of light through a magnetic field each spectral 

 component is broken up into several components which 

 are polarised, the number and polarisation of the components 

 depending upon the direction in which the light passes relatively 

 to the direction of the lines of magnetic force, was applied 

 several years ago to the investigation of the general magnetic 

 field of the sun at the Mount Wilson Observatory. The results 

 obtained were given in the Astrophysical Journal, 38, 27, 191 3. 

 Using a polarising apparatus with the 75-foot spectrograph of 

 the large 150-ft. tower telescope, it was found that four lines 

 in the third order specimen of a Michelson grating gave Zee- 

 man displacements which agreed in sign and closely in magni- 



1 We may compare the deformation of a parallelogram of jointed rigid rods. 

 If one rod is fixed, the opposite rod must move parallel to itself. 



