246 



Mr. W. E. Wilson. 



[Mar. 15, 



March 15, 1894. 

 The LORD KELVIN, D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : 



I. * The Thermal Radiation from Sun Spots. Preliminary 

 Notes of Observations made at Daramona, Streete, Co. 

 Westmeath, 1893." By W. E. WILSON, M.R.I. A. Com- 

 municated by G. JOHNS-TONE STONEY, F.R.S. Received 

 January 4, 1894. 



These observations were made by means of a large heliostat, lent 

 by the Royal Society, and a Boys's radio-micrometer. The heliostat 

 consists of a plane silver-on-glass mirror of 15 in. aperture. It is 

 mounted eqnatorially, and driven by a clock. When in nse, it is 

 adjusted to reflect the sunlight to the north pole, and, as long as the 

 driving clock is kept in motion, the beam of light remains fixed in 

 that position. In the track of this beam, and about 12 ft. from the 

 plane mirror, is mounted a concave silver-on-glass mirror of 9 in. 

 aperture, and about 13 ft. focus. Its axis points to the south pole, 

 so that'the coue of rays formed by it strikes the centre of the plane 

 mirror, and a short distance inside the focus. A small plane mirror 

 mounted on the end of an arm is then so placed as to intercept the 

 cone of rays, and reflect it horizontally into the observatory window ; 

 an achromatic lens enlarges the solar image which is formed on a 

 screen in the room to 4 ft. in diameter. 



Behind this screen, and standing on a pier of concrete, is mount 

 the radio-micrometer. The aperture through which radiant heat 

 reaches the sensitive thermo-couple is a round hole drilled through 

 a thick sheet of brass, and is only 1 mm. in diameter. A white card- 

 board screen is placed in front of the brass one to cut off heat from 

 falling on the latter, and is provided with a hole slightly larger. A 

 beam of lime light is thrown on the mirror of the radio-micrometer, 

 and reflected on to the scale in the usual way. The diagonal mirror 

 of the heliostat is provided with slow motions in two directions, 

 which are moved by long rods and Hook joints inside the observatory. 

 Thus any part of the sun's disc can be placed on the small aperture 



