5i8 



NA TURE 



\_Oct. 14, 1875 



disc raiiiates more powerfully towards the earth than the 

 rest, in which case a higher intensity than 7° C. will be 

 indicated by the actinometer referred to. It will be readily 

 understood that the solar rays entering through the perforations 

 at the upper end of the tube, converge at the lower end and 

 piss through the small perforations, causing maximum indication 

 of the f jcal actinometers as stated. Now, suppose that a cir- 

 cular plate, the area of which is exactly % of the apparent area 

 of the sun, viz. 145-2 millimetres diameter, be inserted concen- 

 trically in either of the two large perforations of the diaphragm 

 at the top of the telescopic tube. The apparent diameter of the 

 sun being as before stated i62'4 millimetres, it will be perceived 

 that the inserted plate will only partially exclude the solar radia- 

 tion, and that the rays from a zone l' 42" wide will pass outside 

 the said plate, converging in the form of a hollow cone at the 



lower end of the tube, and there enter the respective actinometer. 

 The indication of the latter will then show the thermal energy 

 transmitted by radiation from a zone whose mean width extends 

 49" from the sun's border. It should be particularly observed 

 that the three focal actinometers employed will be acted upon 

 simultaneously by the converged rays, (i) from the entire area of 

 the solar disc, (2) from a central region containing \ of the area, 

 and (3) from a zone at the border containing also \ of the area of 

 the solar disc. It is scarcely necessary to point out that an 

 accurate comparison of the intensity of the radiant heat emanating 

 from the central part and from the sun's border calls for simul- 

 taneous observation, in order to avoid the errors resulting from 

 change of zenith distance and variation of atmospheric absorption 

 during the investigation. The great advantage of obtaining also 

 a simultaneous indication of the intensity transmitted by radiation 



Fia 2; 



FIG. 3. 



from the entire solar disc is self-evident, since this indication 

 serves as an effectual check on the observed intensities emanating 

 from the centre and from the border. The latter obviously must 

 be less, while the former must be greater, for a given area, than 

 the indication of the focal actinometer which receives the radia- 

 tion of the entire solar disc. 



The foregoing demonstration, based on hypothesis, having 

 established the possibility of ascertaining by direct observation 

 the temperature produced by the rays projected from certain 

 parts of the solar surface, let us now examine the means actu- 

 ally employed. An observer on the 40th deg. latitude, stationed 

 on the north side of a building 28 metres high pointing east and 

 west, can just see the sun pass the meridian, during the summer 

 solstice, if he occupies a position about 8 metres from such 

 building. Now, if an opaque screen perforated by a circular 

 opening 313 millimetres in diameter be placed on the top of the 



supposed building, the entire solar disc may be seen through the 

 same, provided it faces the sun at right angles. But if the per- 

 foration in the said screen be 140 millimetres in diameter, only \ 

 of the area of the solar disc will be seen. And if the screen be 

 removed and a circular plate 280 miUimetres in diameter put in 

 its place, the observer, ranging himself in line with the plate 

 and the sun's centre, can only see a narrow border i' 42" of the 

 solar disc. Obviously the screen placed on the top of the build- 

 ing might be perforated like the upper diaphragm of the sup- 

 posed telescopic tube, and a plate resembling the lower dia- 

 phragm, secured by appropriate means near the ground, might be 

 made to support the focal actinometers in such a manner that 

 their axes pass through the centres of the perforations of the 

 screen above the building. It is hardly necessary to state that 

 the plate supporting the actinometers should be attached to 

 some mechanism capable of imparting to it a parallactic move- 



