OcL 14, 1875] 



NATURE 



519 



mett, during the observation, corresponding with the sun's decli- 

 nation and the earth's diurnal motion ; and, that some adequate 

 mechanism should be employed for regulating the position of 

 the perforated screen and adjusting the focal distance in accor- 

 dance ".vith the change of the subtended angle consequent on the 

 varying distance from the sun. It will be evident that since the 

 first-named mechanism rests on the ground, while the latter is 

 secured to a massive building, far greater steadiness will be 

 attained by our simple and comparatively inexpensive device, 

 than by employing a telescopic tube of the most perfect con- 

 struction mounted equatorially. 



With reference to the influence of diffraction, it should be 

 stated that before determining the size of the screens intended to 

 shut out certain parts of the solar disc during the investigation, 

 the amount of inflection of the sun's rays was carefully ascer- 

 tained. Two distinct methods were adopted : (i) measuring the 

 additional amount of heat transmitted to the focal thermometers 

 in consequence of the inflection of the rays ; (2) increasing the 

 theoretical size of the screens until the effect of inflection was 

 overcome and the luminous rays completely excluded. Regarding 

 the first-named method of ascertaining the diff"raction, it is im- 

 portant to mention that the temperature transmitted to the focal 

 actinometers by the inflected radiation which passes outside of 

 the theoretically determined screens is not proportionate to the 

 inflection ascertained by the process of enlargement referred to. 

 This circumstance at first rendered the investigation somewhat 



complicated, but it soon became evident that the discrepancy is 

 caused by the comparatively smill inflection of the invisible, heat 

 rays. It will be seen presently that the radiant heat which 

 passes outside of the screens in consequence of diffraction U 

 considerably less than that which would be transmitted to the 

 focal actinometers if the calorific rays were subjected to an 

 amount of inflection corresponding with the enlargement of the 

 screens beyond the theoretical dimensions necessary to exclude 

 the luminous rays. 



Let us first consider the method of ascertaining the inflection 

 of the rays by measuring the additional amount of heat trans- 

 mitted to the focal actinometers. Fig. i, see illustration, repre- 

 sents the solar disc, a being the focal actinometer exposed to the 

 converged rays, d a' representing an imaginary plane situated 

 177 metres from a, at which distance the section of the pencil of 

 converging rays will be 162-4 millimetres in diameter, provided 

 the earth is near aphelion. Fig. 2 also represents the solar disc, 

 and c the actinometer exposed to the converged rays ; but a per- 

 forated screen // b' is interposed, the perforation being of such a 

 size that only the rays projected by the central half of the solar 

 disc (indicated by the circle b b) pass through the same and reach 

 the focal actinometer. The screen b' b' being situated 177 metres 

 from c when the earth is in the position before referred to, the 

 said perforation must be ii4'83 millimetres In diameter, in order 

 that the lines 1^ ;»;' c may be straight Fig. 3 likewise represents 

 the solar disc, its area being divided in two concentric halves by 



the circle dd ; but in place of a perforated screen, an opaque cir- 

 cular screen d' is introduced at the same distance from the focal 

 actinometer as in Fig. 2 ; consequently the lines dy'f will be 

 straight. Now, if the actinometers a, c, and /be exposed to the 

 converged solar radiation simultaneously and during an equal 

 interval of time, c and/ receiving the heat from one half of the 

 solar disc (the former from the central and the latter from the 

 surrounding half), the temperatures of c and / added together 

 should correspond exactly with the temperature transmitted from 

 the entire solar disc to a. Observation, however, shows that 

 the temperatures of c and / together is o'ogi greater than the 

 temperature imparted to a. Hence an increase of temperature 

 of nearly one-eleventh is produced by the inflection of the calorific 

 rays, one-half being the result of the bending of the rays within 

 the perforation of the screen b'b\ the other half resulting from 

 the bending outside of the screen d'. The increment of tempe- 

 rature being thus known, the degree of inflection may be easily 

 determined by drawing a circle x x round the circle b b, covering 



an additional area of — ^^ = 0-0455 5 a"'^ ^J inscribing a circle 



2 

 yy within ddy covering an area of 0-0455 ^^ss than the area of 

 d d. It will be perceived on reflection that xx' b represents the 

 angle of inflection of the calorific rays within the perforation of 

 the screen b' b' , and that d-/ y represents the angle of inflection 

 outside of the screen d'. Demonstration shows that the former 



angle measures I4""57, while the latter measures 14" -86, the 

 mean being I4''-7I. Having thus determined the inflection 

 resulting from invisible radiation, let us now ascertain the inflec- 

 tion of the luminous rays. As before stated, the apparent 

 diameter of the sun at a distance of 177 metres from a given 

 point is 162-4 millimetres when the luminary is furthest from the 

 earth. Now our investigation shows that a screen 167 milli- 

 metres in diameter hardly suffices to exclude the luminous rays ; 



hence their inflection amounts to 



167 - 162-4 



= 2-3 millimetres 



at a distance of 1 7-7 metres. Their angle of inflection will there- 

 fore be 26" -8 r, against 14 "-71 for the dark rays. We have thus 

 incidentally established the fact that the inflection of the luminous 

 and calorific rays differs nearly in the same proportion as the 

 calorific energies of the visible and invisible portions of the solar 

 spectrum. 



Our space not admitting of a detailed account of the result ot 

 the investigation, the leading points only will be presented. 

 The observations have all been made at noDn, the duration of 

 the exposure to the sun having been limited to seven minutes, 

 during which period the actinometers are moved, by the paral- 

 lactic mechanism, through a distance of about 55 centimetres, 

 from west to east. The intensity of the radiant heat imparted 

 to the actinometers has been recorded by the observers at the 

 termination of the fourth, fifth, sixth, and seventh minute, the 



