( 668 ) 



Avlüch either a photometer, or a thermopile, a bolometer or a radio- 

 micrometer was used for exploring an image of the Sun. The results 

 obtained by ditferent observers are rather discordant '). This may be 

 partij' due to instrumental or accidental errors, but there is also a 

 systematical error which must have influenced similarly all of the 

 results thus obtained, and wliich proceeds from the scattering of the 

 rays by the terrestrial atmosphere. In any point of an image of the 

 Sun is not only to be found the radiation coming from the corre- 

 sponding point of the disk, but, besides, some diffused radiation 

 proceeding from other parts of the disk. This disturbing eflect will, 

 of course, vary in magnitude with the condition of our atmosphere, 

 but it will always act in a levelling way, parts of the image lying 

 near the edge receiving more diffused radiation from the middle 

 parts of the disk, than receive the central parts of the image fi-om 

 the marginal parts of the disk. 



We may completely avoid this source of error by using a method 

 in which the radiating power of the different parts of the disk is 

 calculated from observations made on the occasion of a total eclipse 

 of the Sun. 



Let us suppose the curve, representing the intensity of tiie solar 

 radiation from the first until the fourth contact as a function of time, 

 to be exactly known'). The curve will show us by how much the 

 total radiation has increased or decreased between any two epochs. 

 Every (positive or negative) increment is exclusively due to rays 

 coming from that strip of the solar disk through which the Moon's 

 limb has appeared to move between those very epochs. 



Suppose the time after third contact to he divided into equal 

 intervals of, say, 2 minutes, and the position of the Moon's limb 

 at the end of each interval delineated on the solar disk, then the 

 latter will be divided into 39 narrow strips, successively contributing 

 the knoion quantities a, b, c, d , . . to the total radiation. 



Now, let us distinguish n concentric zones on the solar disk and 

 denote by .t^, x^ , . . x-, the nxdiation coming from these zones per 



1) Cf. J. ScHREiNER, Slrahluiig und Teniperatur der Sonne, p. 43—49 (1899). 



-) It is well known that, at Burgos, the observation of the eclipse of August 30, 

 1905, has not been favoured with a clear sky (Gf. the Preliminary Report in the 

 Proceedings of the Meeting of November 25, 1905). Nevertheless, the measurements 

 of total radiation have yielded some results of sufficient accuracy to justify that, 

 in our present investigation, we make use of the radiation curve then secured. 

 Further particulars regarding the observations will soon be published in the 

 complete report on our expedition. 



