Researches in Stellar Photography. 



203 



A comparison of the numbers in the last column affords a definite 

 indication that there does exist in nature a general definite relation 

 between the photographic diameters and the intensities of the stellar 

 lights impressed on the plate, and that this relation is truly repre- 

 sented by the equation — 



D-D'=S{log M'-log M}. 



Nevertheless it is also made evident that although this relation is a 

 fact for stars on the average, there are a few stars which stand out 

 from that average, a result which might have been anticipated on 

 account of the salient character of special spectra occasionally met 

 with in certain stars. In the group examined above, the stars 

 having this abnormal actinic action are Pleione, and Nos. 22 and 31. 

 These stars give the same evidence of peculiarity in all the three 

 plates ; and from this circumstance it becomes clear that if a compari- 

 son is at any time to be made between the photographic and the 

 photometric magnitudes, more than one plate should be taken, in 

 order to be sure that any discordances are due to variations in the 

 actual actinic action, and not to accidental circumstances. Omitting 

 these three stars, the average deviation of the magnitude here com- 

 puted photographically from that photometrically determined, is 

 0'16 mag. If these three stars be included it is 0*19 mag. Had 

 it not been that the Pleiades group was rapidly approaching the 

 sun at the time of these observations, the character of the spectra 

 of the three exceptional stars would have been examined, with the 

 view of explaining, if possible, their peculiar actinic action on the 

 photographic film. 



The general results of the foregoing research appears to be — 

 Pirst. There does in general exist a remarkable and definite relation 

 between the intensity of the actinism of a star on a photographic film, 

 as expressed by the area of the image formed, and the intensity of the 

 light as measured by a photometer. That relation also exhibits a 

 certain analogy to the relation between the photometric magnitude 

 and the actual intensity of the light of a star. These relations being 

 expressed by the formula — 



M 1/ 

 --KlogL" 



D , M' 



where the relation is sufficiently evident. 



Secondly. Both these relations are disturbed in the cases of any 

 salient difference in the colours of particular stars, or at all events 

 present analogous difficulties in dealing with them by way of definite 

 measurement. 



