July 1, 1898.] 



KNOWLEDGE, 



155 



vations. The change in position angle, change in altitude, 

 varying transparency of the sky, and other causes conspire 

 to prevent us determining brightness accurately to one- 

 tenth of a magnitude, let alone anything less than this. 

 Yet if we want to get a light curve with accuracy, we 

 ought to have it to the one-huudredth of a magnitude. 



Wiih the photometer there seems more hope and greater 

 promise. Fig. 3 is a reproduction of a diagram by Prof. 

 Pickering, showing the light curves of four variable stars 

 as resulting from photometrical observations. In the case 

 of W Delphini it will be seen that the dots representing 

 the observations are exceedingly close to the curve, the 

 average deviation being between one and two hundredths 

 of a magnitude.* However, in none of the four stars is 

 the curve absolutely flat at minimum. I think we may 

 conclude that for Algol type stars the shape of the light 

 curve can only be thoroughly determined by continued 

 observations made with some form of photometer. 



Fia. 3. — Light Curves as observed with Photometer (Pickering). 



In the above notes we have supposed each star to have 

 a sharply defined limb. In nature this may not always 

 be the case. If we could see our sun projected on the 

 blackness of space, the corona with its streams and wisps 

 of light would appear to surround it, and the limb would 

 not perhaps be so sharp as we see it through the glare of 

 our illuminated atmosphere, which cuts ofi" the corona. 

 Again, some stars, for all we know, are in an earlier stage 

 of existence than the sun, and only partially condensed 

 from the primitive nebulous matter. So their limb or 

 boundary would be of a cloudy, nebulous nature. In such 

 cases our hard-and-fast light carve would be considerably 

 modified ; the change of light would be more gradual, 

 and there would be a tendency to a rounded curve at the 

 central depression. 



Again, the bright globe might have an absorbing atmo- 

 sphere, so that the light faded away towards the edges and 

 the centre would be brighter than the limb. This might 

 modify the typical light curve, as the eclipsing body would 

 stop out more or less light according to the different 



* Astrophysical Journal, Yul. IV., Xo. 5. 



positions occupied on the disc of the larger globe, irre- 

 spective of the actual area covered. 



We have only referred to the period during which the 

 light changes markedly ; but there has been much dis- 

 cussion of late years as to whether Algol varies when in 

 full light, and the matter seems at present more or less 

 sidi judicc. When we read, however,* that under Prof. 

 Pickering's direction, one thousand eight hundred and 

 fifty-six photometric observations have been made of 

 W Delphini, three thousand two hundred and ninety-six 

 of I' Cephei, two thousand five hundred and ninety-two 

 of S C'ancri,one thousand five hundred and eighty-four of 

 S Cephei, etc., etc., we may be quite certain that the subject 

 will sooner or later be thoroughly thrashed out, probably 

 long before the telescope is made which will reveal to 

 direct vision the occulting globe passing in front of its 

 primary. 



♦ 



THE RECENT ECLIPSE. 

 THE LICK PHOTOGRAPHS OF THE CORONA. 



By E. Walter Maunder, f.r.a.s. 



THE two photographs of the corona which are 

 reproduced in the accompanying plate are copies 

 of transparencies which were most kindly pre- 

 sented to us by Prof. W. W. Campbell. It will 

 be remembered that the late Colonel C. F. Crocker, 

 who had on two previous occasions provided the funds for 

 eclipse expeditions organized from the Lick Observatory, 

 had undertaken the cost of one to observe the late eclipse 

 in India. The astronomer in charge of the expedition 

 was Prof. W. W. Campbell, the spectroscopist of the 

 Observatory, and he was accompanied by Mrs. Campbell 

 and Miss Eowena Beans as volunteer assistants, travelling 

 at their own private expense. Prof. Campbell was also 

 assisted in India by Captain Fleet and Mr. Garwood of 

 the Royal Navy. 



Prof. Campbell's chief instrument was the great photo- 

 graphic telescope of five inches aperture and forty feet 

 focal length. This was firmly fixed, and the sensitive 

 plate was made to follow the sun. With the sun more 

 than fifty degrees high at mid-totality, the mounting of 

 such a monster was a very serious business ; the more so 

 as Prof. Campbell was by no means satisfied with such 

 native carpenters as he could procure. To support the 

 object-glass end, he built a firm wooden tower — well seen 

 in the accompanying photograph — some twenty-three feet 

 in height, whilst the camera end was received in a pit 

 some eight feet deep. The plates used with this great 

 telescope were seventeen inches long by fourteen wide, and 

 the image of the moon was very nearly four and a half 

 inches in diameter. The larger of the two photographs 

 in the plate is copied from one of these, and was given an 

 exposure of one second. 



Beside this great telescope, two other photographic tele- 

 scopes — the Dallmeyer and Floyd — were also employed, 

 with focal lengths of three and five feet. With these two 

 smaller instruments eight beautiful negatives were obtained, 

 and the smaller photograph in the plate is an enlargement 

 from one of these taken with the Floyd telescope with an 

 exposure of ten seconds. The forty-foot telescope gave 

 twelve negatives, of which nine were extremely satisfactory. 

 The exposures varied from an " instantaneous " one up to 

 sixteen seconds. 



Beside the photographic cameras. Prof. Campbell had 

 a number of spectroscopic cameras, his principal objects 



* Fifty-second Annual Report of the Harvard College Obserratory. 



