the Corona during the Total Eclipse, August 7, 1869. 283 



positives especially show that this effect extends over a large part 

 of the sun's disk. The exposure was rendered instantaneous by- 

 passing a diaphragm with a slit in it in front of the camera, the 

 rapidity of motion being regulated by a series of springs. Any 

 irregularity in the motion would cause variations in shade in the 

 photographs; but these would form bands parallel to the slit, 

 while the shade mentioned above was not parallel to it and was 

 curved so as to follow the moon's edge. Since, then, there is an 

 increase both of the actinic power and of the heat, it would seem 

 that these effects are real, since the methods of observing them 

 are so totally different that no error in one could be introduced 

 into the other. The only explanation of the phenomenon that 

 seems possible is to assume the presence of a lunar atmosphere. 

 The corona would then be caused by refraction, light reaching 

 the observer from parts of the sun already eclipsed. Although 

 for various reasons this hypothesis is unsatisfactory, yet it is 

 strengthened by other observations. The protuberances have 

 often seemed to indent the moon's edge, an appearance usually 

 ascribed to irradiation. Several of the photographs, however, 

 show this same effect ; and in some of them the exposure was so 

 short and the edges of the protuberances are so well defined that 

 it cannot be caused by the intensity of their light, but must have 

 its origin outside of the eye of the observer. It is noticeable on 

 all sides of the moon, sometimes in half a dozen protuberances 

 in a single photograph. An atmosphere of rapidly increasing 

 density might produce this effect by reflection, and of course 

 would not influence the corona if it was caused by refraction. On 

 this supposition reliance could not be placed on measurements 

 of the moon's diameter by occultations, or by contacts during 

 eclipses, and would account for the uncertainty of this constant. 

 The principal reason for supposing the corona a portion of the 

 sun is, that during totality it does not appear to move with the 

 moon, but remains concentric with the sun, or, more properly, 

 is brightest where the sun's edge is nearest. Many of the pho- 

 tographs show this very well, the difference on the two opposite 

 sides of the moon being very marked. Now this effect would 

 be explained equally well by supposing the corona caused by 

 refraction. For the centres of the sun and moon never differ 

 during totality by more than half a digit, while the breadth of 

 the corona is sometimes several times as much ; so that merely 

 covering a small portion of it would not produce a greater di- 

 minution of light than would be caused by a slight change in 

 the direction of the sun's rays shining through a lunar atmo- 

 sphere. On the other hand, it is difficult to conceive of an at- 

 mosphere dense enough to produce these effects, and yet so 

 transparent that the edges of the full moon are perfectly di- 

 ll 2 



