412 



Dr. W. Huggins. On a Method of [Dec. 21 y 



refrangible parts of the spectrum does not affect the photographic 

 plates. 



Different times of exposure were given, from so short an exposure 

 that the sun itself was rightly exposed, to much more prolonged 

 exposures, in which not only the sun itself was photographically 

 reversed, but also the part of the plates extending for a little distance 

 from the sun's limb. 



Grelatine plates were used, which were backed with a solution of 

 asphalt um in benzole. 



After some trials I satisfied myself that an appearance peculiarly 

 coronal in its outline and character was to be seen in all the plates. 

 I was, however, very desirous of trying some modifications of the 

 method described with the hope of obtaining a photographic image of 

 the corona of greater distinctness, in consequence of being in more 

 marked contrast with the atmospheric illumination. 



Our climate is very unpropitious for such observations, as very few 

 intervals, even of short duration, occur in which the atmospheric 

 glare immediately about the sun is not very great. Under these cir- 

 cumstances I think it is advisable to describe the results I have 

 obtained without further delay. 



The investigation was commenced at the end of May of this year, and 

 the photographs were obtained between June and September 28th. 



The plates which were successful are twenty in number. In all 

 these the coronal form appears to be present. This appearance does 

 not consist simply of increased photographic action immediately about 

 the sun, but of distinct coronal forms and rays admitting in the best 

 plates of measurement and drawing from them. This agreement in 

 plates taken on different days with different absorptive media interposed, 

 and with the sun in different parts of the field, together with other 

 necessary precautions observed, makes it evident that we have not to 

 do with any instrumental effect. 



The plates taken with very short exposures show the inner corona 

 only, but its outline can be distinctly traced when the plates are 

 examined under suitable illumination. When the exposure was 

 increased, the inner corona is lost in the outer corona, which shows 

 the distinctly curved rays and rifts peculiar to it. 



In the plates which were exposed for a longer time, not only the 

 sun but the corona also is photographically reversed, and in these 

 plates, having the appearance of a positive, the white reversed 

 portion of the corona is more readily distinguished and followed in its 

 irregularly sinuous outline than is the case in those plates where the 

 sun only is reversed, and the corona appears, as in a negative, dark. 



Professor Stokes was kind enough to allow me to send the originals 

 to Cambridge for his examination, and I have his permission to give 

 the following words from a letter I received from him : " The appear- 



