402 



NATURE 



\_AtigMst 15, 187S 



"Assuming the adoption of an equatorial camera 

 twenty inches square, provided with a lens whose inten- 

 sity ratio is one- sixth and whose focal distance is about 

 thirty-three inches, it yet remains to consider how this 

 apparatus should be managed during a totality lasting 

 only three minutes. As the illumination of different 

 parts of the corona varies greatly, there can evidently be 

 no certainty of geting all the details of the phenomenon 

 unless a series of plates are taken, in which the exposures 

 vary from the shortest possible up to the point where it is 

 certain that an increase of time does not improve the 

 picture. On this account it will be desirable to take as 

 many as six plates, the exposures being, respectively — 



3'^, 5 s, 10", 20% 40% and 60*. 



The first four of these plates will receire such short ex- 

 posures that it is unlikely they will show anything but 

 the corona, and therefore their size should be 4^ by 5^ 

 inches. With the last two plates the case is different. 

 Their size should be 17 by 20 inches, because their longer 

 exposures will probably suffice to bring out upon them 

 any bright points which may exist within their field. A 

 lens such as is here under consideration should depict an 

 eighth-magnitude star in about one minute, but of course 

 the intensity of the sky-illumination during totality will 

 determine the limit of brightness at which faint luminous 

 points will cease to impress themselves upon the nega- 

 tives, and what this limit may be it is impossible to 

 predict. The necessity for at least two large plates is 

 evident when it is remembered that the image of a small 

 bright point could not be distinguished from an accidental 

 blemish in the film, and it would only be by finding it 

 upon both plates that its true character could be unmis- 

 takably recognised. It is exceedingly desirable to deter- 

 mine accurately the maximum exposure that the corona 

 will bear with advantage, and it is hoped that on at least 

 one of the large plates it will prove to be over-exposed." 

 It has been proposed to photograph the red promi- 

 nences on a scale of ten seconds of arc to a millimetre. 

 The optical apparatus for the production of such pictures 

 must have an equivalent focal distance of 20627 centi- 

 metres, or 8i2*i inches, and if we take C equal to o"oo2, 

 which is probably very near the truth, the value of / for 

 lenses of various apertures are given as follows in the 

 instructions : — 



As a prominence one minute high could scarcely be 

 photographed with a six-inch objective, because twenty 

 seconds of its height would be covered by the advancing 

 moon before the exposure was over, Prof. Harkness 

 thinks it does not seem possible to photograph pro- 

 minences during eclipses on the scale here contemplated 

 with an aperture much less than ten inches. 



The section relating to telescopic observation is very- 

 full and complete ; full instructions concerning the struc- 

 ture of the corona are given, and the remark is made 

 that " Since the spectroscope furnishes an efficient means 

 of studying the red prominences at any time, it will be 

 very undesirable to waste a single one of the precious 

 moments of totality in examining them." 



To facilitate the work of such astronomers as may 

 desire to search for intra-Mercurial planets with con- 

 siderable telescopic power, a chart is given showing every 

 star so large as the seventh magnitude in that portion of 

 the heavens which will be occupied by the sun on the 29th 

 of July next. The black circle in R.A. 8h. 36m. Dec. -f- 

 18° 39' indicates the position of the sun. Mercury, Regu- 

 lus, and Mars will be pretty close together, and probably 

 quite conspicuous during totality, but they are so far to the 

 eastward that only the last-named comes within the limits 

 of the chart. Venus may also be seen, but she will be 

 low in the western sky. While looking for planets, the 

 possibility of discovering a small comet, or a meteor 

 stream, should be borne in mind. 



" The corona forms a luminous background upon 

 which the moon's limb is sometimes seen projecting- 

 beyond the sun; and a little before totality it is even 

 possible that the complete outline of the moon may 

 become visible. Look for these phenomena, and note 

 the time of their occurrence. It is difficult to assign any 

 reason for the existence of rays, or brushes, of light at 

 the cusps of the sun, but it is said they have been seen. 

 If any such appearances present themselves, they should 

 be carefully scrutinised to ascertain if they change either 

 their position or intensity ; and the interior of the tele- 

 scope should be examined to make sure that they do not 

 originate in reflections, either from the tube or from the 

 lenses." 



The instructions as to the use of the spectroscope and 

 polariscope are so full that they deserve reprinting in 

 extenso. I shall therefore say nothing about them here 

 except to express my belief that no stone has been left 

 unturned to secure results, if results be possible. Spec- 

 troscopically, I suppose Dr. Draper and Prof. Young 

 have the strongest outfit, while, so far as I know, Prof. 

 Harkness is the only one who is equipped for photo- 

 graphing the polarisation of the corona. 



For the first time thermo-electric observation forms 

 part of eclipse work. One of the many points of interest 

 here, to me, has been the observatory in which Mr. 

 Edison has been experimenting on his tasimeter. It is 

 truly a very wonderful instrument, and from the observa- 

 tions made last night on the heat of .Arcturus, it is quite 

 possible that he may succeed in his expectations. For 

 its extreme delicacy I can personally vouch. The instru- 

 ment, however, is so young, that doubtless there are 

 many pitfalls to be discovered, Mr. Edison, however, is 

 no unwary experimenter. 



So much, then, for the present. The day after to- 

 morrow will find us all busier than ever, and if the 

 weather prove fine I hope I shall have, as in 1870 and 

 1871, another distinct advance in solar physics to 

 chronicle. 



J. Norman Lockyer 



Rawlins, Wyoming Territory, July 27 



