May 15, 1885.] 



SCIENCE 



397 



erly winds that precede storms. Sometimes rapid 

 alternations of sunshine and shade, by heating and 

 cooling the wire, cause it to elongate and contract 

 rapidly, and maintain an additional series of musical 

 notes. Sometimes the length and tension of a wire 

 stretched between two telephone supports is such 

 that it can harmonically respond to several classes of 

 waves transmitted from distant parts of the line. 

 We thus obtain the very rich effects of the aeolian 

 harp, which, as is well known, has often been said to 

 ring out the finest notes before a storm, and whose 

 action was also attributed to magnetism and other 

 occult causes, until Chladni gave the correct explana- 

 tion. — Ed.] 



An attempt to photograph the solar corona. 



Mr. W. H. Pickering having called my attention 

 to his letter entitled ' An attempt to photograph the 

 solar corona,' printed in Science for April 3, may I 

 ask you to insert the following lines in the next num- 

 ber of your journal. 



The false coronal effects which Mr. Pickering de- 

 scribes are precisely those which might have been 

 expected to result from his optical and instrumental 

 methods. I have in my papers called special atten- 

 tion to the two principal sources of false effects which 

 are present in the form of apparatus employed by 

 Mr. Pickering; namely, the use of a lens, and the 

 position of the drop-shutter which is said to have 

 been ' attached to the lens.' 



In some early attempts which I made with lenses, 

 any true coronal effect which may possibly have been 

 upon the plates was completely masked by very strong 

 false coronal appearances and rays, similar to those 

 obtained by Mr. Pickering. These were due, prob- 

 ably, in part to outstanding chromatic aberrations 

 of the lenses, though corrected for photographic work, 

 in part to reflections from the surfaces of the lenses, 

 and in part to a diffraction annulus about the sun's 

 image. It was on account of these, and some other 

 probable sources of error when a lens is used, that 

 I had recourse to reflection from a finely polished 

 mirror of speculum metal. When the mirror was 

 used, all these false effects disappeared. 



It is scarcely necessary to remind your scientific 

 readers that the only position in which the drop- 

 shutter can be placed, when an object so bright as 

 the sun is photographed, without introducing strong 

 false coronal effects about the sun's image from dif- 

 fraction, is in, or very near, the focal plane. ' At- 

 tached to the lens,' whether behind or in front of it, 

 a strong diffraction effect is produced upon the plate 

 at the beginning, and again towards the end, of the 

 exposure. If Mr. Pickering will direct his apparatus 

 to the sun, and observe the sun's image on the ground 

 glass of the camera during the time that the drop- 

 shutter is moved very slowly past the lens, he will 

 be the spectator of a succession of fine diffraction 

 effects, which in the aggregate, as far as they were 

 bright enough, must have recorded themselves on his 

 plates. In this way, with care and skill, the sources 

 of other instrumental effects could, no doubt, be 

 tracked out. 



In one of my papers my words are, " The moving 

 shutter, being placed very near the sensitive surface, 

 and practically in the focal plane, could not give rise 

 to effects of diffraction upon the plate." I may now 

 add, that, even with the shutter near the plate, care 

 has to be taken that no light is reflected from the 

 edge of the moving plate of the shutter. 



I state that with my apparatus, when the sky is 

 free from clouds, but whitish from a strong scattering 



of the sun's light, " the sun is well defined upon a 

 sensibly uniform surrounding of air-glare, but with- 

 out any indication of the corona. It is only when 

 the sky becomes clear and blue in color that coronal 

 appearances present themselves with more or less 

 distinctness." Any apparatus intended for photo- 

 graphing the corona must fulfil perfectly these con- 

 ditions before any serious attempts are made to obtain 

 the corona. 



I stated, in a paper presented to the British asso- 

 ciation for the advancement of science in the summer 

 of 1883, that I had discarded the use of colored glass 

 (or cells of colored solutions) because of the danger 

 of false appearances from imperfections in the sur- 

 faces or in the substance of the glass. 



Mr. Pickering does not state that his sensitive plates 

 were ' backed ' with a solution of asphaltum, or other 

 black medium, in optical contact with the glass, — an 

 essential condition. 



No tube, with suitable diaphragms inside, appears 

 to have been used in front of the lens to prevent 

 light falling upon the inside of the telescope tube or 

 camera, and being thence reflected possibly upon the 

 plate. The desirable precaution of using a metal 

 disk, with a suitable surface, a little larger than the 

 sun's image, and placed close in front of the sensitive 

 plate, does not seem to have been taken. 



Mr. Pickering says of the violet glass, " By its use, 

 a negative image of the sun's disk was obtained ; but 

 without it, the plate gave a reversed image." I found 

 no difficulty in obtaining a negative, or a reversed 

 image, when violet glass was used, by a suitable 

 change of the time of exposure; and therefore Mr. 

 Pickering's time of exposure was in fault, if he wished 

 a different result. 



Mr, Pickering says, " Both bromide and chloride 

 plates were provided; but, as with Mr. Huggins, the 

 latter proved to give much the better coronal effects." 

 And again, towards the end of the letter, he says 

 that " chloride plates are more suitable than bromide 

 ones for obtaining an atmospheric corona, just as Mr. 

 Huggins has claimed that they are more suitable for 

 taking a solar one : hence I think one must not rely 

 too much on the ultra-violet sensitiveness of the 

 chloride plate for the separation of the two." Pass- 

 ing by the use of the words ' atmospheric corona ' for 

 the false appearances which were due in great part, 

 if not altogether, to diffraction and other instrumen- 

 tal effects, as I have already pointed out, and presum- 

 ing that Mr. Pickering was not unfamiliar with the 

 greater blackness of chloride plates, especially when 

 developed with ferrous oxalate, he seems to infer 

 some special suitability of the chloride plates to bring 

 out the false effects upon his plates. It may be sug- 

 gested that Mr. Pickering seems to have used the 

 same length of exposure throughout, " giving an 

 exposure which may be estimated at about a fifth of 

 a second." Now, it is scarcely probable that the 

 bromide and chloride plates possessed the same sen- 

 sitiveness ; and it may have been that the (probably) 

 more sensitive bromide plates were thin from exces- 

 sive exposure. It may even have occurred that his 

 lens, if corrected for bromide plates, gave an outstand- 

 ing aberration about H, or a little beyond. Anyway, 

 until these and some other similar points are cleared, 

 it does not seem to me that Mr. Pickering is justified 

 in making the insinuation which seems to lie in the 

 words which I have quoted. 



In conclusion, I cannot refrain from expressing 

 great surprise that Mr. Pickering should have men- 

 tioned my name in connection with experiments car- 

 ried out in complete disregard of the conditions to 

 which I had called attention, as essential in a matter 



