Page:Science vol. 5.djvu/429

 erif wiiiils that precede atortns. SotneLimea rapid altenialiona of suuiblne and shnde, by heating and cooling the wire, cause it to ^loueate and contract rapidly, and maintain an additional aeries of musical notes. Sometimes tlia length and tension of a wire stretched between two teleplione supports is snch that it can hannonicnlly respond to several clnsaes of waves transratlteii from distant pnrts of llie line. We thus obtain the very rich effects of the aeoliaii harp, which, as is well known, has often been said to ring out the finest notes before a storm, and whose action VM also attributed to macnetlam and other occult causes, until Cbladni gave the correct esplana- lion. — Ed.I

Ad attempt to pbotO(^apti the solar oorona.

Mr. W. II. Pickering havin? called my attention to his letLer entitled ' An altcinpl to photograph the solar corona,' printed in Sdmre for April 3, may I ■ak you lo insert the following lines in the next num- ber of your journal.

Tbe false cornual effecls wbicb Ur. Pickering de- scribes are precisely ibose which might have Dceii eipected to result from bis optical and inslrumenlal methods. I hare in my papers called special atten- tion to the two principal sources of latie effects which are present in the fonn of apparatus employed by Mr. Pickering; namely, the use of a letia, and the position of the itmp-sbutter which is said to have been ' attached to the lens.'

Id some early attempts which I ntfide with lenscu, any true coronal effect which may possibly have been upon the plates was comptelelv maaked by very strong false ciirDnal appearances and raya, simitar to those obtained hy Mr. Pickering. These were due, prob- ably, in part to outstanding chromatic aberraiiniis of the lenses, though corrected for pholographic work, in part to reflections from the surfaces of the lenses, and in part to a difFraction aniiulus 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 lo reflection /mm a Bnety polished mirror of tpeculum metal. When the mirror was used, all these false effects disappeared.

It Is scarcely necessary lo rvmlnil your scientiSc readers that the only position in wbicli the drop- shuller can be placed, when an object to bright as the sun Is photographed, without introducing strong false coronal effects about the sun's image from ilif- (raclion, is In, or very near, the focal plane. ' At- taclied to the lens,' whether behind or In front at It, a strong diffraction effect is produced upon the plate at the begiunins, and again towards Ihe end, of the exposure. It Mr. Pickering will direct his apparatus to the sun, and observe the sun's image on the ground glass cif the camera during the time that the ilrxp- shutter is moved very slowly past the lens, be will be the s|)ectator of a succession of flne diffraction effects, which in the aggregate, as far as they were bright enough, mu-t bavi; recorded themselves on his plates. In this way, with care and skill, the sources of other Instrumental effects could, no doubt, be

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 uf diffraction upon the plate," t may now add, that, even with the shutter near the plate, care bas lo be taken that no light is reflected from the edae of (be movinic plate of the sbutler.

f «i.t>i> ih»t with mv apparatus, when the sky is [fee frora clouds, but whitish (mm a strong scallering

��of the sun's light, " the sun is well defined upon a sensibly uniform surrounding of air-glare, but with- out an; 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 dlstincliiess." Any apparatus intended for phoio- Kraphhig the corona must fulfil perfectly these < ditions befa"" ""' — """ " ' ' '

��before any at

��IS attempts ai

��o obtain

��I staled, In a paper presented to the British asso- ciation for the advancement of sdrncein the summer of ISSJ, that I bad discarded the use of colored glass (or cells of colored siilntlons) because of jie danger of false appearances from imperfections In the sur- faces or in the substance of the glasn.

Mr. Pickering does not state that his senaitive plates were 'backed ' with a solution of a»phallum, or other black medium. In optical contact with the glass, — an essential condition.

No tube, with suitable diaphraitms luslde, appears to have l>een used In front of the lens lo prevent light falling upon the inside of the telescope tube or camera, and being thence reflecled possibly upon the plate. Tbe dssirable 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 saiisitive plate, does not seem to liave been taken.

Mr. Pickering says of the violet glas;, " l)y Its ub«, 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 Ihe lime of exposure- and therefore Mr. Pickering's time o( exposure was in fault, if be wished a different result.

Mr. Pickering says, " lloth bromide anil chloride

tlales were provided; but, as with Mr. Huggins, the Liter proved lo give much Ibe better corond effects." And again, towards the end of the letter, he says that " chloride plates are mure suitable than bromide ones for obtaining an atmospheric corona, just as Mr. Sueglns has claimed that they are more suitable for taking a solar one; benco 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 ereat part, if not altogether, to diffraction and other fnstrumen- tal effects, as I bave already pointed out. and presum- ing that Mr. Pickering was not unfamiliar wttfa the Sreater blackness of cntoride plates, especially when eveloped with ferrous oxalate, he seems to infer some special suitability of ihe chloride platesto bring out the false effects upon bis plates. It may be sug- gested that Mr. Pickering aeems lo bave used the same length of exposure throughout, "giving an exposure which may be estimated at alMut a fifui of a second." Now. it h scarcely probable that the bromide and chloride plates possessed the same aen- siliveneaa; and 11 may have been that the (probably) more setisiiive bromide plates were iliin 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 poinu are cleared, it does not seem to me that Ur. Pickering is justified in making the insinuation which seems to lie In Ihe words which I have quoted.

In conclusion, I cannot refrain from expreaibig great 8ur{.,'Lse that Mr. Pickering should have men- tioned my name In connection with esperlmeats car- ried out In complete disregaid of the conditions to which I had called atlejilion, m essential in a matter

��I I

��J

�� �