32 



NATURE 



[May 12, 1887 



of which may be compared to that of a telegraphic relay. 

 The copies therefore represent many hundred times the 

 original energy received from the stars. If care is not 

 taken, the dust and irregularities of the film will give 

 trouble, each foreign particle appearing as a fine spectral 

 line. 



Other methods of enlargement have been considered, 

 and some of them tried, with the object of removing the 

 irregularities of the original spectra without introducing 

 new defects. For instance, the sensitive plate may be 

 moved during the enlargement in the direction of the 

 spectral lines ; a slit parallel to the lines may be used as 

 the source of light, and the original negative separated 

 by a small interval from the plate used for the copy ; or 

 two cylindrical lenses may be used, with their axes per- 

 pendicular to each other. In some of these ways the 

 lines due to dust might either be avoided or so much 

 reduced in length as not to resemble the true lines of the 

 spectrum. 



The 15-inch refractor is now being used with a modifica- 

 tion of the apparatus employed by Dr. Draper in his first 

 experiments, — a slit spectroscope from which the slit has 

 been removed. A concave lens has been substituted for 

 the collimator and slit, and, besides other advantages, a 

 great saving in length is secured by this change. It is 

 proposed to apply this method to the 28-inch reflector 

 thus utilising its great power of gathering light 



The results to be derived from the large number of 

 photographs already obtained can only be stated after a 

 long series of measurements and a careful reduction and 

 discussion of them. An inspection of the plates, how- 

 ever, shows some points of interest. A photograph of a 

 Cygni, taken November 26, 1886, shows that the H line 

 is double, its two components having a difference 

 in wave-length of about one ten-millionth of a milli- 

 metre. A photograph of o Ceti shows that the lines 

 G and /i are bright, as are also four of the ultra- 

 violet lines characteristic of spectra of the first type. 

 The H and K lines in this spectrum are dark, show- 

 ing that they probably do not belong to that series 

 of lines. The star near x^ Orionis, discovered by Gore 

 in December 1885, gives a similar spectrum, which 

 affords additional evidence that it is a variable of the 

 same class as o Ceti. Spectra of Sirius show a large 

 number of faint lines besides the well-known broad 

 lines. 



The dispersion employed in any normal map of the 

 spectrum may be expressed by its scale, that is, by the 

 ratio of the wave-length as represented to the actual 

 wave-length. It will be more convenient to divide these 

 ratios by one million, to avoid the large numbers other- 

 wise involved. If one-millionth of a millimetre is taken 

 as the unit of wave-length, the length of this unit on the 

 rnap in millimetres will give the same measure of the 

 dispersion as that just described. When the map is not 

 normal, the dispersion of course varies in different parts. 

 It increases rapidly towards the violet end when the 

 spectrum is formed by a prism. Accordingly, in this 

 case the dispersion given will be that of the point whose 

 wave-length is 400. This point lies near the middle of 

 the photographic spectrum when a prism is used, and is 

 not far from the H line. The dispersion may accordingly 

 be found with sufficient accuracy by measuring the interval 

 between the H and K lines, and dividing the result in 

 millimetres by 3-4, since the difference in their wave- 

 lengths equals this quantity. The following examples 

 serve to illustrate the dispersion expressed in this way : 

 Angstrom, Cornu, 10 ; Draper, photograph of normal 

 solar spectrum, 3-1 and 5-2 ; Rowland, 23, 33, and 46 ; 

 Draper, stellar spectra, o"i6; Huggins, o'l. Fig. i, 

 o-o6; Fig. 2, o-io; Fig. 3, 0-63; Fig. 4, 1-3; Figs. 5 

 and 6, 6*5. 



The most rapid plates are needed in this work, other 

 considerations being generally of less importance. Ac- 



cordingly the Allen and Rowell Extra Quick plates have 

 been used until recently. It was found, however, that 

 they were surpassed by the Seed Plates No. 21, which 

 were accordingly substituted for them early in December. 

 Recognising the importance of supplying this demand 

 for the most sensitive plates possible, the Seed Company 

 have recently succeeded in making still more sensitive 

 plates, which we are now using. The limit does not 

 seem to be reached even yet. Plates could easily be 

 handled if the sensitiveness were increased tenfold. A 

 vast increase in the results may be anticipated with each 

 improvement of the plates in this respect. Apparatus for 

 testing plates, which is believed to be much more accurate 

 than that ordinarily employed, is in course of preparation. 

 It is expected that a very precise determination will be 

 made of the rapidity of the plates employed. Makecs of 

 very rapid plates are invited to send specimens for trial. 



The photographic work has been done by Mr. W. P. 

 Gerrish, who has also rendered important assistance in 

 other parts of the investigation. He has shown great 

 skill in various experiments which have been tried, and 

 in the use of various novel and delicate instruments. 

 Many of the experimental difficulties could not have been 

 overcome but for the untiring skill and perseverance of 

 Mr. George B. Clark, of the firm of Alvan Clark and 

 Sons, by whom all the large instruments have been con- 

 structed. 



The progress of the various investigations which are to 

 form a part of this work is given below: — 



(i) Catalogue of Spectra of Bright Stars. — This is a 

 continuation of the work undertaken with the aid of an 

 appropriation from the Bache Fund, and described in the 

 Memoirs of the American Academy, vol. xi. p. 210. The 

 8-inch telescope is used, each photograph covering a 

 region 10° square. The exposures for equatorial stars 

 last for five minutes, and the rate of the clock is such 

 that the spectra have a width of about o'l cm. The 

 length of the spectra is about i '2 cm. for the brighter, 

 and o"6 cm. for the fainter stars. The dispersion on the 

 scale proposed above is o*i. The spectra of all stars of 

 the sixth magnitude and brighter will generally be found 

 upon these plates, except in the case of red stars. Many 

 fainter blue stars also appear. Three or four exposures 

 are made upon a single plate. The entire sky north of 

 — 24° would be covered twice, according to this plan, with 

 180 plates and 690 exposures. It is found preferable in 

 some cases to make only two exposures ; and when the 

 plate appears to be a poor one, the work is repeated. 

 The number of plates is therefore increased. Last 

 summer the plates appeared to be giving poor results. 

 Dust on the prisms seemed to be the explanation of this 

 difficulty. Many regions were re-observed on this account. 

 The first cycle, covering the entire sky from zero to twenty- 

 four hours of right ascension, has been completed. The 

 work will be finished during the coming year by a second 

 cycle of observations, which has already been begun. 

 The first cycle contains 257 plates, all of which have 

 been measured, and a large part of the reduction com- 

 pleted. 8313 spectra have been measured on them, 

 nearly all of which have been identified, and the places 

 of a greater portion of the stars brought forward to the 

 year 1900, and entered in catalogue form. In the second 

 cycle, 64 plates have been taken, and about as many more 

 will be required. 51 plates have been measured and 

 identified, including 2974 spectra. A study of the photo- 

 graphic brightness and distribution of the light in the 

 spectra will also be made. 



The results will be published in the form of a catalogue 

 resembling the Photometric Catalogue given in vol. xiv. 

 of the Annals of Harvard College Observatory. It will 

 contain the approximate place of each star for 1900, its 

 designation, the character of the spectrum as derived 

 from each of the plates in which it was photographed. 



