226 



NATURE 



IJan. 22, 1874 



three times the amount necessary to remove my discre- 

 pancy. In any case the photograph is correct, as it is the 

 work of ihe sun, and is only open to errors arising from 

 imperfect flatness in the field of a fine lens, and that field 

 onl\ subtending an angle nf about 4'. The angular aper- 

 ture of the lens, viewed from the sensitive plate, is 20 

 minutes. I tuHt, therefore, that the photograph may be 

 of oermanent value to physicists, for any one can affix 

 another scale if this be slightly erroneous. 



An exaiTiination of the pho ographed spectrum shows 

 manv points ol interest, some o( which are best seen in 

 the SI ectrum wiih the scale above, and some in the por- 

 tion enlarged below. The latter is magnified about twice, 

 and comprises the region from wave-length 3736 104205. 

 I have aho made photographs on the same scale as 

 Angstrom's map, but have not as yet printed them. The 

 capital letters which are attached to the region above H 

 are according to the nomenclature of Mascart, aUhough 

 the wave-lengths assigned by him to those letters do not 

 coincide exactly in all cases with the lines in my photo- 

 graph ; for instance, the line L, which he regards as 

 single, is in reality triple, and does not correspond to 

 3819, but to 3821 ; Pvl is correctly designated by 3728, but 

 it is double ; N is really at 3583, and not 35S0. It has 

 been suggested that it would be pioper to return to the 

 old nomenclature of Becquerel and J. W. Draper, who 

 simultaneously discovered these lines in 1842-43, but the 

 designation of position by wave-length in reality renders 

 the letters unnecessary. 



The spectrum above H,when compared with the region 

 from G to H, is marked by the presence of bolder groups 

 of lines, and most conspicuous are those between 3S20- 

 3860, 3705-3760, 3620-3650, 3568-3590, 3490-3530. The 

 first of these groups is strikingly shown in the enlarged 

 photograph. I am not as yet able to ofler an opinion as 

 to the chemical elements producing these groups, for al- 

 most all the photographs of the ultra-violet spectra of 

 metalline vapours that I have thus far made were pro- 

 duced by a quartz train, and have not yet been reduced to 

 wave-lengths. Indeed, that is a separate field of inquiry, 

 and could not be comprised in a memoir of this length. 

 1 have also tried to utilise the photographic spectra of the 

 late Prof. W. A. Miller, published in the " Transactions " 

 of the Royal Society for 1862, but for some reason, 

 probably insufficient intensity of the condensed induction 

 spark, his pictures do not bring out the peculiarities of 

 the various metals in the striking manner that is both 

 necessary and attainable. The diffraction spectra of 

 metalline vapours that I have made are not yet ready for 

 use. 



The probabilities are that each of these groups will be 

 found to be due to several elements, as is plainly seen in 

 the group H. This compound Une, which is commonly 

 spoken of as being caused by calcium, iron, and aluminum, 

 is in reality much more coinplex, for there can readily be 

 counted in it more than fifty lines in the original negative, 

 and a careful inspection of the accompanying paper pic- 

 ture shows a large proportion of them. This observation 

 leads us to a more general statement. The exact 

 composition of even a part of the spectrum of a 

 metal will not be known until we have obtained photo- 

 graphs of it on a large scale. The coincidences which 

 were so thoroughly examined by Mr. Huggins (Trans. 

 Royal Society, Dec. 1863) will only disappear when we 

 can, in addition to the position of a line, have a clear idea 

 of its size, strength, and degree of sharpness or nebulosity. 

 The eye is not able to see all the fine lines, or even if it 

 does, the observer cannot map them with precision, nor 

 in their relative strength and breadth. For example, in 

 Angstrom's justly celebrated chart, of which the G-H por- 

 tion is copied in this plate, and in the construction of 

 which the greatest pains were taken by him, many regions 

 are defective to a certain extent. The region from 41 01 

 to 4118 is without lines, yet the photograph shows in the 



enlarged copy seventeen that can easily be counted, and 

 the original negative shows more yet. The reader of 

 course understands that a paper print of a collodion pic- 

 ture is never as good as the original ; the coarseness of 

 grain in the paper, want of contact in transferring, &c., 

 effect such a result. Moreover, the Albert-type process 

 depends on a certain fine granulation which is given to 

 the bichromated gelatine, and this forbids the use of a 

 magnifier upon these paper proofs. It is only just, how- 

 ever, to Mr. Bierstadt to state, that without his personal 

 supervision, such sharp and fine-grained proofs could not 

 have been obtained, and that no other printing-press pro- 

 cess that I know of could have accompHshed this work at 

 all.* As an illustration of the difficulty of depicting the 

 relative intensity of lines, we may examine 3998, which 

 in Angstrom's chart is shown of equal intensity with 4004, 

 while in reality it is much fainter, and instead of being 

 single, is triple, as is well seen in the enlarged spectrum. 



When, however, we compare Angstrom's chart with 

 the photograph, it requires, as the above remarks show, a 

 critical examination to detect defects, and we have a 

 striking confirmation of the surprising accuracy of the 

 Swedish philosopher. 



So also in comparing Mascart's excellent map of the 

 prismatic spectrum with the photograph, the difficulty of 

 depicting all the fine lines is seen. In the group L he 

 shows twelve lines, while even in the Albert-type copy of 

 my photograph twenty-five can be counted, and in the 

 original negative many more. From H to L he exhibits 

 seventy lines ; in my plate 138 can be observed, besides 

 many unresolved bands. 



In the earlier part of this memoir it was stated that the 

 relative intensities of the lines in the spectrum were cor- 

 rectly represented if a certain allowance was made. If an 

 unshielded collodion plate were presented to the image 

 of the spectrum, there would be produced a stain very 

 dense from G to H, fainter above H, and still fainter 

 below G. But this stain would not represent the actinic 

 force of the sun ; it would merely be the index of the 

 decomposability of a mixture of iodide and bromide of 

 silver. I have for this reason adopted the idea of J. W. 

 Draper, that force is equally distributed through the spec- 

 trum, and have tried to produce a photograph of equal 

 intensity throughout. This has been accomplished, as I 

 have before stated, by suitable diaphragms. But whether 

 this view be correct or not, lines which are not f;i.r distant 

 from one another are presented virtually without any 

 interference by diaphragms, and must therefore be correct 

 both as to shading and intensity. 



Besides the points above mentioned, there are many 

 theoretical considerations suggested by the photograph 

 which it does not seem expedient to enter upon fully at 

 present. Among such is the possibility of arriving at an 

 estimate of the sun's temperature, by interpreting the 

 apparent bands, such as those near G and H, by the aid 

 of Lockyer's researches on the temperature of dissoci- 

 ation of compounds. No one has yet ascertained whether 

 there are or are not unresolvable bands in the solar 

 spectrum. If they do exist, the compounds to which they 

 belong, and the necessary temperature for dissociation, 

 remain to be determined. 



It would seem also to be possible to find out whether, 

 as asserted by ZoUner, there is a liquid envelope around 

 the sun, by a search for more diffused bands in its photo- 

 graphed spectrum. 



In the hope that this photograph may prove to be of 

 value to scientific men for further investigations upon the 

 sun and the elements, I have caused a number of extra 

 copies to be printed, and shall be glad to present them to 

 anyone who can make use of them. 



Henry Draper 



* From the original negative of the spectrum 12,000 copies have up to 

 the present been printed, and it is not in the slightest degree injured as 



