424 Dr. H. Draper on Diffraction- Spectrum Photography. 



that is both necessary and attainable. The diffraction- spectra 

 of metalline vapours that I have made are not yet ready for use. 



The probability is that each of these groups will be found 

 to be due to several elements, as is plainly seen in the group H. 

 This compound line, which is commonly spoken of as being 

 caused by calcium, iron, and aluminium, is in reality much more 

 complex; for there can readily be counted in it more than fifty 

 lines in the original negative; and a careful inspection of the 

 accompanying paper picture 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 photographs of it on a large 

 scale. The coincidences which were so thoroughly examined by 

 Mr. Huggins (Trans. Roy. Soc. 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 cele- 

 brated chart, of which the G-H portion 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 4101 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 picture 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 Alberttype 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, 

 however, to Mr. Bierstadt to state that, without his personal su- 

 pervision, such sharp and fine-grained proofs could not have 

 been obtained, and that no other printing-press process that I 

 know of could have accomplished this work at all. As an illus- 

 tration 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 shown in the en- 

 larged spectrum. 



When, however, we compare Angstrom's chart with the pho- 

 tograph, it requires, as the above remarks show, a critical exami- 

 nation to detect defects, and we have a striking confirmation of 

 the surprising accuracy of the Swedish philosopher. 



So also id comparing Mascart's excellent map of the prismatic 

 spectrum with the photograph, the difficulty of depicting all the 



