January ii, 191 2] 



NATURE 



0^0 



We must especially note that the newer problems require 

 an enormous resolving power. In the telescope this has 

 been accomplished partly by the construction of giant re- 

 fractors and partly by enormous reflectors : and, curiously 

 enough, the same double path is open to spectroscopy ; for 

 we may employ the analogous dispersive power of refract- 

 ing media or the diffractive power of reflecting media. 

 The increasing cost and difficulty of producing large trans- 

 parent and homogeneous blocks of glass have tended to 

 limit the size and efficiency of lenses and of prisms, and 

 these have been more or less successfully replaced, the 

 former by mirrors and the latter by diffraction gratings. 



These are made by ruling very fine lines very close 

 together on a glass or a metal surface. The effect on the 

 incident light is to alter its direction by an amount which 

 varies with the wave-length — that is, with the colour ; 

 and a spectrum is produced which may be observed to best 

 advantage by precisely the same form of spectrometer, with 

 a substitution of a grating for the prism. 



The dispersion of a diffraction grating depends upon the 

 closeness of the rulings ; but the resolving power is 

 measured by the total number of lines. It is important, 

 therefore, to make this number as large as possible. 



The first gratings made by Fraunhofer, 182 1, contained 

 but a few thousand lines, and had a correspondingly low 

 resolving power — quite sufficient, however, to separate the 

 sodium doublet. A considerable improvement was effected 

 by Nobert, whose gratings were used as test objects for 

 microscopes ; but these were still very imperfect as spectro- 

 scopic instruments, and it was not until Rutherford, of 

 New York, 1879, constructed a ruling engine with a fairly 

 accurate screw that gratings were furnished which com- 

 pared favourably with the best prisms in existence. 



With 30,000 lines (covering more than 40 mm.) the 

 theoretical resolving power would be 30,000 ; practically 

 about J5,ooo — sufficient to separate doublets the com- 

 ponents of which were only one-fifteenth as far apart as 

 those of the sodium doublet. 



An immense improvement was effected by Rowland, 

 i88i, whose gratings have been practically the only ones 

 in service for the last thirty years. Some of them have a 

 ruled surface of 150 mm.x6o mm., with about 100,000 

 lines, and can separate doublets the distance of which is 

 only i/ioo of that of the sodium doublet in the spectrum 

 <if the first order. In the fourth order it should resolve 

 lines the distance of which is only one-fourth as great. 



Practically, however, it is doubtful if the actual resolving 

 power is more than 100,000, the difference between the 

 theoretical and the actual performance being due to the 

 defect in uniformity m the spacing of the grating furrows.' 



The splendid results obtained by Rowland enabled him 

 to produce the magnificent atlas and tables of wave-lengths 

 of the solar spectrum which are incomparably superior in 

 accuracy and wealth of detail to any previous work ; so 

 that until the last decade this work has been the uni- 

 vfrsally accepted standard. With these powfjrful aids it 

 was possible not only to map the positions of the spectral 

 lines with marvellous accuracy, but many lines before sup- 

 posed simple were shown to be doublets or groups ; and a 

 systematic record is given of the characteristics of the 

 individual lines, for example, whether they are intense or 

 faint, nebulous or sharp, narrow or broad, symmetrical or 

 unsymmetrical, reversed. Sec. — characteristics which we 

 recognise to-day as of the highest importance, as giving 

 indications of the structure and motions of the atoms the 

 vibrations of which produce these radiations. 



One of the most difficult and delicate problems of modern 

 stronomy is the measurement of the displacement of 

 ~^p<'ctral lines in consequence of the apparent change of 

 wave-length due to " radial velocity " or motion in line 

 of sight. This is known as the Doppler effect, and had 

 hr-on well established for sound waves (a locomotive whistle 

 appears of higher pitch when approaching and lower when 

 1. ceding) ; but it was only confirmed for light by Huggins 

 nd by Vogel in 1871, by the observation of displacements 

 'f the solar and stellar spectral lines. 



It may be worth while to indicate the accuracy necessary 

 ti such measur'-ments. The velocity of rotation of the 



'This applies to all the "Rowland entities which have come under my 

 no icp, with the exception of one which I had the opportunity of testing at 

 ihc Physii:al Ijiboratory, University, Gottingen. The resolving power of 

 this grating was about ;oo,ooo. 



NO. 2202, VOL. 88] 



sun's equator is approximately 2 kilometres per second, 

 while the velocity of light is 300,000 kilometres per second. 

 According to Doppler 's principle, the corresponding change 

 in wave-length should be i : 150,000 — a quantity too small 

 to be " resolved " by any prism or grating then in exist- 

 ence. But by a sufficient number of careful micrometer 

 measurements of the position of the middle of a given 

 spectral line, the mean values of two such sets of measure- 

 ments would show the required shift. It is clear, however, 

 that if such radial velocities are to be determined with 

 any considerable degree of accuracy, nothing short of the 

 highest resolving power of the most powerful gratings 

 should be employed. 



Another extremely iinportant application of spectroscopy 

 to solar physics is that which, in the hands of Hale and 

 Deslandres, has given us such an enormous extension of 

 our knowledge of the tremendous activities of our central 

 luminary. 



The spectroheliograph, devised by Hale in 1S89, consists 

 of a grating spectroscope provided with two movable slits, 

 the first in its usual position in the focus of the collimator, 

 and the second just inside the focus of the photographic 

 lens. A uniform motion is given to the two slits so that 

 the former passes across the image of the solar disc, while 

 the other exposes continually fresh portions of the photo- 

 graphic plate. 



If the spectroscope is so adjusted that light of the wave- 

 length of a particular bright line in a solar prominence 

 (say, one of the hydrogen or the calcium lines) passes 

 through the instrument, then a photograph of the promin- 

 ences, or sun-spots, or faculae, &c.. appears on the plate. 

 But the character of this photograph depends on the por- 

 tion of the bright spectral " line " which is effective, and 

 as the entire range of light in such a line may be only a 

 thirtieth part of the distance between the sodium lines, it 

 would require a resolving power of at least 100,000 to sift 

 out the efficient radiations so that they do not overlap. 



As another illustration of importance of high resolving 

 power in attacking new problems, let us consider the beau- 

 tiful results of the investigations of Zeemann on radiation 

 in a magnetic field. The effect we know is a separation 

 of an originally simple radiation into three or more, with 

 components polarised at right angles to each other. This 

 is one of the very few cases where it is possible actually to 

 alter the vibrations of an atom (electron), and the fact 

 that the effect is directly calculable, as was first shown by 

 Lorentz, has given us a very important clue to the structure 

 and motions of the atoms themselves. 



The experiment is made by placing the source of radia- 

 tion (any incandescent gas or vapour) between the poles 

 of a powerful electromagnet and examining the light 

 spectroscopically. Now this experiment h.ad been tried 

 long before by Faraday, but the spectroscopic appliances at 

 his disposal were entirely inadequate for the purpose. 



Even in the original discovery of Zeemann only a broaden- 

 ing of the spectral line was observed, but no actual separa- 

 tion. In fact, the distance between components which had 

 to be observed was of the order of a hundredth of the 

 distance between the sodium lines, and in order to effect a 

 clear separation, and still more to make precise measure- 

 ments of its amount, requires a higher resolving power 

 than was furnished by the most powerful gratings then in 

 existence. 



.\s a final illustration, let us consider the structure of 

 the spectral " lines " themselves. Rowland's exquisite 

 maps had shown many of these, which were then thought 

 simple, to be double.' triple, or multiple, and there are 

 clear indications that even the simpler lines showed differ- 

 ences in width, in sharpness, and in symmetry. But the 

 general problem of the distribution of light within spectral 

 lines had scat eel v been touched. Here, also, the total 

 " width " of the line is of the order of i/ioo of the distance 

 between the sodium lines, and it is evident that without 

 more powerful appliances further progress in this direc- 

 tion was hopeless. 



Enough has been said to show clearly that these modern 

 problems were such as to tax to the utmost the poweTS 

 of the best spectroscopes and the experimrntnl «;kin of the 

 most experienced investigators. 



Some twenty vears ago a method u 1 \yhich, 



though somewhat laborious and indirect. i;..v. j.iomise of 



