496 



NATURE 



[August 21, 1919 



wave-lengths, for this is well known to vary in some 

 cases very greatly according to the circumstances in 

 which the atom is excited. I shall describe, in the 

 first instance, a method designed by Dr. Merton for 

 investigating the distribution of energy among spec- 

 trum lines, or in the breadth of an individual line, with 

 great accuracy. It is possible by this means to obtain 

 tne long-desired object of an absolute scale of spectral 

 intensity, independent of all the ordinary difficulties 

 determined by such matters as the unequal behaviour 

 of the photographic plate for light in different regions 

 of the spectrum. Dr. Merton and I have been work- 

 ing together on this subject for the past three years, 

 and I shall conclude the present lecture with an account 

 of some of the more interesting results which have 

 been reached, after an explanation of the method. 



The intensities of spectrum lines have usually been 

 recorded on an arbitrary scale, ranging between lo 

 and zero, the numbers assigned being at the discretion 

 of the observer, and varying so greatly among different 

 observers as frequently to be of little value for exact 

 knowledge. Thej, depend also very much on the 

 nature of the observation, whether . visual or photo- 

 graphic, and in the latter case on the region of the 

 spectrum to which the line belongs. The sensitivity of 

 a photographic plate varies with the wave-length of 

 the light in a curious manner, and apparently an irre- 

 gular one not following any simple law. The sensi- 

 tivity of the eye is also different for different colours. 

 When the line is outside the visible spectrum, in the 

 infra-red or dark heat region, measurements of in- 

 tensity can be made with some accuracy by a thermo- 

 pile or a bolometer. But they are needed more ur- 

 gently in the visible region at present, not only for the 

 information they will afford regarding the nature of 

 the atom, but also for application to other problems. 

 The subject is very important, for instance, in the inter- 

 pretation of celestial spectra, and more particularly 

 those spectra of great complexity and variability which 

 are associated with the birth of new stars, from which 

 most of our knowledge regarding such stars must be 

 constructed. 



Previous knowledge of changes in spectral intensity 

 under varying conditions was of necessity limited to the 

 great changes. Those changes, which are of especial 

 value in connections guch as I have mentioned, are 

 liable to be of a less conspicuous type, not readilv 

 capable of detection by the ordinary photographic or 

 the. visual method, and, if detected, not capable of. 

 accurate measurement. 



In the visual region of the spectrum observations 

 with the bolometer are not satisfactory. The source 

 of light must be very intense in order to produce large 

 deflections in the galvanometer, and only the brightest 

 lines could be dealt with even in this way. Only one 

 line in the spectrum can be experimented upon at one 

 time, and the source of light cannot be maintained 

 constant over a protracted period. The method is, in 

 fact, quite unsuitable, and the spectrophotometer has 

 been tried instead, but no great accuracy is possible, 

 and its use is confined to a very narrow region of wave- 

 length. Moreover, the variability of the source of light 

 is again present. 



In adopting any photographic method for quantita- 

 tive work we must remember that not onlv does the 

 sensitivity of the plate vary with the wave-length, but 

 also that there is no very definite relation between the 

 density of a photographic image and either the in- 

 tensity of the light or the time of exposure. If we 

 halve the former and double the latter, we do not get 

 the same density of the image, but another which 

 depends on the particular plate used. The grain of a 

 plate also scatters light, and ' the actual size of the 

 NO. 2599, VOL. 103] 



image thus depends on the exposure and the intensity 

 of the light. We were early compelled to conclude that 

 accurate measurements of intensity by a photographic 

 method involve the necessity of an equal exposure on 

 the same plate for all the sources of light to be com- 

 pared, and the method to be described satisfies this 

 necessity. 



The spectrograph for producing and photographing 

 the lines of a spectrum is set up in the usual way, 

 which requires no description. A wedge of neutral- 

 tinted glass, cemented to another of clear glass so as 

 to form a plane parallel plate, is mounted in front of 

 the slit. The image of the slit formed by light of any 

 wave-length is thus attenuated towards the part of the 

 slit opposite the thick end of the wedge, where the 

 absorption of light is greatest, and the image ceases 

 to be strong enough to affect the plate beyond a certain 

 specific height, which depends on the original in- 

 tensity, in the beam from the source, of this particular 

 wave-length. 



The photograph thus consists, not of the usual spec- 

 trum with all lines or slit-images of the same length, 

 but of a spectrum in which all the lines are cut down 

 to specific heights depending on the original intensi- 

 ties, and thus it gives a simultaneous record of all the 

 intensities in the spectrum at any one instant. All spec- 

 trum lines have a breadth, due to the Doppler effect of 

 the atomic motions in the kinetic theor\, and to other 

 agencies. The shape of one of the truncated lines 

 depends on the original law of intensity across the 

 line, and they may be wedge-shaped, or bounded by a 

 more or less rounded curve, from the nature of which, 

 if the. boundary can be sharply defined, we can deduce 

 mathematically the law of intensity across the original 

 line. Sharp changes of intensity, such as o«cur when 

 the line has several close components overlapping one 

 another, are detected as peaks or kinks in this bound- 

 ing curve. The original photograph can be enlarged 

 with considerable magnifying power, and if the bound- 

 ing curve on this enlargement is sharply, defined, we 

 can obtain its mathematical shape very accurately, and 

 deduce an esfimate of the intensity in any part of the 

 line with a great degree of precision. We have been 

 able to show that in most of our experiments such 

 accura(v as i part in loo has been reached, and it 

 could readily be increased, if desired, by the use of 

 greater magnification of the original photograph. 



The determination of the exact boundary of a patch 

 of dark on a white ground is a matter in which "per- 

 sonal equation " is important, We overcame this diffi- 

 culty bv enlarging positives, prepared from the nega- 

 tives, on to bromide paper through a ruled "process " 

 screen. The resulting photograph consists in this way 

 of an assemblage of very minute dots, fading away 

 towards the boundary into invisibility. It is a simple 

 matter to prick out the last dots visible all round the 

 contour, and in this way personal equation can appa- 

 rently be entirely eliminated. We adopted usually 

 about 100 dots to the inch on the final photograph. If 

 comparisons of different lines with one another are 

 required, onlv the central heights of the figures are 

 ne-^essarv, and the topmost dot can be seen at once._ 



The first application of the method was to the in- 

 tensity distribution in the lines of the hydrogen spec- 

 trum when a condensed discharge was passed through 

 the exciting tube. It was known that with' a con- 

 densed discharge the lines always appeared much 

 broader, and we concluded that the best method of 

 obtaining information as to the source of the effect 

 was to examine the intensity distribution across the 

 lines. Some remarkable contours were obtained, show- 

 ing at once a clear distinction between this source of 

 broadening and that associated with the Doppler effect. 



