MacTutor
JOHN WILLIAM STRUTT, 3rd Baron Rayleigh, was born in 1842. As a child he was extremely delicate, and was not able to go through the ordinary public school education. He was prepared for the university by Mr. Warner, who took pupils at Torquay. In 1861 he went up to Trinity College, Cambridge, where he gained the Sheepshanks Exhibition for astronomy. He graduated as Senior Wrangler in 1865, and was elected to a Fellowship at Trinity College in 1866. He did not, however, undertake any college teaching work. His private circumstances left him free for scientific pursuits, and to these he devoted a life of almost unremitting labour.
Lord Rayleigh could scarcely be called an astronomer, though he always took an interest in some branches of the subject. Much of his work had little or no astronomical bearing, but other parts bore closely on the work of astronomers, and it is to these latter that attention will here be drawn. Other more general accounts of his career have appeared, e.g. in Nature, to which reference may be made.
A paper appears in the Monthly Notices for 1872 (33, 59-63) on "The Diffraction of Object Glasses," in which it is proposed to block out the centre rather than the circumferential parts of an object glass used on the sun, so as to reduce light without loss of resolving power. The idea was, however, less novel than the author then thought. A central stop increases the diameter and intensity of the diffraction rings, and calculations of this effect were given. This was the only paper he presented to the Society.
In his early experimental work much attention was paid to the duplication of diffraction gratings by photography (contact printing). From boyhood onwards he had become expert in the various photographic processes which were in vogue before the use of silver-gelatine plates, and indeed throughout life he took more pleasure in photography and its scientific applications than in any other kind of experimental technique. These early processes give a very fine grain, well adapted to the 'problem. Very successful reproductions of the early glass gratings by Nobert and Rutherfurd were obtained. They were not appreciably inferior to the originals in resolving power, and some of them gave much brighter spectra, owing, no doubt, to a more favourable width of the opaque bars in the copy. Rowland's speculum gratings, far superior to their predecessors, appeared shortly afterwards, and these did not lend themselves well to photographic reproduction; thus the method has not been of much actual service. If the proposal to make transparent objective gratings for large telescopes should be realised, a better field for Rayleigh's methods of reproduction might be found.
These experiments on gratings brought the author's mind into intimate contact with the subject, and suggested the train of thought which led to some of his most important optical work, the work by which he is perhaps best known to astronomers: this is his theoretical study of the resolving power of spectroscopes. Without attempting any critical study of the state of opinion on such matters at that time (1879), it is enough to say that it was extremely vague and confused. For example, in the case of gratings there was a notion among spectroscopic workers that the abrupt limitation of the grating aperture had an unfavourable effect on definition, and that a great improvement might be obtained if this abruptness could be in some way rased off. Again, in the case of prism spectroscopes no clear notions were entertained of the distinction between the angular dispersion and the resolving power of spectroscopes, though the analogous problem for the telescope was well understood, at least by some.
In Rayleigh's treatment of the subject, the separation of two close spectrum lines was comparable with the separation of two double stars in the telescope. The resolving power was defined as the ratio of the whole wave-length to the smallest difference that could just be detected. The latter was taken as a definite displacement of the diffraction pattern. For the grating the resolving power was shown by quite simple considerations to be equal to the product of the order of spectrum examined and the total number of rulings.
In his experimental work he had occasionally obtained photographic copies of gratings which threw a less fraction of the light of a soda flame into the central image than into the first-order spectrum on either side. This was at the time an unexpected result, and, as he pointed out, it showed that the gratings in question could not act merely by blocking out part of the wave-front, but that they must introduce differences of phase. This conception is now familiar, and has its highest development in Michelson's echelon.
His theory of the prism spectroscope was also the result of experimental suggestion. In testing the performance of his grating copies, the very superior results they gave compared with a prism raised in his mind the question of what the essential condition is for high resolving power. The dispersive power of the glass used must of course enter. But what more? The answer generally given at that time would almost certainly have been the number of prisms used, assuming them of course to be of the largest angle practicable. He showed by an argument of characteristic simplicity and directness that the resolving power for a given glass depended solely on the aggregate length of glass traversed at the thick ends of the prisms, assuming of course that the beam was broad enough to extend to their thin ends. The numerical resolving power was found to be simply the product of this thickness by the dispersive power. No consideration is more essential to the design of spectroscopes than this. Indeed, the matter is so simple and fundamental, that it now seems a matter of surprise that it was not discovered earlier in the history of optics.
In the same series of papers is discussed the question of what error of phase is admissible in optical instruments without appreciably distorting the diffraction pattern, and thus doing harm to the resolving power. The question is not of course one which admits of a perfectly precise answer, but it was shown that in many important cases an extreme error of of a wave-length does no appreciable harm, but beyond this the definition suffers. This rule, which has stood the test of time, is of great value in practical optician's work, as it shows the limit beyond which improved accuracy is not necessary or useful in figuring the surfaces of objectives or mirrors, as the author pointed out shortly before his death. It is important to avoid the mistake of applying this rule blindly, and without due discrimination as to the limitations under which it holds. It would, for example, be altogether beside the mark to apply it to instruments of the interferometer class, where the requirements are much more exacting.
Another of Rayleigh's optical investigations may be mentioned: this concerns the broadening of spectrum lines as conditioned by the Doppler effect of moving molecules. He drew attention to this in 1873, though the idea had been mooted three years earlier by Lippich, Rayleigh, however, was the first to show quantitatively what would be the limit to interference on this basis. This line of thought has been developed by Fabry and Buisson, and has led in their hands to an interesting determination of the effective temperature of the great nebula in Orion.
Rayleigh's best-known work is of course the discovery of argon, which resulted from his accurate weighings of gases. Sir William Ramsay collaborated in the later stages of the work, which led afterwards in the hands of the latter to the isolation of terrestrial helium.
Rayleigh wrote on many other subjects which bear directly or indirectly on astronomical work, such as the origin of the blue sky, stellar scintillation, and the rationale of the grinding and polishing processes of the optician.
Most of his work was carried out at his country seat, Terling Place, near Witham, Essex, where he had a laboratory distinguished by the homely simplicity of the appliances used. During the years 1880-1884, however, he was Professor of Experimental Physics at Cambridge, and worked mainly at the Cavendish Laboratory. Afterwards (1887-1905) he was Professor of Natural Philosophy at the Royal Institution, and worked there during a part of the year.
He served as President of the Royal Society for the years 1905-1908. A characteristic sentence from one of his Presidential Addresses may be worth quoting. Referring to Hale's discovery of the Zeeman effect in sun-spots, he said, "Until 'we understand better than we do these solar processes, on which our very existence depends, we may do well to cultivate a humbler frame of mind than that indulged in by some of our colleagues."
Rayleigh married in 1871 Miss Evelyn Balfour, sister of the Rt. Hon. A. J. Balfour. He had four sons, of whom two survive. He died after an illness of a few weeks on 1919 June 30.
Many honorary distinctions were accorded to him. Amongst others, he was one of the original members of the Order of Merit, and Chancellor of the University of Cambridge.
He was elected a Fellow of the Society on 1866 January 12.
R.
Lord Rayleigh could scarcely be called an astronomer, though he always took an interest in some branches of the subject. Much of his work had little or no astronomical bearing, but other parts bore closely on the work of astronomers, and it is to these latter that attention will here be drawn. Other more general accounts of his career have appeared, e.g. in Nature, to which reference may be made.
A paper appears in the Monthly Notices for 1872 (33, 59-63) on "The Diffraction of Object Glasses," in which it is proposed to block out the centre rather than the circumferential parts of an object glass used on the sun, so as to reduce light without loss of resolving power. The idea was, however, less novel than the author then thought. A central stop increases the diameter and intensity of the diffraction rings, and calculations of this effect were given. This was the only paper he presented to the Society.
In his early experimental work much attention was paid to the duplication of diffraction gratings by photography (contact printing). From boyhood onwards he had become expert in the various photographic processes which were in vogue before the use of silver-gelatine plates, and indeed throughout life he took more pleasure in photography and its scientific applications than in any other kind of experimental technique. These early processes give a very fine grain, well adapted to the 'problem. Very successful reproductions of the early glass gratings by Nobert and Rutherfurd were obtained. They were not appreciably inferior to the originals in resolving power, and some of them gave much brighter spectra, owing, no doubt, to a more favourable width of the opaque bars in the copy. Rowland's speculum gratings, far superior to their predecessors, appeared shortly afterwards, and these did not lend themselves well to photographic reproduction; thus the method has not been of much actual service. If the proposal to make transparent objective gratings for large telescopes should be realised, a better field for Rayleigh's methods of reproduction might be found.
These experiments on gratings brought the author's mind into intimate contact with the subject, and suggested the train of thought which led to some of his most important optical work, the work by which he is perhaps best known to astronomers: this is his theoretical study of the resolving power of spectroscopes. Without attempting any critical study of the state of opinion on such matters at that time (1879), it is enough to say that it was extremely vague and confused. For example, in the case of gratings there was a notion among spectroscopic workers that the abrupt limitation of the grating aperture had an unfavourable effect on definition, and that a great improvement might be obtained if this abruptness could be in some way rased off. Again, in the case of prism spectroscopes no clear notions were entertained of the distinction between the angular dispersion and the resolving power of spectroscopes, though the analogous problem for the telescope was well understood, at least by some.
In Rayleigh's treatment of the subject, the separation of two close spectrum lines was comparable with the separation of two double stars in the telescope. The resolving power was defined as the ratio of the whole wave-length to the smallest difference that could just be detected. The latter was taken as a definite displacement of the diffraction pattern. For the grating the resolving power was shown by quite simple considerations to be equal to the product of the order of spectrum examined and the total number of rulings.
In his experimental work he had occasionally obtained photographic copies of gratings which threw a less fraction of the light of a soda flame into the central image than into the first-order spectrum on either side. This was at the time an unexpected result, and, as he pointed out, it showed that the gratings in question could not act merely by blocking out part of the wave-front, but that they must introduce differences of phase. This conception is now familiar, and has its highest development in Michelson's echelon.
His theory of the prism spectroscope was also the result of experimental suggestion. In testing the performance of his grating copies, the very superior results they gave compared with a prism raised in his mind the question of what the essential condition is for high resolving power. The dispersive power of the glass used must of course enter. But what more? The answer generally given at that time would almost certainly have been the number of prisms used, assuming them of course to be of the largest angle practicable. He showed by an argument of characteristic simplicity and directness that the resolving power for a given glass depended solely on the aggregate length of glass traversed at the thick ends of the prisms, assuming of course that the beam was broad enough to extend to their thin ends. The numerical resolving power was found to be simply the product of this thickness by the dispersive power. No consideration is more essential to the design of spectroscopes than this. Indeed, the matter is so simple and fundamental, that it now seems a matter of surprise that it was not discovered earlier in the history of optics.
In the same series of papers is discussed the question of what error of phase is admissible in optical instruments without appreciably distorting the diffraction pattern, and thus doing harm to the resolving power. The question is not of course one which admits of a perfectly precise answer, but it was shown that in many important cases an extreme error of of a wave-length does no appreciable harm, but beyond this the definition suffers. This rule, which has stood the test of time, is of great value in practical optician's work, as it shows the limit beyond which improved accuracy is not necessary or useful in figuring the surfaces of objectives or mirrors, as the author pointed out shortly before his death. It is important to avoid the mistake of applying this rule blindly, and without due discrimination as to the limitations under which it holds. It would, for example, be altogether beside the mark to apply it to instruments of the interferometer class, where the requirements are much more exacting.
Another of Rayleigh's optical investigations may be mentioned: this concerns the broadening of spectrum lines as conditioned by the Doppler effect of moving molecules. He drew attention to this in 1873, though the idea had been mooted three years earlier by Lippich, Rayleigh, however, was the first to show quantitatively what would be the limit to interference on this basis. This line of thought has been developed by Fabry and Buisson, and has led in their hands to an interesting determination of the effective temperature of the great nebula in Orion.
Rayleigh's best-known work is of course the discovery of argon, which resulted from his accurate weighings of gases. Sir William Ramsay collaborated in the later stages of the work, which led afterwards in the hands of the latter to the isolation of terrestrial helium.
Rayleigh wrote on many other subjects which bear directly or indirectly on astronomical work, such as the origin of the blue sky, stellar scintillation, and the rationale of the grinding and polishing processes of the optician.
Most of his work was carried out at his country seat, Terling Place, near Witham, Essex, where he had a laboratory distinguished by the homely simplicity of the appliances used. During the years 1880-1884, however, he was Professor of Experimental Physics at Cambridge, and worked mainly at the Cavendish Laboratory. Afterwards (1887-1905) he was Professor of Natural Philosophy at the Royal Institution, and worked there during a part of the year.
He served as President of the Royal Society for the years 1905-1908. A characteristic sentence from one of his Presidential Addresses may be worth quoting. Referring to Hale's discovery of the Zeeman effect in sun-spots, he said, "Until 'we understand better than we do these solar processes, on which our very existence depends, we may do well to cultivate a humbler frame of mind than that indulged in by some of our colleagues."
Rayleigh married in 1871 Miss Evelyn Balfour, sister of the Rt. Hon. A. J. Balfour. He had four sons, of whom two survive. He died after an illness of a few weeks on 1919 June 30.
Many honorary distinctions were accorded to him. Amongst others, he was one of the original members of the Order of Merit, and Chancellor of the University of Cambridge.
He was elected a Fellow of the Society on 1866 January 12.
R.
John William Rayleigh (Strutt)'s obituary appeared in Journal of the Royal Astronomical Society 80:4 (1920), 350-353.