December 2, 1922] 



NA TURE 



74i 



bourg at Easter of the latter year to work under August 

 Kundt. He followed Kundt to Berlin in May 1888, 

 and obtained his Ph.D. there the year following. His 

 early post-graduate career was spent as Assistent under 

 Kundt at the Physical Institute of the University of 

 Berlin, where he remained until 1896, when he was 

 invited to the Charlottenburg Technical High School, 

 and in 1900 he was officially elected professor at that 

 institution. In the autumn of 1906 he was elected 

 to a full chair of experimental physics at the University 

 of Berlin, and to the directorship of the Physical 

 Institute, which posts he filled during the remainder 

 of his life. He died of leuchaemia on July 17 last. 



Rubens was a member of the Berlin Academy of 

 Science, and of many other similar bodies in his own 

 countrv and abroad, including the Royal Institution, 

 of which he was an honorary member. He held 

 doctor's degrees {honoris causa) of the Universities 

 of Leeds and Cambridge, and was a recipient of the 

 Rumford Medal of the Royal Society. 



Most of Rubens' scientific investigations were con- 

 cerned with the infra-red region of the spectrum, 

 and the logical connexion of his numerous researches 

 is a noteworthy feature of his scientific activity. Many 

 of the instruments used in the prosecution of his work 

 were of his own construction, including the Rubens 

 thermopile, and the Rubens-Du Bois spherical sheath 

 galvanometer. He was led to the discovery of residual 

 rays as a result of his work and measurements on the 

 optical properties of various substances with regard 

 to heat rays. He succeeded in reducing the previously 

 unexplored region of about twelve octaves (from 

 A = 0-005 t0 5° mm.) between the infra-red region of 

 the spectrum and electrical waves, by his discovery 

 of about seven of the missing octaves. 



After his observation that a number of minerals 

 strongly reflect infra-red waves of certain definite 

 wave-lengths, and transmit the rest of the rays, Rubens 

 was able to isolate rays up to a wave-length of about 

 o-oi mm. Repeated reflection of the radiation from 

 such surfaces results in a residual radiation which 

 contains certain definite wave - lengths only ; e.g. 

 from fluorspar (0-022 and 0-033 mm.), rock salt (0-052 

 mm.), sylvine (0-063 mm.), potassium bromide (0-083 

 mm.), potassium iodide (0-094 mm.). In part col- 

 laboration with Wood, Rubens isolated still greater 

 wave-lengths by the quartz-lens method, in which, by 

 virtue of the higher refractive index of quartz for 

 these long waves than for the shorter infra-red and 

 visible rays, and by the use of suitable diaphragms, 

 he succeeded in obtaining rays with a wave - length 

 of about o-no mm. from an incandescent mantle. 

 Using a quartz mercury lamp he extended this limit 

 to beyond 0-3 mm. In continuation of his earlier 

 measurements on wave - lengths in the near infra- 

 red, Rubens and his co-workers examined the dis- 

 persion and absorption of the whole range of the infra- 

 red in numerous substances. By making use of the 

 refractive indices of numerous substances found for 

 these long wave-lengths, or the values extrapolated 

 for infinite wave - length, he tested the validity of 

 Maxwell's law (n 2 = k) between the refractive index 

 for these waves, and the corresponding dielectric 

 constant of the substance in question. Several series 

 of measurements on the absorption of infra-red waves 



NO. 2770, VOL. I TO] 



in water vapour supplied him with the material requisite 

 for the comparison of Bjerrum's theory of rotation 

 spectra with experiment, and for calculating the main 

 moment of inertia of the water vapour molecule. 



In addition to his fundamental work on residual 

 rays, Rubens accomplished much in other branches 

 of radiation. He carried out measurements in col- 

 laboration with Hagen at the Physikalisch-Technische 

 Reichsanstalt on the reflecting power (R) of metals, which 

 led to the empirical result that for metals the coefficient 

 of penetration (P = 1 - R) for very long waves can be 

 represented by the relation P = 0-365 Jo-jX, where o- is 

 the specific resistance of the metal, and A the wave- 

 length of the rays in terms of the unit o-ooi mm. 

 This result is in agreement with deductions from the 

 electromagnetic theory of light. His investigations on 

 the validity of the law of radiation are of primary 

 importance. Conjointly with Kurlbaum he carried out 

 measurements on black body radiation of long wave- 

 length, and this work was largely responsible for a 

 revision of Planck's first radiation formula, and thus 

 supplied one of the experimental bases of the quantum 

 theory. Only last year, Rubens again applied his 

 great experimental ability in an endeavour to test 

 Planck's law of radiation in its final form. The 

 results of this work led to the complete confirmation 

 of the theory. They were communicated to a Congress 

 of Physicists at Jena in the autumn of 192 1, and 

 Rubens was acclaimed by the congress in a manner 

 seldom met with in scientific life. 



Rubens, whose wife survives him, was in failing health 

 for some years prior to his death. To those who knew 

 him well, it seemed that the privations attendant upon 

 war-time conditions were in a large measure responsible 

 for hastening the end. In addition to his great powers 

 and achievements, his active nature and kindly disposi- 

 tion bound him closely to his colleagues, who realise that 

 in Rubens they have lost much more than a valued 

 colleague. The loss to science will be appreciated by 

 those of other countries who came in contact with 

 him, for one could not meet Rubens without feeling 

 the forcefulness of a striking personality. Until his 

 death he maintained none but the friendliest of feelings 

 towards his colleagues in England, and during the 

 long years of the great war he took a human interest 

 in the well-being of those of our scientific nationals 

 whose lot it was to be detained in enemy territory. 

 For these he did what he could. Science mourns 

 his loss, and the record of his active life will occupy 

 a prominent place in the annals of science. 



R. W. L. 



The opportunity is most welcome to add my expres- 

 sion of deep regret for the loss of Prof. Rubens at an 

 age when much might still have been expected from 

 his scientific activity. I well remember the enjoyment 

 of the hospitality of himself and his family in days now 

 past, in the residence attached to the Physical Institute 

 of the University of Berlin, where memories of Helm- 

 holtz were evoked at every turn. One can recall the 

 simplicity of the apparatus used in his personal investi- 

 gations, in keeping with the directness of his main 

 results. In these respects he retained throughout his 

 career the stamp of the school of his early master Kundt. 



The existence of sharply defined ranges of intense 



