January 9, 1896] 



NA TURE 



237 



I 



calcium and silver, attaining results which accord well with the 

 fiest previously attained by those who had employed the 

 lK)lometer, and which demonstrate the exceeding delicacy of 

 the radiomicrometer as an instrument of research. 



The Visible Spectrum. 



To follow out all the applications of the spectroscope that 

 have resulted in recent additions to our knowledge would carry 

 us far beyond the scope of a single paper. It is possible only 

 10 make brief mention of a few. 



For a number of years Rowland {ibid., January to August 

 1895) has been investigating the spectra of all the chemical 

 elements, photographing them in connection with the normal 

 solar spectrum, and reducing them to his table of standards, 

 which is now accepted everywhere. The work is of such 

 magnitude that years more must elapse before its completion. 

 It now includes all wavelengths from 3722 to 7200, and of 

 these the list already published extends as far as wave-length 

 5150, or from ultra-violet nearly to the middle of the green. 



Through the spectroscope chiefly has been established during 

 the present year the discovery of the new atmospheric element, 

 argon, by Lord Rayleigh and Prof. Ramsay (Proc. Royal 

 Society, January 31, 1895); its remarkable property of green 

 fluorescence when the electric spark is passed through it in pre- 

 sence of benzene, by Berthelot and Deslandres ( Comptes rendus, 

 June 24, 1895) ; and its association in meteoric iron and 

 various minerals with helium, now proved to be a terrestrial as 

 well as solar element, by Ramsay (Nature, April 4, May 16, 

 July 4 and 25, 1895), Crookes, Lockyer, and others. 



With the diffraction spectroscope Rydberg ( Wiedemann' s 

 Annalen, 1893-94) and Kayser and Runge {ibid., 1888-95) have 

 discovered interesting relations among the spectral lines of a 

 large number of terrestial elements, arranging them into series 

 whose distribution manifests chemical relationship quite analo- 

 gous to that indicated in Mendelejeff s periodic law. 



By photographing the spectrum of Saturn's rings and noting 

 the relative displacement of the different parts of a spectral line, 

 Keeler {Astropkysical Journal, May 1895, P- 4^6) has obtained 

 I beautiful direct proof of the meteoric constitution of tWese 

 ■ ings, a confirmation of the hypothesis put forth by Maxwell in 

 1859, that the outer portion of the rings must revolve more 

 Uowly than the inner portion, and yet not satisfy the conditions 

 f fluidity. His work has been repeated and confirmed by 

 Campbell {ibid., August 1895, p. 127) at the Lick Observatory. 



The spectroheliograph devised by Hale (Astronomy and 

 ,Utrophysics, March, 1893, P- 256) has enabled him to photo- 

 t;raph, on any bright day, not only the solar photosphere and 

 pots, but also the chromosphere and protuberances. He has 

 Miade some remarkable attempts with this instrument to photo- 

 graph the corona without an eclipse, unsuccessfully thus far, but 

 not without promise of future success. 



Polarised LrcHT, 



In the domain of polarised light, there have been several note- 

 worthy recent researches. Nichols and Snow {Philosophical 

 Magazine (5), vol xxxiii. p. 379) have shown that calcite, 

 though readily transparent for the brighter rays of the spectrum, 

 rapidly diminishes in power of transmission for waves of short 

 j^eriod, so that for the extreme violet this power is scarcely half 

 so great as for the yellow. The transmissive power of this 

 crystal for the infra-red rays, between the wave-length limits 

 of I micron and 5*5 microns, has been investigated with the 

 bolometer i)y Merritt (Physical Review, May-June 1895, p. 

 424) who reaches the interesting result that the transmission 

 curve for the ordinary ray is wholly independent of that for the 

 extraordinary, the absorption being in general much greater for 

 the former. Several sharp absorption bands are found for each 

 ray. For radiations whose wave-length exceeds 3*2 microns, 

 the absorption of the ordinary ray is almost complete, so that 

 calcite behaves for such radiation just as tourmaline does for the 

 rays of the visible spectrum. The independence of the two 

 transmission curves is found to exist also for quartz and tourma- 

 line, these curves for the latter crossing each other twice in the 

 infra-red region. 



The application of polarised l^ht to the investigation of 

 internal stress in transparent media was made more than forty 

 years ago by Wertheim (Comptes rendus, 32, p. 289, 1851), who 

 demonstrated that the retardation of the rays is proportional to 

 the load. An extended series of such experiments has been 

 ilately made in America by Marston {Physical Review, Sep- 



NO. 1367, VOL. 53] 



tember, October, p. 127, 1893) who, besides confirming 

 Wertheim's conclusion, shows that, •' for small strains at least, 

 the colours seen in a strained glass body, when polarised 

 light is passed through it in a direction parallel to one of the 

 axes of strain, are measured by the algebraic difference of the 

 intensities of those two principal strains whose directions are 

 perpendicular to the direction of the polarised light." 



A new substance with double rotatory power, like quartz, 

 has been discovered by Wyrouboff (journal de Physique (3), 3, 

 452, 1894), the neutral anhydrous tartrate of rubidium, which is 

 unique in one respect. The rotatory power of the substance in 

 the crystalline state becomes reversed in solution. This wholly 

 new phenomenon introduces some perplexity in connection with 

 certain molecular theories that have been formulated to account 

 for double rotatory power. 



Crehore ( Transactions of the American Institute of Electrical 

 Engineers, October 1894, p. 91) has ingeniously applied Fara- 

 day's principle of electro-magnetic rotation of the plane of polar- 

 isation in carbon bisulphide to the photographing of alternate 

 current curves. Every variation in the magnetic field causes 

 variation in the amount of light transmitted through a pair of 

 crossed Nicol prisms. The combination becomes a chrono- 

 graph with an index as free from inertia as the beam reflected 

 from a galvanometer mirror. The same instrument has been 

 applied to measurement of the velocity of projectiles (Journal of 

 the United States Artillery, p. 409, July 1895), with results of 

 exceeding interest to the student of gunnery. 



Physiological Optics. 



The temptation to dilate upon recent progress in physiological 

 optics has to be resisted. The revision of Ilelmholtz's great 

 book on this subject was interrupted by the death of the dis- 

 tinguished author, but the last part is now approaching com- 

 pletion under the care of his pupil, Arthur Konig, who, in con- 

 junction with Diederici, has done much important work in this 

 domain. The selection of hues for the three primary colour 

 sensations has been slightly modified. Young selected the two 

 extremes of the spectrum, red and violet, together with green, 

 which is about midway between them. The hues now accepted 

 by Helmholtz and those who follow his lead, including the great 

 majority of physicists, are a highly saturated carmine red, an 

 equally saturated ultramarine blue, and a yellowish green, cor- 

 responding somewhat to that of vegetation. The red and blue 

 agree with those previously determined by Hering, but the 

 rivalry between the two schools on the subject of colour 

 sensation continues, and perhaps will last through a period 

 commensurate with the difficulty of devising crucial experiments. 



Independent theories of colour sensation have been brought 

 out by Mrs. Franklin (Christine Ladd Franklin, " Eine neue 

 Theorie der Lichtempfundungen," Zeitschrift fiir Psychologic 

 und Physiologie der Sinnesorgane, 1892) in America, and by 

 Ebbinghaus ("Theorie des Farbensehens," ibid., 1893) in 

 Germany. The former particularly is worthy of much more 

 extended notice than can here be given. It may perhaps be 

 quite properly called a chemical theory of vision. Light is 

 always bringing about chemical changes in external objects, and 

 the eye is the one organ whose exercise requires the action of 

 light, while such chemical action is implied in the performance 

 of most of the bodily functions, such as the assimilation of food 

 and the oxidation of the blood. The bleaching action of light 

 upon the visual purple, which is continually formed on the retina, 

 has been known ever since the discovery of this in 1877 by 

 Kiihne, who secured evanescent retinal photographs in the eyes 

 of rabbits. Mrs. Franklin considers that light sensafon is the 

 outcome of photo-chemical dissociation of two kinds of retinal 

 molecules that she denominates grey molecules and colour 

 molecules, of which the latter arise from the grey molecules by 

 differentiation in such a way that the atoms of the outer layer 

 group themselves differently in three directions, and the cor- 

 responding action of light of proper wave-length gives rise to 

 the three fundamental colour sensations. She develops the 

 theory with much skill, applying it particularly to the 

 phenomena of retinal fatigue and colour blindness. To the 

 objection that there is no direct proof of the existence of the 

 assumed grey and colour molecules, it may be answered that 

 Helmholtz himself fully recognised the uncertainty of the 

 assumption that three different sets of nerves respond to the 

 three fundamental colour sensations, and he admitted that these 

 may be only different activities in the same retinal cone. The 

 supposition of three adjacent cones, responding respectively to 



