JULV 30, 1908J 



NA TURE 



III 



was also used in this way, and it was found that n is not 

 the same for blue as for red ; for the first n = o-36, and 

 for the second n = o-25, nearly. Thus it appears that the 

 method of measuring light which consists of various colours 

 by exposing a selenium resistance to the compound light 

 is erroneous; the light must be broken into a spectrum, 

 and the intensity measured in each part. 



\ selenium bridge possesses the peculiarity (which was 

 noticed by Adams and Day in their experimentsj that, once 

 it has been exposed to light, while a current is passing 

 through it, its resistance is not the same to currents pass- 

 ing in one direction as to currents passing in the oppo- 

 site direction, and apparently the two resistances never 

 again become equal. 



Moreover, the resistance to a current of given direction 

 depends on the voltage. It was found that if c is the con- 

 ductivitv of the bridge when the voltage of the battery 

 is V, 



c=iv+/y, (2) 



where k, k' are constants. Thus the conductivity is a 

 linear function of the voltage, but this will not be found 

 unless the observer allows the current to run for several 

 minutes. When light falls on the bridge, the current 

 produced increases as the exposure is continued. With red 

 light the current rises very rapidly, and after, perhaps, 

 fortv-five seconds moves slowlv towards an asymptotic value. 

 With blue light the rise of current at the instant of e.x- 

 posure is much slower. The curve the ordinates of which 

 are the values of C and abscissce the times, (, has an 

 ecjuation of the form 



(H-C)-"'-(II-C„)-"' = A/ (3) 



where H (the final value of C), 111, and A. are constants 

 depending on the colour of the light. 



The curve is hyperbolic in appearance. 



Finally, a spectrum was formed by passing the light of 

 the Nernst lamp through quartz lenses and prisms, and 

 it was found that the effect is a maximum in the red near 

 the yellow, and that effects are produced in and a little 

 beyond the violet, while at the infra red end the effects 

 extended to more than a whole spectrum length. The 

 radiation of a very hot, but invisible, metal ball produces 

 scarcely any effect, even at a small distance from the 

 bridge. 



Geological Society, June 17.— Prof. W. J. Solla?, F.R.S., 

 president, in the chair. — The hornblendic rocks of Glenda- 

 iough and Greystones (co. Wicklow) : J. .'Mian Thomson. 

 Both these rocks are intrusive into Ordovician strata in 

 the east of county Wicklow, the former occurring as a 

 small boss in the south side of Camaderry, a ridge which 

 separates the V'ale of. Glendalough from the valley of 

 Glendrosan, while the latter occur as three dykes travers- 

 ing the sedimentary rocks on the shore at Greystones. 

 The Glendalough rock is older than the Great Wicklow 

 l-jranite, and e.xhibits much heterogeneity in composition. 

 The Ordovician sediments are converted into hornfels at 

 the conta,ct with the igneous rock, and this type of rock 

 lias resisted the dynamic metamorphism which occurs else- 

 where in the district. The Greystones rock shows a trans- 

 formation from peridotite into amphibolite, but with a 

 greater development of talc. Olivine and rarely mica are 

 present in the original rock. — On the occurrence of foot- 

 prints in the Lower Sandstones of the Exeter district : 

 A. W. Clayden, Suitable exposures in the " Lower 

 Sandstones " of the Geological Survey map are very rare. 

 Dr. Shapter has recorded "claw-like footmarks," &:c., 

 from a locality about half a mile north-east of Broadclyst. 

 .Another quarry has been recently re-opened here for build- 

 ifig-stone. and, on a search being made, slabs with foot- 

 prints were found by the author and his students. Later, 

 a slab with a track containing thirty pairs of footprints 

 was found. In all, five specimens have been secured, and 

 three of the sets of prints may have been made bv the 

 same individual, one with fore and hind feet about the 

 same size and bearing about the same weight. The two 

 other sets of prints were made bv smaller and different 

 individuals. — The basic intrusion of Bartestree, near Here- 

 ford : Prof. S. H. Reynolds. The Bartestree dyke, which 

 has a thickness of about 35 feet, strikes in an east-north- 

 easterly direction through the Old Red Marls and Sand- 



MO. 2022. VOL. YS^ 



stones, which for a distance of at least 10 feet from the 

 contact are strongly metamorphosed, the marl being con- 

 verted into a hard purplish-grey rock with yellow patches, 

 while in the sandstone the felspars are re-crystallised and 

 the quartz-grains corroded. The dyke itself is not a single 

 uniform intrusion, but a multiple dyke composed of several 

 allied though differing types of dolerite and basalt. 



Challenger Society, June 24. — Prof. d'A. W. Thompson 

 in the chair. — Oceanography in America : Prof. C. A. 

 Kofoid. The recent traverses of the interesting stretch 

 of ocean lying between the Galapagos and Easter Island, 

 on the one hand, and the S. American coast on the other, 

 made by Prof. .Agassiz in the U.S. Fish Commission 

 steamer Albatross, with which the speaker had been 

 associated, were described in detail, and valuable con- 

 clusions drawn as to the influence of currents, up-welling, 

 and eddies on the richness or poverty of both plankton and 

 benthos. Some of the more important marine stations of 

 the United States, and the character of their work, were 

 also dealt with. 



Paris. 



Academy of Sciences, July 20. — M. Bouchard in the 

 chair. — The minerals from the fumaroles of the recent 

 eruption of Etna, and on the existence of boric acid in the 

 existing fumaroles of Vesuvius ; A. Lacroix. One of the 

 peculiarities of the recent eruption of Etna was the slight 

 intensity of the fumaroles. The latter offered all the 

 usual phenomena, with the exception that the warmest 

 fumaroles contained no copper salts. Those containing 

 ammonium chloride as the principal constituent also con- 

 tained a notable quantity of fluorine. In the fumaroles 

 of Vesuvius a small quantity of the mineral sassolite was 

 found ; this was identified by its hexagonal form, its optical 

 properties, and its chemical properties, the last correspond- 

 ing to normal boric acid. — The hydrates of strontia and 

 baryta : M. de Forcrand. Strontium hydrate can be con- 

 verted into the anhydrous SrO by heating for a long time 

 in a current of hydrogen at a temperature of 850° C. 

 BaO can be obtained in a similar manner in two or three 

 hours at 780° C. Various intermediate hydrates are de- 

 scribed and thermochemical data given. — Remarks on the 

 note of M. Lebedew. The apparent dispersion of light in 

 interstellar jpace : G. A. TikhofT. Some new observations 

 on the star RT Perseus, and a reply to the criticisms of 

 M. Lebedew. — Ruled surfaces : M. Tzitzeica. — Algebraic 

 functions of two variables : H. W. E. Jung:. — The points 

 of equilibrium of a fluid in movement : M. Popovici. — 

 The periodic solutions of a functional linear equation : 

 Ernest Esclangron. — The calculation of the tensions in 

 articulated systems of three dimensions : B. Mayor. — A 

 safety apparatus against continuous disturbing sparks in 

 wireless telegraphy ; Edouard Branly. — The flame spectra 

 of calcium : G. A. Hemsalech and C. de Watteville. 

 The finely divided substance, pulverised by the electrical 

 mefhod previously described by the authors, was introduced 

 into various flames, air-coal gas, air-hydrogen, oxygen-coal 

 gas, and oxygen-hydrogen, and the observed spectra com- 

 pared with the arc spectrum (Kayser and Runge). — Varia- 

 tions in the fringes in the photochromes of the spectrum : 

 E. Rothe. A study of the conditions necessary to free the 

 photographs from the results of secondary phenomena. 

 Photographs of spectra are submitted to the academy, in 

 which the colours are pure, the exact reproduction of the 

 colours of the spectrum, all the accessory reflections having 

 been suppressed. — Electric and magnetic double refraction 

 of nitrobenzene. Variation with the wave-length : A. 

 Cotton and H. Mouton. In nitrobenzene, Kerr's pheno- 

 menon is exceptionally large, being ninety-seven times that 

 of carbon bisulphide for the yellow mercury line. Within 

 the experimental error of the experiments, the dispersion 

 of the electric double refraction of nitrobenzene is the same 

 as the dispersion of the magnetic double refraction. — ^ 

 case of anomalous rotatory dispersion ; application of the 

 measurements of rotatory dispersion to the study of the 

 composition of essence of turpentine : Eugene Darmois. 

 Some mixtures of dextro- and la;vo-rotatory turpentine 

 show an anomalous dispersion : the rotation becomes zero 

 for one colour of the spectrum, and passes through a 

 minimum for another colour. — The reduction of alkaline 

 chloroiridates by oxalates : Marcel Delepine. A reply to 



