us PHYSICS 



Jeans. It appears that a sound wave 0.4 mm long decreases to one-hundredth of its 

 strength within 10 cm, while a wave o.i mm long does so within 6 mm. Neklepajef, 

 using spark waves concentrated by a mirror and analysed by means of a Fraunhofer 

 grating of silver-steel rods, measured the absorption of waves down to 0.8 mm length 

 by means of an Altberg pressure apparatus. 



The measurement of the physical intensity of sound has been brought nearer to its 

 full realisation. Heindlhofer (1912) has worked out the theory of a method of deter- 

 mining it by the heating of gold-leaf, while Zwaardemaker has found the energy required 

 to sound ah to be about 3 megergs per second. 



Measurement of Visible Light. R. A. Houstoun 1 proposes to measure light absolutely 

 by means of a thermopile provided with an ideal light-filter to give it the same curve 

 of sensitiveness as the human eye. The filter proposed is an aqueous solution of 

 CuSO^. 5 HsO of strength 0.2 gram-mols per litre, in a layer 3 cm thick, followed by an 

 aqueous -solution of K^ Cr2 Ov of strength 0.0025 gram-mols per litre, in a layer i cm thick. 

 The light would then be measured in ergs per cm 2 per second. 



Meanwhile, Buisson and Fabry, 2 using a Heraeus mercury lamp, have determined 

 the minimum possible expenditure of energy on a lamp which entirely transforms the 

 energy it receives into the most advantageous luminous radiations. They find the 

 value 0.018 watt per Hefner candle, which is about one-fiftieth of the expenditure on a 

 metallic filament lamp. This figure might be called the " mechanical equivalent of light." 



As regards the maximum of visibility, we now have the wave-lengths indicated by 

 Nutting, of the Bureau of Standards (1911). They are 544/i/* for strong light and 

 SO^IJLH for very feeble light. 



Advances in Spectroscopy. The recent great activity of spectroscopists has been 

 accompanied by the discovery of new methods of investigation and the devising of new 

 aids. Thus we have Anderson's copies in collodion of Rowland's original gratings, 

 rendered accurate by mechanical stretching where required; Fery's prisms with curved 

 surfaces (1910), which offer a valuable saving of precious material like fluorspar, and 

 eliminate the loss at additional optical surfaces; Lehmann's ultra-violet filter (1910) 

 consisting of Jena " uviol " glass and a layer of dimethylnitrosoaniline, which only trans- 

 mits light of wave-lengths below 380 /x;u; and Wood's " echelette " grating, consisting 

 of lines ruled about 1,000 to the inch on metal. This new grating has been applied with 

 great success by Trowbridge and Wood to the resolution of the extreme infra-red resid- 

 ual rays from quartz. 



Longest Light Waves. The gap between the longest light waves and the shortest 

 mechanical waves observable has been definitely bridged by Rubens and von Baeyer, 3 

 who isolated a beam of the mean wave-length 313^ from the radiation of a mercury 

 lamp consuming 4 amperes at 100 volts. This wave-length of about \ mm coincides 

 with that of the shortest sound waves and closely approaches the shortest observable 

 Hertzian waves. 



The infra-red spectrograph recently devised by Lebedef, and Trowbridge's. simple 

 demonstration apparatus for the infra-red spectrum, in which the sensitive receiver is 

 a stretched rubber band provided with a mirror to indicate its thermal expansion, will 

 no doubt contribute to the advance of infra-red spectroscopy both in the laboratory and 

 the lecture-room. 



The theoretical analysis of spectrum lines by series has hitherto suffered from the 

 presence of numerous lines which did not fit into any of the series specified by Kayser 

 and Runge. This is now in a fair way towards solution, since Ritz indicated a principle 

 of combination which enables us to deduce new wave-lengths from lines already fitted 

 into a series. The principle in question has been very successfully applied by Paschen, 4 

 with the aid of the Zeeman effect, which gives a valuable index to the mutual connection 



1 Royal Society, Proceedings A 85, p. 275 (1911). " Comptes Rendus 153, p. 254 (1911). 



3 Preussiche Akademie, Sitzungsberichte, 14, p. 339 (1911). Philosophical Magazine, 21, 

 p. 689 (1911). 



4 Annalen de Physik, 36, p. 191 (1911). 



