Intelligence and Miscellaneous Articles. 79 



holds for iron from 0° to -92°, with a=0-0049, but it does not 

 hold for platinum. The value of a deduced from the formula (2), 

 which near 0° would be about 0*0030, increases as the temperature 

 sinks, and becomes 0-00342 for a lower limit equal to — 94°*57; 

 hence platinum approaches the other pure metals as the temperature 

 sinks^ 



In conclusion, our experiments prove that the electrical resistance 

 of most metals decreases regularly when the temperature sinks 

 from 0° to —123°, and that the coefficient of variation is appreci- 

 ably the same for all. It seems probable that this resistance would 

 become extremely small, and therefore the conductivity very great, 

 at temperatures below —200°, although our experiments do not 

 enable us to form any precise idea of what would take place in 

 those conditions. — Comptes JRendus, May 11, 1885. 



OPTICAL METHOD FOR THE ABSOLUTE MEASUREMENT OF SMALL 

 LENGTHS. BY M. MACE DE LEFINAY. 



M. Mouton* gave an ingenious method for measuring, in wave- 

 lengths, the thickness of a quartz plate cut parallel to the axis, 

 which depends on a determination, by means of a known grating, 

 of the wave-lengths of Eizeau and Foucault's dark bands. Un- 

 fortunately it depends on our really very imperfect knowledge of 

 the values ri— n of the two indices of quartz for different radia- 

 tions, and cannot therefore give numbers which are exact to within 

 Y^jtj-. On the other hand, the corrections for temperature are 

 considerable : nrJroir ^ or a degree Centigrade. 



I. The method which I have used, which is analogous in prin- 

 ciple to the preceding, depends on an observation of Talbot's bandsf , 

 obtained by intercepting half the pencil of sunlight which falls on 

 a known grating by a quartz plate with true parallel faces. The 

 spectra deflected on the side of the plate are then channelled with 

 lines, in general extremely fine, but which for that very reason 

 tend to a point as exact as that of the spectrum. By using the 

 third spectrum of a grating of g-J^- millim., I have been able to 

 apply the method directly to a quartz plate parallel to the axis, about 

 4 millim. in thickness. It is known that the wave-lengths corre- 

 sponding to the centre of each of these dark bands are related to 

 the thickness of the plate e, and the index n, by the formula 



p being an even whole number, which measures the number of the 

 order of the fringe observed, and which increases by two units in 

 passing from one band to the other, counting from red to violet. 

 We thus obtain as many values very near the desired thickness as 

 are observed, and the mean is taken. 



In the preceding formula the index is directly given to within 



* Journal de Physique, vol. viii. p. 393 (1879), 

 t Journal de Physique, vol. i. p. 177 (1872). 



