226 Prof. W. Ramsay and Mr. M. W. Travers. 



And as a check on these measurements, the hydrogen was compared 

 with oxygen and subsequently with nitrogen free from argon. It 

 was noticed, after some of these experiments had been made, that the 

 ref ractivity of air could not be accurately calculated from the given 

 data for oxygen, nitrogen, and argon ; and it appeared therefore 

 worth while to examine more minutely the refractivity of these gases 

 for white light, and to see whether any error could be detected in 

 previous measurements. Moreover, as physicists perhaps do not 

 always devote sufficient care to the chemical purity of their mate- 

 rials, an additional reason was furnished for the inquiry. 



Apparatus. — It will be seen, on consulting Lord Rayleigh's paper, 

 that the refractivity is measured in the following manner : — Light 

 from a paraffin lamp passes through a fine slit, cut with a razor in 

 tin-foil pasted on glass. The beam is made parallel by passage 

 through an achromatic plano-convex lens of about 1 foot focal 

 length. It then divides ; the upper portion passes through air, and, 

 after extraneous light is cut off: by passage through two wide slits, it 

 is brought to a focus by a lens similar to the first, and the bands 

 produced are viewed by a cylindrical lens of very short focus. The 

 lower portion of the beam traverses two tubes, 9 inches long and one- 

 quarter of an inch in diameter, placed close together, and closed at 

 each end with plates of optically worked glass. Each of these tubes 

 contains one of the gases to be examined ; and each is connected with 

 a manometer and a movable reservoir ; so that, on raising or lower- 

 ing the reservoir, the pressure of the gases can be so adjusted that 

 the interference- bands formed in the lower half of the field can be 

 accurately brought into line with the stationary bands in the upper 

 half. Readings of pressure are taken on both manometers at pres- 

 sures not differing greatly from that of the atmosphere ; then, on 

 lowering the reservoirs, readings on both manometers are taken at 

 lower pressures, the bands being again made to coincide in position 

 with the upper fiducial bands. The ratio of the refractivities is 

 inversely as the differences of pressure in the two gases. The in- 

 fluence of temperature does not appear, for the tubes of the mano- 

 meter lie side by side, and may be regarded as equally affected by 

 variations of temperature. 



The accuracy of this method varies with the value of the refrac- 

 tivity of the gas. For, if the gas has a low refractivity, then a great 

 difference of pressure produces the passage of fewer bands across the 

 field than if it has a high one ; and, as the accuracy of reading may 

 safely be taken as the twenty-fifth of a band, and as between thirty 

 and forty bands passed the field with such gases as oxygen, nitrogen, 

 and argon, the error may be taken in such cases as from 1 in 750 to 

 1 in 1000. 



The tubes containing the gases to be examined were connected 



