204 



Lord Rayleigh. 



[Jan. 16, 



short length of stout rubber tubing G, to which was applied a 

 squeezer H. The object of this attachment was to cause a rise of 

 mercury in both limbs immediately before a reading, and thus to 

 avoid the capillary errors that would otherwise have entered. A 

 similar pipette and manometer were connected with the air tube B. 

 In order to be able, if desired, to follow with the eye a particular 

 band during the changes of pressure (effected by small steps and 

 alternately in the two tubes), diminutive windlasses were provided by 

 which the motions of the reservoirs (E) could be made smooth and 

 slow. In this way all doubt was obviated as to the identity of a 

 band ; but after a little experience the precaution was found to be 

 unnecessary. 



The manner of experimenting will now be evident. By adjustment 

 of pressures the centre of the middle band was brought to a definite 

 position, determined by the web or otherwise, and the pressures were 

 measured. Both pressures were then altered and adjusted until the 

 band was brought back precisely to its original position. The ratio 

 of the changes of pressure is the inverse ratio of the refractivities 

 (a* — 1) of the gases. The process may be repeated backwards and 

 forwards any number of times, so as to eliminate in great degree 

 errors of the settings and of the pressure readings. 



During these observations a curious effect was noticed, made 

 possible by the independent action of the parts of the object-glass 

 situated at various levels, as already referred to. When the ban Is 

 were stationary, th^y appeared straight, or nearly so, but when in 

 motion, owing to changes of pressure, they became curved, even in 

 passing the fiducial position, and always in such a manner that 

 the ends led. The explanation is readily seen to depend upon the 

 temporary changes of temperature which accompany compression or 

 rarefaction. The full effect of a compression, for example, would 

 not be attained until the gas had cooled back to its normal tempera- 

 ture, and this recovery of temperature would occur more quickly at 

 the top and bottom, where the gas is in proximity to the metal, than 

 in the central part of the tube. 



The success of the measures evidently requires that there should 

 be no apparent movement of the bands apart from real retardations 

 in the tubes. As the apparatus was at first arranged, this condition 

 was insufficiently satisfied. Although all the parts were carried upon 

 the walls of the room, frequent and somewhat sudden displacements 

 of the bands relatively to the web were seen to occur, probably in 

 consequence of the use of wood in some of the supports. The obser- 

 vations could easily be arranged in such a manner that no systematic 

 error could thence enter, but the agreement of individual measures 

 was impaired. Subsequently a remedy was found in the use of a 

 second system of bands, formed by light which passed just above the 



