98 Lord Rayleigh. On the Character of the 



desirable to give further details of the apparatus employed, referring 

 for explanation of the principles involved to the former communication 

 already cited. 



The optical parts, other than the tubes containing the gases, are 

 mounted independently of everything else upon a bar of T-iron 90 cm. 

 in length over all. The telescopes are cheap instruments, of about 

 3 cm. aperture and 30 cm. focus, from which the eye-pieces are 

 removed. At one end of the T-iron and in the focus of the collimating 

 telescope the original slit is fixed. This requires to be rather narrow, 

 and was made by scraping a fine line upon a piece of silvered glass. 

 At the further end the object-glass of the observing telescope carries 

 two slits which give passage to the interfering pencils, and are 

 situated opposite to the axes of the tubes holding the gases. The sole 

 eye-piece is a short length of glass rod the same as formerly described 

 of about 4 mm. diameter, which serves as horizontal magnifier. The 

 gas tubes are of brass, about 20 cm. long and 6 mm. in bore. These 

 are soldered together side by side and are closed at the ends by plates 

 of worked glass, so cemented as to obstruct as little as possible the 

 passage of light immediately over the tubes. There are two systems 

 of bands, one formed by light which has traversed the gases within the 

 tubes, the other by light which passes independently above ; and an 

 observation consists in so adjusting the pressures within the tubes that 

 the two systems fit one another. Unless some further provision be 

 made, there is necessarily a dark interval between the two systems of 

 bands corresponding to the thickness of the walls of the tubes and any 

 projecting cement. It is, perhaps, an improvement to bring the two 

 sets of bands into closer juxta-position. The interval can be abolished 

 with the aid of a bi-plate (fig. 1), formed of worked glass 4 or 5 mm. 

 thick.* This is placed immediately in front of the object-glass of the 

 observing telescope, the plane of junction of the two glasses being hori- 

 zontal and at the level of the obstacles which are to be blotted out of 

 the field of view. 



FIGL 1. 



The objects sought in the design of the remainder of the apparatus 

 were (i) the use of a minimum of gas, and (ii) independence of other 

 pumping appliances. To this end the glass tubes associated with each 

 optical tube were arranged so as to serve both as manometer tubes and 



* Compare Mascart, "Traite d'Optiquc,' vol. 1, p. 495, 1889. 



