1896.] 



More about Argon. 



departure from the operation of a spherical eye-piece is an 

 advantage, inasmuch as optical defects show themselves by deformation 

 of the bands instead of by a more injurious encroachment upon the 

 distinction between the dark and bright parts. 



The collimating lens A (Fig. 1) is situated 23 feet (7 metres) 

 from the source of light. B, C are the tubes, one containing dry air, 

 the other the gas to be experimented upon. They are 1 foot 

 (30-5 cm.) long, and of J inch (1*3 cm.) bore, and they are closed at 

 the ends with small plates of parallel glass cut from the same strip. 

 E is the object-glass of the telescope, about 8 inches (7*6 cm.) in 

 diameter. It is fitted with a cap D, perforated by two parallel slits. 

 Each slit is ^ inch (6 mm.) wide, and the distance between the 

 middle lines of the slits is 1 J inches (38 mm.). 



The arrangements for charging the tubes and varying the pres- 

 sures of the gases are sketched in Fig. 2. A gas pipette, D E, com- 

 municates with the tube C, so that by motion of the reservoir E and 

 consequent flow of mercury through the connecting hose, part of the 

 gas may be transferred. The pressure was measured by a U-shaped 



n ji 



tfc 



Fig. 1. 



manometer F, containing mercury. This was fitted below with 

 a 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 in the inverse ratio of the refractivities 



