202 



Lord Bayleigh. 



[Jan. 16, 



field is traversed by bauds of width inversely as the distance between 

 the slits. If from any cause one of the portions of light be retarded 

 relatively to the other, the bands are displaced in the usual manner, 

 and can be brought back to the original position only by abolishing 

 the relative retardation. 



When the object is merely to see the interference bands in full per- 

 fection, the use of a telescope is not required. The function of the 

 telescope is really to magnify the slit system,* and this is necessary 

 when, as here, it is desired to operate separately upon the two por- 

 tions of light. The apparatus is, however, extremely simple, the 

 principal objection to it being the high magnifying power required, 

 leading under ordinary arrangements to a great a.ttenuation of light. 

 I have found that this objection may be almost entirely overcome by 

 the substitution of cylindrical lenses, magnifying in the horizontal 

 direction only, for the spherical lenses of ordinary eye-pieces. For 

 many purposes a single lens suffices, but it must be of high power. 

 In the measurements about to be described most of the magnifying 

 was done by a lens of home manufacture. It consisted simply of a 

 round rod, about -| in. (4 mm.) in diameter, cut by Mr. Gordon from 

 a piece of plate glass. f This could be used alone ; but as at first it 

 was thought necessary to have, a web, serving as a fixed mark to 

 which the bands could be referred, the rod was treated as the object- 

 glass of a compound cylindrical microscope, the eye-piece being a 

 commercial cylindrical lens of 1^ in. (31 mm.) focus. Both lenses 

 were mounted on adjustable stands, so that the cylindrical axes could 

 be made accurately vertical, or, rather, accurately parallel to the 

 length of the original slit. The light from an ordinary paraffin lamp 

 now sufficed, although the magnification was such as to allow the 

 error of setting to be less than 1/20 of a band interval. It is 

 to be remembered that with this arrangement the various parts of 

 the length of a band correspond, not to the various parts of the 

 original slit, but rather to the various parts of the object-glass. This 

 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 distinc- 

 tion between the dark and bright parts. 



The collimating lens A (fig. 1) is situated 23 ft. (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 ft. (30"o cm.) 

 long, and of ^ in. (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 3 in. (7'6 cm.) in diameter. It is fitted 



* ' Brit. Assoc. Ker>ort,' 1893, p. 703. 



f Preliminary experiments had been made with ordinary glass cane and with 

 tubes charged with water. 



