4 Lord Bayleigh, [Jan, 17, 



purpose absolute measurements were not required. It sufficed to 

 compare the pressures necessary in two columns of air and argon of 

 equal lengths, in order to balance the retardations undergone by 

 light in traversing them. 



The arrangement was a modification of one investigated by 

 Fraunhofer, depending upon the interference of light transmitted 

 through two parallel vertical slits placed in front of the object- 

 glass of a telescope. If there be only one slit, and if the original 

 source, either a distant point or a vertical line of light, be in focus, 

 the field is of a certain width, due to "diffraction," and inversely 

 as the width of the slit. If there be two equal parallel slits whose 

 distance apart is a consitlerable multiple of the width of either, the 

 field is traversed by bands 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 

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

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

 sary when, as here, it is desired to operate separately uj^on tlie two 

 portions 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 attenuation 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 suihces, but it must be of high power. 

 In tiie 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 ^ inch (4 mm.) in diameter, cut by Mr. Gordon from 

 a piece of plate glass.j 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-j^iece being a 

 commercial cylindrical L ns of IJ inch (31 mm.) focus. Both lenses 

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

 be made accurately vertical, or, rather, accurately 2>arallel to the 

 length of the original slit. The li<iht 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 



* Brit. Assoc. Keport, 1893, p. 703, 



t Preliminary experiments liad been made with ordinary glass cane and with 

 tubes charged with water. 



