478 Mr. J. L. W. Gill on the Distribution of 



and the limit of visibility for that wave-length may be 

 calculated. In most cases, however, diffraction complicates 

 the phenomenon, and if great accuracy be required, a more 

 complete investigation would be necessary. 



4. Lloyd's Bands. — The case of Lloyd's Bands is slightly 

 different (Lord Rayleigh, loc. cit. p. 80). If f be the distance 

 of the luminous point from the plane of the mirror, the 

 relative retardation of the reflected and direct streams at 

 a point of the screen distant x from its line of intersection 

 with the plane of the mirror is (measured in length) 2x£/d. 

 where d is the distance of the luminous point from the screen, 

 supposed at right- angles to the plane of the mirror. 



The central line of the slit being at a distance c from the 

 plane of the mirror, the intensity due to the whole slit of 

 width k may be written 



r . 2tt xk 

 = hk < 1 H - — . cos 



X. ' 2tt 2cx ( 



*-• 



27r xk 

 'X'l 



The visibility is here given by +sin (7rnk/c)/(7rnk/c), where 

 n is the order of the bands, and the condition for maximum 

 distinctness is that the width of the slit must be a small 

 fraction of c/n, and thus depends upon the order of the bands : 

 in other respects the phenomenon is the same as in the 

 former cases. 



This dependence of the visibility on the order of the bands 

 and their periodic disappearance may be observed with 

 homogeneous light by leaving the width of the slit unaltered 

 and moving backwards the eyepiece with which the bands 

 are observed, keeping it all the time in the doubly illuminated 

 field. 



XLIX. On the Distribution of Magnetic Induction in Straight 

 Iron Rods. By J. L. W. GrlLL, B.A.Sc, 1851 Exhibition 

 Scholar*. 



TOURING- the summer and autumn of 1897 the author was 

 J-^ engaged in working out the details of a new method 

 of measuring magnetic hysteresis in iron. The fundamental 

 principle of this method is the measurement of the mechanical 

 work expended when a specimen of iron is passed through 

 a magnetic field (see ' Electrician,' Sept. 24, 1897). 

 * Communicated by the Author, 



