Theoretical Optics since 1840. 537 



longitudinal magnetic induction of 17,500 C.G.S. lines per 

 sq. cm., the value 10*2 per cent, was found; and this value 

 is in tolerable agreement with those quoted in an earlier part 

 of the paper as obtained by the direct method. 



Summary of Results. 



The change in the thermal conductivity of bismuth pro- 

 duced by magnetic field is very small ; for a transverse field 

 of 3550 it is considerably less than J per cent., and it is 

 scarcely measurable even in higher fields by any of the 

 methods used. 



As tested by the bridge method, mild steel suffered a re- 

 duction of its conductivity of 3' 3 per cent, for an increase of 

 transverse field of 7500, and a reduction of 2'1 per cent, for 

 an increase of 6700. The reduction 3'1 per cent, was ob- 

 served for the same material by the method of direct measure- 

 ment on application of a transverse field of 6500. 



The effect of a longitudinal field of 51 c.G.s. on the con- 

 ductivity of mild steel was to diminish it by about 4 per cent. 



In the case of soft iron, as tested by direct measurement, 

 a longitudinal field producing a magnetic induction of 16,000 

 per sq. cm. diminished the conductivity by about 10'5 per 

 cent.; while the effect of transverse field is comparatively 

 small — about 1 per cent, for a field of 7850 ; by the bridge 

 method the effect of a longitudinal induction of 17,500 was 

 found to be a diminution of conductivity of 10*2 per cent. 



I beg to express sincerest thanks to Prof. J. J. Thomson 

 for continual encouragement and assistance. 



Cavendish Laboratory, Cambridge. 



I/VT. theoretical Optics since 1840. — A Survey. 

 By R. T. Glazebrook, D.Sc., F.R.S.* 



SIR GEORGE STOKES took his degree in 1841 ; the 

 first of the papers contained in his Collected Works was 

 read in 1842 : he became Lucasian Professor in 1849. 



Speaking as I do so soon after his death, it is, perhaps, not 

 unnatural to look back over the progress of our Science 

 during the sixty years for which he has been one of the most 

 prominent of its exponents. 



To attempt such a task in any completeness would need a 

 fuller knowledge and an abler pen than mine ; will yon. 

 however, bear with me if I take one corner of the field 

 covered by his activities and attempt a brief survey of this. 



* Communicated by the Physical Society, being a portion o£ the 

 Presidential Address delivered on February 13, 1903." 



