SCIENTIFIC THOUGHT. 



Newton's 

 authority 

 on the 

 side of the 

 emission 

 theory, 



as that theory failed for a long time to explain the 

 apparently fundamental fact, viz., that light travels in 

 straight lines, accompanied by well - marked shadows. 

 The contrary view, according to which light is a tremor 

 propagated like sound, was unable to explain the ex- 

 istence of clearly marked shadows. And so it came 

 about that Xewton, to whom both theories were quite 

 familiar, and to whom we owe great discoveries telling 

 severally in favour of each of these theories, in the 

 end threw the weight of his authority into the scale 

 of the corpuscular or emission theory. For many this 

 was quite sufficient to suppress for a long time all 

 claims which the tremor or wave theory put forward, 

 the fact being forgotten or overlooked that Xewton 

 himself had pronounced the pure emission theory to 

 be insufficient, and had modified and complicated it by 



tion to crystallography, it became a 

 desideratum to reach the geometri- 

 cal conception of the wave-surface 

 by purely geometrical methods. 

 This has been done in an admir- 

 able treatise entitled 'The Optical 

 Indicatrix,' by Mr L. Fletcher. He 

 has shown that the construction of 

 the ray, a conception easily denned 

 geometrically, gives an easier ap- 

 proach than the construction of 

 the wave, which introduces physi- 

 cally doubtful definitions ; and 

 he demonstrates how " a simple 

 generalisation, involving no refer- 

 ence either to the constitution 

 of the luminiferous ether or to 

 the nature of the physical change 

 involved in the transmission of 

 light," will lead to the ray surface 

 (p. 18). For his purpose he starts 

 from a surface of reference, which 

 in singly refractive substances is 

 a sphere, in uniaxial crystals a 



spheroid, and by inference in biaxial 

 crystals an ellipsoid with three un- 

 equal axes. This beautiful con- 

 struction was arrived at, as the 

 author tells us, before the detailed 

 history of Fresnel's theory had 

 come to his notice. It is now 

 known through Verdet, one of the 

 editors of Fresnel's ' Works ' (1868), 

 that Fresuel arrived at his wave- 

 surface by a purely geometrical 

 generalisation of Huygens' con- 

 struction, and that the conception 

 of the ether was subsequently 

 fixed so as to allow the wave surface 

 to be deduced therefrom (p. 24) ; 

 surely an interesting case in the 

 history of scientific thought. As 

 to the insufficiency of purely geo- 

 metrical optics for explaining the 

 phenomena connected with optical 

 instruments, see Czapski, 'Theorie 

 der optischen Instrumente,' Bres- 

 lau, 1893, p. 2. 



