402 



Prof. J. H. Poynting on 



P = total pull on surface. 



M= momentum per second in incident beam. 



R= reflexion coefficient for angle i. 



fi = 1/5. 



i. 



R. 



P/M. 







•0400 



•4000 



20 



•0402 



•4240 



40 



•0458 



•4925 



50 



•0572 



•5310 



60 



•0893 



•5720 



65 



•1205 



•5771 Maximum. 



70 



•1710 



•5683 



75 



•2531 



•5329 



80 



•3878 



•4521 



89 



•9044 



•0738 



$0-cW 





2/^0 



90 



robob 



•0000 



If a ray of light passes obliquely through a parallel plate, 

 there is a normal pull outwards ait incidence and a normal 

 pull outwards at emergence ; and if the refraction were 

 total, this would result in a couple. But since some of the 

 light returns into the first medium, it is easy to see that 

 the net result is a normal repulsion and a couple. 



An experiment which I have lately made in conjunction 

 with Dr. Barlow will serve as an illustration of the idea of a 

 beam of light regarded as a stream of momentum. A rect- 

 angular block of glass, 3 cm. x 1 cm. x 1 cm., was suspended 



Fig. 6.— Plan. 



by a quartz fibre so that the long axis of the block was hori- 

 zontal. It hung in a case with glass windows, which was 

 exhausted to about 15 mm. of mercury. A horizontal beam 

 of light from either a Nernst lamp or an arc was directed 

 on to one end of the block so that it entered centrally as AB 



