506 Mr, P. V. Bevan on Lloyd's 



investigation shows that there is a difference of phnse of half 

 an undulation in the cases of internal and external reflexion, 

 but this investigation only applies to the case of incidence 

 internally at an angle less than the critical angle. 



The experiment of this paper shows that at grazing 

 incidence in internal reflexion there is a loss of phase of half 

 a period in the case of both kinds of light just as in the case 

 of external reflexion. 



The experiment is of the same kind as Lloyd's, and is only 

 of interest in that it affords further confirmation of the 

 applicability of the Fresnel formulae. 



Lloyd * concluded that the successive distances of the 

 dark fringes from the centre of the system, obtained by the 

 single mirror, were in accordance with the assumption that 

 a loss of half an undulation took place at the reflexion, in 

 other words that the geometrical centre of the fringe system 

 was a dark fringe. This result is quite clear if the fringes 

 be observed with an eyepiece close up to the mirror, so that 

 the fringes seen have their centre actually on the mirror. 

 The dark fringes appear clearly at distances from the edge 

 of the mirror in the ratios of their numbers 1, 2, 3, 4, &c, 

 and not in the ratios 1, 3, 5, &c, as would be the case 

 were a half undulation not lost at the reflexion. Fringes of 

 the same type as Lloyd's can easily be obtained by means of 

 a reflecting surface of water and internal reflexion. A trough 

 of about 20 cm. length was made with glass ends, so that 

 light from a slit could be observed through v\ ater in the trough 

 directly and after reflexion at the surface. In this way two 

 pencils of light can be obtained which are in the condition 

 for interference. The water meniscus gives no trouble as the 

 glass is wetted, and so the surface is turned upwards at the 

 ends of the trough and a large perfectly plane mirror is 

 formed. 



The fringes can be seen on a screen, or with an eyepiece 

 or microscope in the ordinary way. A fine slit was used 

 illuminated by an arc-lamp. If the microscope be near 

 enough to the water-trough a line of the surface will appear 

 in focus, and this will be the centre of the fringe system. 

 With a travelling microscope measurements of the distances 

 of the dark fringes from the centre of the system can easily 

 be made. It was found that these distances were in the ratios 

 1, 2, 3, &c, showing that the centre of the system is a dark 

 fringe. The position of the surface can be determined very 



* kloyd * Papers,' p. 149 ? 



