:36 



Sir J. Conroy. 



[Feb. 15, 



Principal Incidence. Principal Azimuth. 



73-38 38-39 



73-37 38-40 



The theoretical intensities calculated by Cauchy's formulae from the 

 value of the principal incidence and principal azimuths are — 



Incidence. Observed. Calculated. 



20° .... 62-90 .... 78-09 



40 .... 65-93 .... 77-95 



60 .... 69-29 • .... 77-46 



80 .... 79-74 .... 81-34 



These numbers do not agree any better than those previously 

 obtained, and it therefore seems necessary to assume that Cauchy's 

 formulas (at least in their simplified form) do not express the facts of 

 the case, except, perhaps, for speculum metal. 



The values for the intensity of the light calculated by the formulae 

 given by Professor James MacCullagh (" Collected Works," p. 133) 

 decrease slowly up to a large angle of incidence, and then increase 

 again just as is the case with the similar formulae of Cauchy. 



Professor Stokes suggested that very probably the discrepancy 

 between the observed and calculated results was due to imperfect 

 polish, as the differences were greater with the soft metals, silver 

 and tin, to which it is more difficult to give a good polish than 

 for the hard metals, steel and speculum metal, and also as the 

 observed intensities fell short of the calculated ones at moderate 

 incidences, whilst sometimes even exceeding them at high incidences, 

 for which deficiencies of illumination due to defects of polish might 

 possibly be expected to disappear. 



Professor Stokes also suggested a method for examining the polish 

 of the mirrors. 



In accordance with his suggestion a cylindrical tinned iron canister, 

 closed at one end, about 9 centims. in diameter and 27 centims. deep, 

 was blackened internally and supported on a table at an angle of 

 about 30° with the horizon, and with the lower edge of the open end 

 about 4 centims. above the surface of the table, which was covered 

 with a black cloth. 



The table was placed out of doors, so that there might be plenty of 

 light coming from all round, and the metal plates laid on the black 

 cloth in front of the canister, so that an observer standing in front 

 could see, by reflection, into the perfect darkness. If the polish were 

 perfect, the surface of the metal plate would appear perfectly black ; 

 and if such should not be the case, the illumination of the plate would 

 afford an estimate of the defect of polish. 



The reflection in the plate was examined through a small hole in a 



