38 



Sir J. Conroy. 



[Feb. 15, 



that with the silver and the tin mirrors the difference between theory 

 and observation may be due to imperfect polish, but that such 

 can hardly be the case with the steel. 



Under these circumstances it appears desirable that a fresh series of 

 observations should be made by some independent method, which 

 would either confirm or disprove the results contained in this paper, 

 and this I hope to attempt. 



Received December 18, 1882. 



All previous determinations of the reflective power of metals 

 having been made in air, it appeared desirable to make some observa- 

 tions with the sfceel and speculum metal mirrors in water, the polish 

 of these mirrors being satisfactory. 



A glass trough about 7*6 centims. square and 3*7 centims. deep, was 

 filled with distilled water and placed on the stage of the goniometer in 

 such a position that the light from the lamp passed normally through 

 two of its opposite sides, and then fell on the photometer. The 

 distance of the sliding lamp was then altered till both papers appeared 

 equally illuminated ; four such readings were made, and then the 

 mirror, which had been previously adjusted, placed in the trough and 

 clamped at an angle of 45° with the incident light. 



The trough was then so adjusted that the incident and emergent 

 light passed normally through two of its adjacent sides, the gonio- 

 meter turned until the light fell on the photometer, and the readings 

 made in the usual way. The mirror was then removed and the 

 intensity of the light which passed through the trough again deter- 

 mined. 



The partial reflections from the glass sides of the trough, and the 

 length of the path of the light in the water (the reflecting surface 

 coinciding with the diagonal of the trough), being the same in both 

 cases, the difference in the intensity of the light could only be due to 

 the loss caused by reflection. 



The light not being parallel, but forming a slightly divergent beam, 

 the effect of introducing the trough of water between the source and 

 the photometer was equivalent, in an optical sense, to slightly re- 

 ducing the distance between them ; it was therefore necessary that 

 the light when falling directly on the photometer, and when doing so 

 after reflection, should have traversed in both cases an equal thickness 

 of water. This condition prevented observations being made at angles 

 other than 45°. 



The results of four observations with the speculum metal, and three 

 with the steel mirrors, show that, as might have been anticipated, the 

 percentage of light reflected was less than when the mirrors were in 

 air ; the numbers are — 



