Water and Mercury at nearly Perpendicular Incidence. 315 



When not in use the reflecting-surfaces are protected by 

 a sort of cap of tin-plate, which fits loosely over them. The 

 improvement thus obtained was very remarkable, the mirrors 

 not suffering so much in a month as they formerly did in a 

 day before the protection was provided. 



The following are the measures of distances required for 

 the calculation. 



From the division-line C to the axis of rotation of the lamp 

 A (fig. 1), 



AC = 82-21 inches; 



AB = ll-28, BC = 93\L5, 

 so that 



AB + BC= 104-43. 



The factor expressing the ratio of the squares of the distances 

 is thus 1-6137. 



The angle of incidence is best obtained from a measurement 

 of the horizontal distance between C and A. This proved to 

 be 11^ inches ; so that 



""^ToA^' 11 ' 



and 



This applies to all the experiments referred to in the present 

 paper. 



The estimation of the angular opening in the disk used for 

 the water experiments depended upon measurements of corre- 

 sponding chord and diameter. The chord, measured by means 

 of the screw of a travelling-microscope, was -7574 inch. The 

 radius, expressed in terms of the same unit, was found to bo 

 7*79. Hence, if a be the angular opening, 



or£ a =2°47' = 167'. 



The ratio in which the direct light is reduced is thus 



167 167 



180 x 60 ~ 10800 



= •01546. 



It will now be necessary to give some details with respect to 

 the actual matches as determined photographically. At first 

 the intention was to employ ordinary plates (Uford), which 

 worked very satisfactorily. But when the attempt was made 

 to compare the result with theory, the comparison was found 



