Water and Mercury at nearly Perpendicular Incidence, 311 



A few calculations from the original expressions will serve 

 to indicate the field of these approximations. 



l M=y 0=10°, ^ = 7° 29', 

 S 2 =^x 1-0467, T»=-gX-9541, 



S 2 + T 2 = 2x ~ x 1-0004. 



49 



From (5) we get as the" last factor 1-00050. 



^=| = 20°, ^=14°51'-8, 



S»=~x 1-2021, T 2 =lx-8158, 



S 2 +T 2 =2x-^x 1-0090. 

 49 



By (5) the last factor is 1-0080. 

 Again, 



^=1, = 30°, 1 = 22°l'-4, 

 o 



S 2 =lx 1-5189, T*=^x-5866, 



S 2 + T2 = 2x 1 xl . Q527 



49 



According to (5) the last factor is here 1*0405. 



It appears that in the case of water the aggregate reflexion 

 scarcely begins to vary sensibly from its value for 0=0 until 

 0=20°, a property of some importance for our present 

 purpose, as it absolves us from the necessity of striving after 

 very small angles of incidence. 



I will now describe the actual arrangement adopted for the 

 experiments. The source of light at A, (fig. 1, PI. X.), is a 

 small incandescent lamp, the current through which is con- 

 trolled with the aid of a galvanometer. It is so mounted 

 that its equatorial plane coincides with the (vertical) plane of 

 the diagram. Underneath, upon the floor, is placed the 

 liquid (B) whose reflecting power is to be examined. At C, just 

 under the roof, the direct ray AC and the reflected ray BG 

 are turned into the same horizontal direction by two mirrors 

 silvered in front and meeting one another at C under a small 



Z2 



