Cooper. — An Experiment on the Wave Theory of Light. 489 



graduated circle vertical and its collimator-tube horizontal. 

 A small glass tank with approximately parallel sides was 

 now placed at the centre of the circle, and the deviation 

 it produced was noted. The tank was now half-filled with 

 gasoline, and carbon-bisulphide was run in underneath by 

 means of a funnel. In the region where they mix we have 

 then a liquid slab fulfilling the conditions stated in the first 



Slit 



ce 



Fig. 2. Diagrammatic Plan op Spectrometer. 

 a a Region of mixture. 



sentence of this paper. These liquids mix freely in all pro- 

 portions without turbidity. The beam now passed through 

 the mixture-layer, and the deviation was noted. At the same 

 time a notable dispersion was observed. It then occurred to 

 the author that these elifects might be produced by prisms 

 formed by capillary attraction in contact with the glass sides 

 of the tank. If this were so, the introduction of a glass slip 

 (a microscope- slide) would make four such prisms instead of 

 two, thus doubling the deflection. A microscope-slide was 

 carefully introduced in a vertical position, and the observed 

 deflection was sensibly diminished. The introduction of 

 another slab caused a further reduction of the deviation 

 which was in sensible proportion to the thickness of the 

 liquid traversed. 



On now mixing the liquids thoroughly there was only such 

 deviation as was accountable by the inclination of the sides of 

 the tank, and that in a contrary direction to the previous 

 deviations. There was no prismatic dispersion observable. 



Details of One of the Experiments. 

 Deviation produced by glass sides of empty tank 

 „ by tank witli gasoline 



„ by region of mixture it 



7"5mm.)... 

 with microscope- slip 2 mm. 

 thickness {t = 5-5 mm.) 



m 



= - 10' 



= - 1' 



= + 62' 

 = + 44' 



