and the Phenomena of the Rainbow. 



325 



f^O\ 



rainbow, lead us to conclude, that if the cylinder of water had 

 been at rest, the extreme visible light might not have extended 

 even up to the caustic. 



In resuming the experiments lately, I have had as a reservoir 

 for the water, a common glass bottle with 

 the bottom broken off, as in the figure : a 

 glass tube drawn with the blowpipe to a 

 fine tapering aperture passed through the 

 cork in its neck. When the water in the 

 reservoir was at some height, as a b, and 

 the stream c d was formed, it was received 

 in a catch-tube, as in the figure, and led 

 away. The part of c d which appeared 

 cylindrical and without ripples, either from 

 the lower end falling on the catch-tube, 

 or from its breaking into drops, was ad- 

 justed so as to be over the centre of the 

 horizontal protractor, and level with the axis of the telescope, 

 carried on the arm of the vernier, before mentioned. The stream 

 of water, where used, was very nearly j^th of an inch diameter, 

 and its distance from the narrow slit of yi^dth of an inch breadth 

 between metallic edges, through which the sun's light was re- 

 flected by a mirror of a solar microscope fixed in the shutter of 

 a dark room, was 22 feet. 



Preliminary measures on the 5th of April last gave the distance 

 from the perpendicular ray, to which the extreme red extended 

 for the primary iris, with the height of the surface of the water 

 in the reservoir 4| inches above the aperture, equal to 43° 26' ; 

 and with a height of water 7^ inches, equal to 43° 30'. 



When the side of the tube had been oiled and the water had 

 ceased running out, a drop was obtained hanging from the end 

 of the tube, when readings for the extreme red were taken, which 

 gave in one experiment the distance from the normal ray 42° 1', 

 and in another 41° 51^'. Calculating with the refractive index 

 of the fixed ray B, the radius of the primary bow, if at the caustic, 

 is 42° 22'. So that with the vertical stream the light is visible 

 outside the caustic, and with the stationary drop it does not 

 extend to it. 



With the adjustments more perfect, I obtained on the 7th of 



April as follows : — 



o ; 

 With 4^ inches water pressure the angle was 43 3| 

 AVith average 1\ inches ... ... 43 17 



8 t 7 tt 43 24£ 



844 43 20| 



ns 



I conclude that with this fine aperture the difference of the 



