and the Phenomena of the Rainbow. 323 



that this solution did not represent the natural phenomena. 

 Professor Miller, however, undertook an experimental examina- 

 tion of the analogous phenomena, in M. Babinet's experiment, 

 in which they are observed in a small cylindrical vertical stream 

 of water, and he has published a paper in vol. vii. of the Cam- 

 bridge Transactions, detailing the measures of some of the results. 

 He found the light to extend outside the calculated place of the 

 caustic, in accordance with Mr. Airy's investigation, and that 

 the dark bars or minima near the caustic were nearly in the rela- 

 tive places as given by that investigation ; but he has not given 

 the places of the bright bars or maxima, which I find do not 

 agree with their calculated places. 



At that time I commenced a repetition of these experiments, 

 and found like results with Professor Miller as to light extend- 

 ing outside the caustic. I found, however, as will be seen below, 

 that the light extended further and further outside it as the 

 velocity of the stream was greater. The discrepancy between 

 these results and those from the natural phsenomena evidently 

 arose from the refractive index being slightly different, and a 

 little less for water falling in such a stream and about to break 

 into drops, from that for still water. 



I did not at that time think of any method of experimenting 

 with water at rest, which I now find is so easy to be employed, 

 but 1 prepared a prism and small cylinder of plate-glass ; the 

 prism served to determine the refractive indices of the rays to be 

 used, and the cylinder gave the irises in position in accordance 

 with those from the natural phamomena of the rainbow, and not 

 in accordance with those from the running stream. The expe- 

 riments extended to within a few days of my leaving Cambridge 

 to commence the duties of my Professorship at University Col- 

 lege. They have been only resumed lately in consequence of 

 my arriving at this part of physical optics in a general investi- 

 gation of the whole science, which I hope before much longer 

 time has elapsed to lay before the scientific public in a continuous 

 treatise. 



The following were obtained in Queens' College, Cambridge. 

 A heliostat reflected the sun's light through a narrow vertical 

 aperture, which falling at a distance of about 5 feet on a fine 

 Munich prism with its edge vertical, furnished a very pure pris- 

 matic beam of light. Of this beam the rays from orange to 

 azure were reflected at right angles by a plane metallic mirror 

 one room and into another which was dark, and there fell 

 at about 41 feet from the prism on the small vertical stream of 

 water. This stream of water was directly over the centre of the 

 circle of an excellently divided protractor, which, being laid 

 horizontal, had on the arms which carried the verniers a support 



Y2 



