XV. On Spurious Rainbows. By W. H. Miller, M.A. F.R.S. Professor 

 of Mineralogy in the University of Cambridge. 



[Read March 22, 1841.] 



The sixth volume of the Transactions of the Cambridge Philo- 

 sophical Society contains a Memoir by the Astronomer Royal, on the 

 Intensity of Light in the neighbourhood of a Caustic, in which the 

 relative distances of the brightest parts of the first spurious bow, and of 

 the first and second dark rings, from the geometrical place of the bow, 

 are determined by calculations founded on the undulatory theory. The 

 numbers to which he finds these distances proportional are, 



Brightest part of bow 1.08 



Dark ring between the bow and the first spurious bow 2.48 



Brightest part of the first spurious bow 3.47 



Dark ring between the first and second spurious bows 4.4 (probably). 



It appears also, that the illumination extends beyond the place of 

 the geometrical bow, the intensity of the light there being about 0.442 

 of the intensity at the point of maximum brightness. 



In order to compare these results with observation, I employed the 

 method of exhibiting rainbows and the accompanying spurious bows, 

 invented by M. Babinet (Poggendorff's Annalen, B. xli. S. 139). When 

 a beam of light admitted horizontally through a narrow vertical slit falls 

 upon a vertical cylindrical stream of water, portions of the primary and 



Vol. VII. Paet III. Gg 



