Mr. J. J. Walker on the Iris seen in Water. 439 



then 



E _ Ti-.Tg 



It follows from this, that that which is called by Prof. William 

 Thomson Carnofs Function, has the following value : — 



^= ^. 



T—K 



From the specific heat of steam of saturation, as determined 

 by M. Regnault, viz. 0*475, it is to be anticipated, as various 

 authors have inferred from other circumstances, that the relations 

 between its pressure, density, and temperature for high pres- 

 sures, will be found to deviate widely from those of a perfect gas. 

 But it would be premature to discuss this subject further until 

 the details of M. Regnault^s experiments are published. 



Glasgow, April 25, 1853. 



LXIX. On the Iris seen in Water, By J. J. Walker*. 



IN those parts of optical and meteorological treatises usually 

 devoted to the phsenomena of rainbows, and to the rational 

 explanation and mathematical demonstrations of their theory, a 

 singular omission occurs ; an omission common to all such works 

 from Newton's time down to the present day, as far as the 

 writer's examination of them, for the purpose of satisfying him- 

 self on this head, has led him. The omission alluded to is the 

 absence of all mention of the iris seen on the horizontal surface 

 of an extent of sufficiently calm water, lying between the spec- 

 tator and a falling shower of rain, and opposite the sun. Under 

 favourable circumstances, in such a case, an inverted arch is seen 

 on the water, which is popularly called a " reflexion of the rain- 

 bow,'' and to cursory observation bears a considerable resem- 

 blance to what such a " reflexion " — supposing it for a moment 

 possible — would be. A very slight acquaintance, however, with 

 the Cartesian or Newtonian theory of the rainbow suggests at 

 once the correct explanation of the phsenomenon ; that it is a 

 secondary iris, formed by pencils of parallel rays, which, emerging 

 from rain-drops, after undergoing one reflexion within the drops, 

 are reflected a second time at the horizontal surface of the water, 

 and so reach the eye. It is proposd briefly to investigate the 

 true figure of this '^primary horizontal iris/^ as it may for the 

 sake of distinction be called, and its position on the surface of 

 the water with reference to the spectator's eye. 



Let p be the angle which the emergent parallel pencil of red 



* Communicated bj^ the Author. 



