NATURAL PHILOSOPHY. 159 



The angular dimensions of the iris were obviously the same with those 

 of a rainbow of the first order ; its colors were complete, from red to 

 violet, and very bright and distinct, especially where the mud was 

 softest and moistest ; where a sheet of water, how thin soever, covered 

 the mud, the iris vanished. No trace of an iris could be seen on the 

 grass, in the sky, or anywhere but on the mud ; and on those parts 

 of the turnpike road where the mud had been much disturbed no iris 

 was visible. 



The necessary conclusion from this appearance is, that the surface 

 of the mud must have been thickly covered with globules of pure 

 water, perfectly spherical, and not in absolute contact with the mud, 

 although resting on it ; but those globules must have been extremely 

 minute, for they were invisible to the closest inspection with the 

 naked eye. 



THE SUPERNUMERARY BOWS IN THE RAINBOW. 



In a communication on the above subject to the British Associa- 

 tion, in 1862, by the Rev. J. Dingle, the author stated that he had 

 investigated a method of approximating to the size, of the drops of 

 rain corresponding to any given position of the supernumerary bows 

 produced by the interference of the two luminiferous surfaces pro- 

 ceeding from each drop. It appeared from his tables appended to 

 the paper that the size which Dr. Young (without giving his method 

 of calculation) had assigned to the drops under certain conditions 

 was within -g^ppth of an inch of the truth, and was more accurate 

 than that assigned subsequently by Mr. Potter, whose method was 

 not quite satisfactory. The subject was interesting as illustrating the 

 marvellous accuracy with which the operations of nature are carried 

 out, and the delicate adaptation of our organs for discerning them. 



THE RELATIVE AMOUNT OF SUNSHINE FALLING ON THE TORRID 



ZONE OF THE EARTH. 



At the British Association, 1862, Prof. Hennesey stated that he 

 had ascertained that the amount of sunshine falling on the outer 

 limits of the earth's atmosphere between the tropics is very nearly 

 equal to that which falls on the remaining portions of the earth's 

 surface. If we reflect that, according to Forbes's researches, the 

 amount of heat extinguished by the atmosphere before a given solar 

 ray reaches the earth, is more than one-half for inclinations less than 

 25, and that for inclinations of 5 only the twentieth part of the 

 heat reaches the ground, we immediately see that the torrid zone of 

 the earth must be far more effective than all the rest of the earth's 

 surface as a recipient of solar heat. It follows, therefore, that the 

 distribution of the absorbing and radiating surfaces within the torrid 

 zone must, upon the whole, exercise a predominating influence in 

 modifying general terrestrial climate. 



VELOCITY OF LIGHT. 



M. Foucault, to whom we owe the physical demonstration of the 

 earth's movement, has announced a discovery respecting the velocity 



