Mr. Potter on the Primary and Secondary Rainbotios. 9 



II. On the Radii and Distance of the Primary and Secondary 

 Rainhoisos, as found by Observation, and on a Comparison of 

 their Values with those given by Tlteory. By R. Potter, 

 Esq., M.A* 



THHE explanation of the rainbow which Sir Isaac Newton 



has given in the second part of *the first book of 



Opticks,' appeared one of the happiest appHcations of his 



freat optical discovery, the unequal refrangibility of light. It 

 as, however, been long observed that there are frequently 

 supernumerary bows attending the principal ones, and of 

 these the theory of Sir Isaac Newton gives no solution. It 

 was reserved for Dr. Young to show that they were results of 

 the principle of interferences. (See his Lectures on Natural 

 Philosophy, or Phil. Trans, for 1803.) But notwithstanding 

 the satisfactory manner in which all the phaenomena of the 

 rainbow appeared to be accounted for, when the interference 

 of the light was taken into consideration, yet the problem 

 continued to be discussed in our optical treatises according 

 to the old method, and Dr. Young's theory, like most of his 

 other fine discoveries, did not receive the notice it deserved. 



In the year 1835, I re-discovered that the solution of these 

 phaenomena involved the principle of interferences, without 

 knowing what Dr. Young had written, and presented a paper 

 to the Cambridge Philosophical Society, which is printed in 

 the sixth volume of the Transactions, in which I undertook 

 to show that the problem belonged to physical optics. Near 

 the close of that paper will be found an intimation that the 

 ordinary rainbows might not eventually be found in the posi- 

 tions which had hitherto been assigned to them, by Dr. 

 Young as well as by all others. 



My views with respect to interference on the corpuscular 

 theory of light (see Lond. and Edinb. Phil. Mag. vol. ii. p. 81. 

 and ' Correspondance Mathematique et Physique de I'Obser- 

 vatoire de Bruxelles,' 2^ livraison, tome 8.) had led me to ex- 

 pect this, and I had, at the time of writing that paper, exa- 

 mined Sir Isaac Newton's discussion sufficiently to convince 

 myself, that there were abundant grounds for anticipating a 

 full confirmation when I had time to pursue the investigation 

 more completely. 



In the beginning of the year 1836, I compared the mea- 



Geol. Soc. whose Prospectus on the subject will be found in Phil. Mag., 

 First Series, vol. Ixvi. p. 137, and in his " Records of Mining." Mr. H. 

 English has also repeatedly brought the same subject before the public in 

 his Mining Review. — Edit. 

 * Communicated by the Author. 



