1 12 Mr. Coddington's Reply to Dr. Goring. 



groups of very obtuse rhomboids, masses resembling pearl- 

 spar, acute scalene dodecahedrons, and six-sided prisms with 

 flat summits, or terminated by planes of the equiaxe crystal of 

 Haliy ; and in one instance that lies before me, the calcedonic 

 cast represents one of those crystals of carbonate of hme 

 which are frequently observed, in which a change from a flat 

 to a modified termination of the hexagonal prism has only 

 partially taken place. 



XII. On the Theories of Achromatisation, Sfc. in reply to Dr. 

 Goring. By The Rev. H. Coddington. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 



MY name having been brought before the public by Dr. 

 Goring, in an article contained in the last number of the 

 Edinburgh Journal of Science, in a manner which, though 

 apparently complimentary, must lead readers to consider me 

 as a framer of apparently fine theories, which answer no good 

 purpose in practice, a character for which those who know me 

 are well aware that I entertain very little respect, I would re- 

 quest leave to make a iew observations in reply, through the 

 medium of your Journal, which I presume must find its way 

 wherever any scientific periodical work is taken in. 



Dr. Goring states that my work on the Reflexion and Re- 

 fraction of Light, whether considered as a work of originality, 

 or as a compilation from the wi'itings of our first opticians, 

 is admitted to be the best publication of the kind at present 

 extant. So far I am obliged to him : but in the next article 

 he " ventures, though well aware of what he is doing, to im- 

 pugn the infallibility of exact science in the case of the theories 

 of achromatisation of Professor Robison, Professor Airy, and 

 Mr. Coddington, on the ground that no artist is able to make 

 an achromatic instrument according to them." 



Now, Sir, the plain truth of the matter is, that my work 

 contains, to the best of my knowledge, exactly those theories 

 which have guided all artists, from the elder DoUond down- 

 wards, who have been competent to refer to a theory at all. 

 To these I have added one other, (to which Dr. Goring alludes 

 in a note,) out of respect to its inventor Professor Airy. It is 

 one of singular ingenuity and beauty, and gave every promise 

 of a good result, but is probably imperfect, like many other 

 speculations of the present day, since it certainly does not 

 answer the end proposed. The author is well aware of this, 



and 



