34?2 Mr. Ivory o?i the Theory of the Astronomical Refractions. 



from which the above, anci other results are deduced, will be 

 laid before the Royal Society. 



The salts of the resins have also occupied much of my at- 

 tention, but I have not yet been able to satisfy myself as to 

 their constitution. So far is clear, that in combining with 

 bases the resins do not give off the elements of water. There 

 is a difference I suspect in the number of equivalents of hy- 

 drogen present in the resin when combined and uncombined, 

 but the exact nature of that difference requires further eluci- 

 dation. 



Though you do not favour symbolic expressions in che- 

 mistry very much, I hope you will allow that there is consi- 

 derable beauty in the hasty generalizations I have here given 

 you. Believe me, my dear Phillips, 



Yours very truly, 



James F. W. Johnston. 



LIV. The BaJcerian Lecture. — On the Theory of the Astro- 

 nomical Refractions. By James Ivory, K.H., M.A., 

 F.R.S. L. 8f E., Instit. Reg. Sc. Paris, Con-esp. et Reg. Sc. 

 Gottin. Corresp. 



[Continued from p. 287-] 



3. T1 appears that Newton himself was the first to apply 

 ■■■ this new method to the problem of the astronomical 

 refractions. A table, which he had computed, and which he 

 gave to Dr. Halley, is published in the Philosophical Trans- 

 actions for 1721. Nothing is said as to the manner in which 

 the table was constructed ; and it has always been a curious 

 and interesting question among astronomers, whether it is the 

 result of theory, or is deduced from observations alone with- 

 out the aid of theory. Some original letters of Newton to 

 Flamsteed, published in 1835 at the expense of the Board of 

 Admiralty, have put an end to all doubts on this point. These 

 letters prove that Newton studied profoundly the problem of 

 the refractions ; that for several months in succession he was 

 occupied almost entirely in repeated attempts to overcome the 

 difficulties that occurred in this research ; during which time 

 he calculated several tables with great labour, namely, the 

 one he gave to Halley, and another, or rather three others, 

 which have been found in his letters to Flamsteed lately 

 printed. 



Admitting that the refractive power of the air is propor- 

 tional to its density, which Newton had established in his 

 Optics on speculative principles, and which Hawksbee after- 

 wards was the first to demonstrate experimentally, the mathe- 



