SUPERPOSITION OF PARAGENIC SPECTRA, ETC. 225 



of grooves is very difficult. Sir John Barton, with all his experience, failed 

 in producing good specimens ; but even with those which he executed for me, 

 phenomena of a remarkable kind were exhibited, not only on the .middle or 

 colourless image, but upon all the paragenic spectra, varying with the number of 

 grooves, but still more remarkably with the angle of incidence* 



P.S. — The preceding experiments were made in 1823 and 1827, and those 

 described in p. 223, were repeated in 1838. Having lost or mislaid the glass 

 gratings which I then employed, I am not able to compare the bands which they 

 produced with a more remarkable series which I have recently obtained with new 

 gratings, and which will be the subject of another communication 



* See Phil. Trans. 1829, p. 301. 



VOL. XXIV. PART i. 3 P 



