TRANSACTIONS OF THE SECTIONS. 11 



of discoloration, which would be complete if prolonged sufficiently. 

 For I have found that if, instead of using a prism, a strong sunshine is 

 transmitted through a combination of glasses carefully prepared so as 

 to transmit absolutely no ray but that definite red at the extreme of 

 refrangibility, a paper previously darkened by exposure under a green 

 glass has its colour heightened from a sombre neutral tint to a bright 

 red ; and a specimen of paper rendered almost completely black by ex- 

 posure to daylight, when exposed for some time under the same glass, 

 assumed a rich purple hue ; the rationale of which effect I am disposed 

 to believe consists in a very slow and gradual destruction, or stripping 

 off as it were, of layers of colour deposited or generated by the other 

 rays, the action being quicker on the tints produced by the more refran- 

 gible rays in proportion to their refrangibilities. 



It seems to me evident that a vast field is thus opened to further in- 

 quiries. A deoxidizing power has been attributed to the red rays of 

 the spectrum, on the strength of the curious experiments of WoUaston 

 on the discoloration of tincture of guaiacum, which ought to be re- 

 peated ; but in the sensitive papers, and still more in Daguerre's mar- 

 vellous ioduretted silver, we have reagents so delicate and manageable, 

 that everything may be expected from their application. 



I remain, my dear Sir, 



Your very obedient servant, 



J. F. W. Herschel. 



P.S. — I inclose a picture of the spectrum formed as above described. 

 It must be viewed by lamp- or candle-light — not being fixed. In this 

 way it may be examined by any number of persons, which by daylight 

 would be impracticable, as a few minutes' exposure would obliterate all 

 its peculiar characters. The larger pencil dot indicates the centre of 

 the sun's image formed by the extreme red rays, at which point the 

 maximum of whiteness will be observed to occur. 



Prof. Whewell communicated some tide observations, forwarded to 

 him by the Russian Admiral Liitke. These observations supplied — 

 first, the tide hours of various places on the coasts of Lapland, the 

 White Sea, the Frozen Sea, and the coasts of Nova Zemlia. These 

 observations enable us to follow the progress of the tide-wave further 

 than had hitherto been done. Mr. Whewell's map of Cotidal Lines 

 (the second approximation contained in the Phil. Trans. 1836,) follows 

 the tide only as far as the North Cape of Norway. Prof. Whewell 

 stated, that he was informed by Admiral Liitke, that in the Frozen Sea 

 east of Nova Zemlia there is little or no perceptible tide. The obser- 

 vations communicated by Admiral Liitke, offered various other results, 

 and especially the existence of the diurnal inequality in the seas ex- 

 plored by Russian navigators, as on the coast of Kamscatcha, and the 

 west coast of North America. 



Prof. Whewell made some observations on Capt. Fitzroy's views of 

 the tides. In the account of the voyage of H.M.SS. Adventure and 



