212 Sir D. Brewster on the Connexion between 



tion of the colouring matter of the green leaves of vegetables. 

 The spectrum which it forms consists of six luminous bands, 

 separated by five dark ones*, and the phaenomena have the 

 same character as those of the blue glass. 



When the spectrum is viewed through nitrous acid gas the 

 phaenomena are still more remarkable. While the gas exerts 

 a general absorbent action over the violet extremity of the 

 spectrum, it attacks it when in a diluted state in definite lines 

 as sharp and distinct as those in the solar spectrum ; and what 

 is still more important, it acts upon the same parts of light as 

 the cause which produces the fixed lines in the sun's spec- 

 trum. In other respects the character of its action is similar 

 to that of the blue glass and the green sap of plants. 



In thus comparing the phasnomena of absorption with those 

 of thin plates, we find no connecting link but that of giving 

 a divided or a mutilated spectrum ; and even this common 

 fact has not the same character in both. In coloured media 

 the bands of light and darkness have no fixed relation, as in 

 periodical colours; and the light removed from the dark por- 

 tions, as well as the tints from some of the coloured spaces, 

 have wholly disappeared, in place of being found in the re- 

 flected beam. 



I have already mentioned, that by the aid of two substances 

 I have been able to study this subject under a new aspect, 

 and that the nacreous substance described by Mr. Horner was 

 the one which first exhibited to me the connexion between 

 absorption and periodical action. 



This substance when it contains no thin plates acts generally 

 in absorbing the violet and blue end of the spectrum; but 

 when it includes within it, or has on its surface thin films 

 which act like thin plates, it exercises an additional 'action 

 upon the spectrum. In some cases when the thickness of the 

 plate is small, it produces bands perfectly identical with those 

 of thin plates, but in other cases the bands are exactly similar 

 to those of coloured media. In one specimen I obtained a 

 dark and distinct band in the orange space at D, with another 

 feint band in the red. These bands were parallel to the fixed 

 line D at a vertical incidence, but by inclining the plate the 

 bands moved towards the green space, and became inclined 

 to the line D. In a recent specimen I obtained the darkest 

 band in the green space, with other lesser bands of unequal 

 size and breadth in the other spaces, all of which moved 

 along the spectrum, while new ones advanced from the red ex- 



* A full account of this experiment, and a coloured drawing of the di- 

 vided spectrum, will be found in the Edinburgh Transactions, vol. xii. 



