248 
SIR DAVID BREWSTER ON THE CONNEXION BETWEEN CERTAIN 
phenomenon and those of thin plates. The bands diminish in number as the thickness 
of the plate increases, and their colour suffers no other change by inclining the plate 
but that which arises from the small increase of thickness which the ray traverses. 
There is one remarkable point of difference between the two classes of phenomena 
which requires to be specially attended to. The colours of some of the luminous hands 
are not the same as those of the spectrum, and therefore the glass has removed certain 
colours while it has left others of exactly the same refrangibility. The green , for ex- 
ample, is changed into yellow by the removal of blue rays, and in certain glasses a 
band, almost ivhite , is produced. The colours thus removed are said to be absorbed; 
and by an extensive series of experiments with such absorbing substances I have been 
able to insulate white light in the spectrum, which no prism can decompose, and to 
establish the existence of three equal and superposed spectra of red yellow and blue 
light. 
Analogous phenomena are exhibited in an alcoholic solution of the colouring matter 
of the green leaves of vegetables. The spectrum which it forms consists of six lu- 
minous bands separated by Jive dark ones*, and the phenomena have the same cha- 
racter as those of the blue glass. 
When the spectrum is viewed through nitrous acid gas the phenomena 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 spectrum. 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 phenomena 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 portions, as well as the tints from some of the coloured spaces, 
have wholly disappeared, in place of being found in the reflected 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 absorp- 
tion 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 
* A full account of this experiment, and a coloured drawing of the divided spectrum, will be found in the 
Edinburgh Transactions, vol. xii. 
