90 
FIRST REPORT— 1831 . 
2. The solar spectrum and that of artificial flames, whether 
formed by prisms of transparent solids and fluids, or by 
grooves in metallic and transparent bodies, or by the diffrac¬ 
tion of light passing through a narrow aperture, consist of 
three spectra of equal length, beginning and terminating at 
the same points (viz. a Red, a Yellow, and a Blue Spectrum), 
and having their maximum of illumination at different points of 
their length, and their minimum at their two extremities. 
3. All the seven colours in the solar spectrum, as they were 
observed by Newton and Fraunhofer, are compound colours, 
each of them consisting of Red, Yellow, and Blue Light in 
different proportions. 
4. A certain portion of White Light incapable of being 
decomposed by the prism, in consequence of all its component 
rays having the same refrangibility, exists at every point of the 
spectrum, and may at some points be exhibited in an insulated 
state. 
(Since this paper was read, an abridgement of it has been 
published in the Edinburgh Journal of Science, No. X. New 
Series, pp. 197—207; and the original Memoir, illustrated with 
coloured drawings, will appear in the next Part of the Transac¬ 
tions of the Royal Society of Edinburgh, Yol. XII. Part I.) 
Mr. Wm. Gray, jun. read the Translation of a memoir by 
Professor Gazari of Florence, on a method of detecting the traces 
of writing which has been fraudulently erased. 
The Author of this paper having been frequently appointed 
by the Tribunals to give professional evidence in trials of this 
nature, instituted experiments on the subject, which, by showing 
him the possibility of removing entirely not only the ink, but 
also the materials employed in its removal, proved that cases 
might arise, when the fraud could not be detected in any other 
manner than by examining the condition of the paper or other 
material written on. For this purpose optical means were tried 
in vain, and immersion in water did not show such a dif¬ 
ference in the absorptive power of the written and unwritten 
parts as happens in the employment of certain sympathetic inks; 
but on exposure of the suspected paper near to a moderate fire, 
the paper, which in consequence of the corrosive effects of the 
ink, was in those parts altered in its nature, was unequally 
acted on by the process of carbonization, and thus the number 
and length of the lines, and often the whole of the erased por¬ 
tion was distinctly revealed. 
