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VIII. On the Action of the Rays of the Spectrum on Vegetable Juices. Extract of a 
Letter from Mrs. M. Somerville to Sir J. F. W. Herschel, Bart., dated Rome, 
September 20, 1845. Communicated by Sir J. IIerschel. 
Received November 6, — Read November 27, 1845. 
In the following experiments the solar spectrum was condensed by a lens of flint 
glass of inches focus, maintained in the same part of the screen by keeping a pin- 
hole, or the mark of a pencil constantly at the corner of the red rays, which were 
sharply defined by using blue spectacles to protect my eyes from the glare of light, 
and the apparatus was covered with black cloth in order to exclude extraneous 
light. 
Thick white letter paper, moistened with the liquid to be examined, was exposed 
wet to the spectrum, as the action of the coloured light was more immediate and 
more intense than when the surface was dry. As I had not access to the morning 
sun, the observations were made between noon and three in the afternoon. 
The lavender rays came vividly into view under a condensed spectrum on white 
paper washed with a solution of sulphate of quinine in dilute sulphuric acid: they 
were narrow, and their length, by rough measurement, was equal to the distance 
between the upper edge of the violet and the lower edge of the blue. They were very 
brilliant on black silk or other dark surfaces, and invariably of lavender colour; and 
even on paper stained with turmeric, the pale yellow rays which you had observed, 
were tipped with lavender on being washed with this liquid, though its duration was 
momentary, as it vanished as the surface became dry; but they were permanent in 
other instances. 
The lavender rays change their colour with a change of the liquid, for instance 
they are lavender coloured on nitrate of silver discoloured by light to a very pale 
brown, washed with a solution of sulphate of quinine in dilute sulphuric acid ; 
whereas, on a similar surface of pale brown nitrate of silver, washed with the juice 
of the petals of pale blue Plumbago auriculata in distilled water, to which sulphuric 
acid was added, they appeared of a vivid apple-green, and acquired a tip of lavender 
colour on the surface being washed with a solution of sulphate of quinine in dilute 
sulphuric acid of considerable strength ; the effect, however, was transient. After 
several unsuccessful attempts to repeat this experiment next day, I at length dis- 
covered that its success depended upon the acid being strong enough to decompose 
the juice and give it a reddish orange hue ; and even then the rays are not vivid till 
the paper. has been frequently washed with the juice and become nearly dry; and 
MDCCCXLVI. Q 
