relative to physical Optics. 1 5 
red and the blue. In the blue light of a candle, expanded by 
the prism, the portions of each colour appear to be narrower, 
and the intervening dark spaces wider, than in the analogous 
spectrum derived from the light reflected from a thin plate. 
The light of burning alcohol appears to be green and violet 
only. The pink dye sold in the shops, which is a preparation 
of the carthamus, affords a good specimen of a yellow green 
light regularly reflected, and a crimson probably produced by 
transmission. 
VI. EXPERIMENT ON THE DARK RAYS OF RITTER. 
Exper. 6. The existence of solar rays accompanying light, 
more refrangible than the violet rays, and cognisable by their 
chemical effects, was first ascertained by Mr. Ritter : but Dr. 
Wollaston made the same experiments a very short time after- 
wards, without having been informed of what had been done 
on the Continent. These rays appear to extend beyond the violet 
rays of the prismatic spectrum, through a space nearly equal to 
that which is occupied by the violet. In order to complete the 
comparison of their properties with those of visible light, I was 
desirous of examining the effect of their reflection from a thin 
plate of air, capable of producing the well known rings of colours. 
For this purpose, I formed an image of the rings, by means of 
the solar microscope, with the apparatus which I have described 
in the Journals of the Royal Institution, and I threw this image 
on paper dipped in a solution of nitrate of silver, placed at the 
distance of about nine inches from the microscope. In the course 
of an hour, portions of three dark rings were very distinctly 
visible, much smaller than the brightest rings of the coloured 
image, and coinciding very nearly, in their dimensions, with the 
