20 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
they produce oil my own eye, as well as on that of a friend to whom I have shown 
them, and who proposed that term, or that of simple grey, to express it. It was in 
the course of my experiments on the polarized rings in 1819, made with M. Biot’s 
apparatus, that I was first led to something approaching to conviction of the existence 
of such rays. While following out the measurements of the angular diameters of the 
polarized rings in those experiments, it was found easy to arrive at a maximum dia- 
meter of each ring, corresponding to an extreme red illumination, but by no means 
so for the other end of the spectrum. Long after any sensible violet illumination 
continued to fall on the reflector of the apparatus, that spark of light, increasing, di- 
minishing, vanishing, reappearing and vanishing again, as the crystal was turned 
round, on which the observation in question depends, would still remain obstinately 
visible, and the angular diameter of the polarized ring deduced from it would still go 
on decreasing till it became evident that there was no assigning any definite limit to 
the violet end of the spectrum, otherwise than by the use of a definite absorptive me- 
dium, — I mean a limit where the visible phenomena depending on continually de- 
creasing breadths of undulation should be held to cease. It was equally evident in 
those experiments, that the colour of this outstanding ultra-violet ray was not pro- 
perly that of any received prismatic tint ; but this, at the time, I attributed to the 
great and progressively increasing faintness of the rays in question not making suffi- 
cient impression on the eye to allow of any decision as to their colour. This view of 
the matter, however, is incorrect. The following experiment will show, that these 
rays when so concentrated as to possess an unequivocal illuminating power, still show 
no colour, but that sort of imperfect white which is best distinguished by the terms 
grey, ash-colour, lavender-colour, or such expressions. As orange, indigo, and violet, 
vegetable tints, are used for those of the prismatic hues, I may be allowed to express 
by the epithet lavender the rays which produce the tint in question, rather for the 
purpose of abbreviating the uncouth appellation of ultra-violet , and avoiding the am- 
biguity attaching to the term chemical rays (which exist in all regions of the spec- 
trum) than for that of laying any undue stress on the observed fact. 
5 7- A spectrum from a highly refractive and dispersive flint-glass prism, the com- 
panion to the crown prism before mentioned, and, like it, the personal gift of Fraun- 
hofer, of faultless purity and perfect workmanship, was thrown on the same crown 
lens as in the last experiment. This was purposely placed at once oblique to the in- 
cident light, and out of the plane of dispersion of the prism, or that plane to which 
its refracting edge is perpendicular. Moreover, it was so arranged that the spectrum 
might fall on any part of the lens, above, below, or on either side of its centre. Under 
these circumstances, it is easy to give excessive concentration to light of any given 
degree of refrangibility, the rest being thrown aside into long, coloured caustics. In 
fact, the image of the spectrum is thus bent and distorted into curvilinear forms, the 
general type of which may be assimilated to the letter C lengthened and flattened, 
with ends more or less unequal. And upon these curves, so opened or contracted, 
