RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 17 
a priori conclusion prepare us for differences in the kind of action at different points 
of the spectrum, as manifested by differences of colour produced, or for what at first 
sight appears yet more singular, the shifting upon the spectrum itself of the points 
where these differences of action commence and terminate. Nor, lastly, could any 
penetration enable us to anticipate a fact so totally at variance with all our ideas, as 
that two rays of different refrangibilities, (and therefore of different lengths of undu- 
lation) acting at once, should produce an effect which neither of them acting sepa- 
rately is competent to produce at all; a fact which seems difficult to deal with on 
any theory of light. These are but a few of the many singular results which promise 
to render this field of research, hardly yet entered upon, as replete with novelty, va- 
riety, and interest, as any other department in the vast field of physical optics. 
51. My first experiments on this subject were directed to the detection of inactive 
spaces (if any) in the chemical spectrum, analogous to the dark lines of Wollaston 
and Fraunhofer in the luminous one. Possessing at the time no means of fixing a 
sunbeam of large area, I operated coarsely with a prism placed horizontally. The 
light was introduced through a narrow slit, and the time chosen for experiment when 
the sun was on the meridian, so that no motion of the spectrum in the direction of 
its length might confound the result. This arrangement proved, as indeed I ex- 
pected, quite inadequate to the end proposed ; but it sufficed to reveal several lead- 
ing facts, viz. 1st, That with the paper employed (which happened to be a specimen 
prepared by the process in art. 31. on Mr. Talbot’s principle) the maximum of ac- 
tion was not beyond the violet, as I had always understood it to be, but rather about 
the confines of the blue and green, nearly where Fraunhofer’s ray F is situated. 
2ndly, That the extent of the chemical spectrum beyond the visible violet light far 
surpassed any idea I had previously formed of it. For, in fact, the visible termination 
of the violet rays nearly bisected the photographic image impressed on the paper. 
Srdly, That in the visible violet rays there occurred a sort of minimum of action, 
about one third of the way from Fraunhofer’s ray H towards G. 4thly, That the 
whole of the red, up to about Fraunhofer’s line C, appeared to be inactive. And 
5thly, That where the orange-red rays, or the less refrangible half of the portion from 
C to D in Fraunhofer’s spectrum, fell, there was communicated to the paper a sort 
of ruddy tint, a dull brick red. 
52. From this experiment, coarsely performed as it was, several practical conclu- 
sions came to be deduced. 1st, The absolute necessity of an achromaticity in the 
object-lens of a photographic camera, of a very perfect kind, and in some sort sui 
generis *, is rendered evident by the great extension of the chemical spectrum. And 
it is equally clear that this very extension affords the most perfect and certain means 
* A lens photographically perfect, or which unites all the chemical rays into one focus, may be called am a- 
cralic (apci together, k paros power ) or amasthenic (oderos force). If this nomenclature be adopted, a diacratic or 
diasthenic medium will be one which transmits the chemical power or force ; diacratescence that quality in virtue 
of which it does so, &c. &c. In place of the awkward word diathemaneity , however, I would venture to propose 
its Latin version, transcalescence. 
MDCCCXL. 
D 
