RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
21 
the colours may be made to occupy any spaces, by receiving- the spectrum higher or 
lower on the lens, and by inclining its axis more or less to the plane of incidence. 
Without calling coloured figures to aid our description, we may conceive the matter 
perhaps most readily by considering the spectrum as painted on a ring, or on some 
oval curvilinear form, having the eye more or less elevated above its plane, and ca- 
pable of being made to revolve about its centre, the spectrum, including the che- 
mical rays, occupying about two thirds of its circumference. If we conceive this 
ring, oval, or curve, projected on any plane, it is clear that the colours as depicted 
on that plane will have more or less concentration, according as the element of the 
arc corresponding to each is more or less inclined to the plane, and that where that 
inclination is perpendicular, the intensity at the point of projection will be (mathe- 
matically speaking) infinite in comparison with that of any other portion of the spec- 
trum. 
58. If we conceive the eye to be situated in the plane of our fictitious ring, its pro- 
jection will be rectilinear, and we shall have a spectrum, as it were, doubled back on 
itself at any particular part of its length, and the density of the rays where this redu- 
plication takes place will be vastly greater than at any other point. By placing 
the lens, therefore, so that this condition shall hold good, (which happens when its axis 
lies in the plane of refraction of the prism, and the spectrum falls on its principal 
section,) the photographic impression will exhibit in its highest intensity the effect of 
that particular chemical ray which is thus violently concentrated. Its illuminating 
power, if any, will thus also be brought into corresponding evidence. 
59. By this arrangement, the colours described in Art. 54. were brought out in 
great distinctness, and some very striking appearances produced. Now when the ray 
whose chemical and optical qualities were thus especially forced into prominence 
was among those whose place in the spectrum, though far beyond the ordinarily vi- 
sible violet, was yet within the limit of the chemical action, the portion of it thus con- 
centrated became veryapparent to the eye as a greyish-white insulated oval spot, which, 
when first noticed, was for the moment attributed to some accidental achromatic re- 
flection of light backwards and forwards within the prism, an idea which the smallest 
movement of the prism or the lens sufficed to dissipate. But if such an idea could 
have been entertained after a moment’s thought, the reception of the spectrum on a 
slip of prepared paper completely refuted it, by the intense chemical action exercised 
at that particular spot. The blackening, indeed, was there instantaneous, as if a red- 
hot body had been applied behind, or a smoky flame directed on the paper over all 
the space so visibly illuminated, and faithfully marking its form and degradation. 
A few moments of exposure sufficed to connect this spot (at first insulated) with the 
general impression of the spectrum, which by rapid degrees came into view. We 
come now to other and more remarkable results. 
