7 
‘ 
and on the Elementary Colors of the Solar Spectrum. 9 
there may remain no doubt on this point, I will transcribe from — 
my memoir, communicated to the Royal Academy on November 
24th, 1843, the part that relates to this subject. 
“Let one of the three surfaces of an ordinary glass prism be 
covered with a layer of India ink; let it dry, and then divide it 
into three equal portions at right angles to its axis. Remove with 
a penknife the ink from the middle portion, and also a band four 
or five millimetres wide on the sides of the two lateral compart- 
ments, so that these two bands from which the ink has been re- 
: moved, may be on opposite sides, and form by their junction with 
a the central force a kind of Z. It will be understood, that a solar 
Ee beam issuing from a prism thus arranged will produce three col- 
ored images side by side; the middle one is very luminous,—it 
arises from the part of the prism from which all the ink is remov- 
ed; the two others, which are much paler, arise from the lateral 
bands. It will also be perceived, that the middle image or spec- 
; trum has each of its extremities on the boundary of one of the 
. extremities of the lateral spectra; and that when, for example, 
its red extremity is in the same line with the red extremity of the 
left spectrum, its violet extremity will be upon the same line as 
the violet extremity of the right spectrum, and vice versi. As 
a to the other two extremities of the lateral spectra, they will not 
found corresponding to the extremities of the central spectrum, 
but to some one of the interior colors, and they will evidently be 
more distant as the width of the uncovered bands is less in pro- 
portion to the width of the prism. In one of my experiments 
with an equilateral prism of crown glass, the width oi 
which was twenty-four millimetres, and that of the lateral bands 
five, I found, at a distance of two metres, that the red extremity 
of the left spectrum was upon the same line as the upper part of 
the yellow belonging to the central image, and the violet extrem- 
ity of the right spectrum was upon the blue of the same image. 
A prism of water, prepared in like manner, of which the refract- 
mg angle was 79°, gave analogous results. In one, as in the oth- 
er case, the red extremity of the left spectrum was ona level with 
the green of the central image, when the observations were made 
at the distance of a metre from the prism. aad 
_ “Let us imagine the central unpainted part of our prism to be 
divided into a series of longitudinal elements, the width of each 
being equal to that of the lateral bands. It is clear, that each of 
ese elements will produce a refracted image similar to the two 
pale spectra arising from the lateral bands; and that the two last 
mages of the series will be as it were continuations of those spec- 
tra. Therefore, the red and the violet which we see by the side 
of the yellow and blue of the central colored spectrum, equally 
 €Xist in that central spectrum, and enter into the composition of 
those tints. This argument is unanswerable; it establishes that 
_ Sconp Srrims, Vol. V, No. 13.—Jan., 13848. 2 
