ACCORDING TO THE UNDULATORY THEORY. 4.93 
imitate with éwo pieces of mica the phaenomena arising from two retarda- 
tions, &e. To perform this it is only necessary that the light which 
is reflected from one plate of mica on the prism must first have been re- 
flected from the one plate of mica on the other plate. According to what 
has already been proved, a spectrum is then obtained which contains 
all the absorptions which have been caused by each single retardation. 
The following is, as I have found, the easiest method of performing 
this experiment, which indeed, properly speaking, explains nothing, but 
which deserves to be mentioned as a beautiful experiment : I generally 
take a plate of mica, whose surfaces, besides being even and without 
faults, incline one towards the other, so that the plate is thicker at one 
border than at the other. Among the plates of mica which I have ex- 
amined in this respect, I have found one which possesses these properties 
in a high degree. AsI bent this plate into the form of the surface of 
a vertical cylinder, and placed it so that the light was reflected on the 
prism from its thick end, I obtained a perfectly regular prism, with 
about 120 black stripes; but the surface of the cylinder being turned 
round its axis, so that the reflecting element gradually advanced 
toward the other end, the distance between the stripes gradually in- 
ereased, while at the same time their number diminished, till at last 
from the thin end I received but a few more than 20. In order to 
bring the surface of the cylinder to any desired position, I fastened it 
on to a small cylindrical pillar, A B (fig. 7), which was fixed by wax, 
or any other glutinous substance, to an even support. 
In order to produce spectra which contain two series of absorptions, 
I placed two such cylinder surfaces in the manner shown in A and B 
(fig. 8). The light from the lamp C, concentrated by means of a 
great convex lens D, is carried to the first cylinder surface A ; 
from this it is thrown on to the second, B, and from this further on to 
the prism E. The light of the flame is hindered from falling on the 
surface B, by means of a sliding screen F; and by another similar 
screen G that light is received, which otherwise might easily be re- 
flected from the surface A to the prism. By turning both cylinder 
surfaces round their axes, I can give to the two retardations any de- 
sired relation to one another; and in this manner, as just described, I 
can change ad infinitum the phenomena of absorption. 
Very small retardations, such for instance as are smaller than a wave- 
length, cannot, according to this method, be accomplished, because it 
is almost impossible to give to the mica the necessary degree of thin- 
ness. But in order to produce also the phenomena in which small re- 
tardations are presupposed, I use coloured fluids which are inclosed in 
a cylindrical tube, between two plates of glass whose distance from one 
another can be altered at pleasure. I have completely imitated, with a 
red absorbing fluid and a plate of mica, not only the spectrum of iodic 
