38 Frauenliofor on the R^r active and Dlsperfivc Power 
If we make the sun’s rays pass through a small round aper- 
ture in the window-gutter, nearly 15" in diameter, and cause it 
to fall on a prism placed before the telescope of the theodolite^ 
it IS obvious that the spectrum seen by the telescope can only 
have a very small width, and consequently will form only a line. 
In a line, however, of almost no breadth, it is impossible to see 
the fine and delicate lines which traverse it ; and, on that ac- 
count, the fixed lines are not seen in a spectrum of this kind. 
In order, liowever, to see all the lines in this spectrum, it is 
necessary only to widen it an object-glass, without altering its 
length. I obtained this effect, by placing against the object- 
glass a glass having one of its faces perfectly plane, and the 
other ground into the segment of a cylinder of a very great dia- 
meter. The axis of the cylinder was exactly parallel to the base 
of the prism. The spectrum could not, thereforej change in 
its length, and w^as therefore only widened. In the spectrum 
thus altered, I recognized all the lines occupying the very same 
position that they had when the aperture was long and narrow. 
I employed the same apparatus for examining, in the night 
time, the planet Venus, without allowing the light to Jail upton a 
small aperture. 
In the spectrum formed by tliis light, I found the same lines 
such as they appeared in the light of the sun. That of Venus, 
however, having little intensity compared with that of the sun re- 
flected from a mirror; the brightness of the violet and the exte- 
rior red rays is very feeble. On this account, we perceive even 
the strongest lines in these two colours with some difficulty ; but, 
in the other colours, they are easily distinguished. I have seen 
the lines D, E, 5, F, Fig. 5. Plate VII., Vol. IX., very well 
terminated ; and I have recognized that those in 5, are formed 
of , two, namely, a fine and a strong line. The weakness of the 
light, however, prevented me from seeing that the strongest of 
these two lines consisted of two ; and, for the same reason, the 
other finer lines could not be distinguished. By an approximate 
measure of the lines DE and EF, I am convinced that the light 
of Venus is, in this respect, of the same nature as that of the sun. 
With the same apparatus, I have also made several observa- 
i;ions on some of the brightest fixed stars. As their light was 
much fainter than that of Venus, the brightness of their spec- 
trum was consequently still less. I have nevertheless scon, with- 
