33 
different Species of Glass. 
of the spectrum more or less illuminated, the edge of the mirror 
becomes, in both cases, more distinct ; because, in the first case, 
the mirror appears to be placed in the shadow, and, in the se- 
cond case, it is the colour of the spectrum that is found there. 
The experiment with the mirror is a little difficult and uncer- 
tain, if we perceive clearly the lines of the spectrum, because 
the brightest and the darkest lines touch one another almost in 
every colour. On this account, the aperture in the window- 
shutter is made so broad, that only the strongest lines are just 
visible, and the fine ones not at all. In place of the mirror 
outside of the shutter by which the light entered, I put a 
white plane surface illuminated by the sun, because by any im- 
perfection of the mirror the light is irregularly dispersed, which 
renders the observations more dubious. 
In order to vary the experiments, I at one time enlarged the 
round aperture before the flame, and at other times I contracted 
it. I placed also at the end c of the wide tube a piece of ground 
glass, through which the mirror received its light. In this case 
I measured the distances of the flame from the ground glass. To 
avoid all illusion, the aperture before the eye-glass ought to be 
small, and to be at the place where the principal rays, or the axes 
of the rays coming from the edge of the field, cut the axis of 
the telescope. With the prism of flint-glass No. 13, having an 
angle of S6° I obtained the following results. Though 
the experiments were made in clear weather, and at noon, I 
sometimes perceived, in the course of the observations, a slight 
change in the density of the light which the prism received. 
The differences in the four sets of experiments may have been 
partly owing to this change, and the flame may also have changed 
its intensity in the course of the observations. If we call the 
intensity of the light at the brightest part of the spectrum 1, 
we shall then have. 
Experiment I. 
Exp. II. 
Exp. III. 
Exp. IV. 
Mean of Four 
Experiments. 
Intensity of Light 
At . B - O.OlO 
At - C - 0.048 
At - D - 0.61 
Between D and E = 1.00 
At - E = 0.44 
At - F = 0.084 
At - G = 0.010 
At - H = 0.0011 
Int. of Light 
0.044 
0.096 
0.59 
1.00 
0.38 
0.14 
0.029 
0.0072 
Int. of Light 
0.053 
0.15 
0.72 
1.00 
0.61 
0.25 
0.053 
0.0090 
Int. of Light 
0.020 
0.084 
0.62 
1.00 
0.49 
0.19 
0.032 
0.0050 
Int. of Light, 
0.032 
0.094 
0.64 
1.00 
0.48 
0.17 
0.031 
0.0056 
vor. X. NO. 19. JAN 
. 1824. 
C 
