298 M. Frauenhofer on the Existence of numerous 
bands at H are of a very singular nature. They are both near- 
ly equal, and are formed of several lines, in the middle of which 
there is one very strong and deep. From H to I they likewise 
occur in great numbers. Hence it follows that in the space B H 
there are 574 lines, the strongest of which are shewn in the 
figure. The relative distances of the strongest lines were mea- 
sured with the theodolite, and placed in the figure from obser- 
vation. The faintest lines only were inserted from estimation 
by the eye. 
Various experiments and changes to which I have submitted 
these lines, convince me that they have their origin in the nature 
of the light of the sun, and that they cannot be attributed to il- 
lusion, to aberration, or any other secondary cause. In trans- 
mitting the light of a lamp through the same aperture, we ob- 
serve only the line shewn at It, in Fig. 4. It occupies, how- 
ever, exactly the same place as D in Fig. 5 ; so that the index 
of refraction of the line D is the same as that of II. 
It is easy to understand why the lines are not well marked, 
and why they disappear, if the aperture of the window becomes 
too large. The largest lines occupy nearly a space of from 5" to 
10". If the aperture is not such that the light which passes 
through it cannot be regarded as a single ray, or if the angle of 
the width of the aperture is greater than that of the width of 
the line, then the image of the same line will be projected several 
times parallel to itself, and will consequently become indistinct, 
and disappear when the aperture is too great. The reason why, 
in turning the prisms, we cease to see the lines, unless the tele- 
scope is lengthened or shortened, may be thus explained. 
The emersion of the rays, in respect to their divergence, is 
similar to their immersion only in the case where the angles of 
incidence and emergence are equal. If the first angle is greater, 
the rays after refraction will diverge, as it were, from a more 
distant point, and, if it is smaller, from a nearer point. The 
reason of this is, that the path of the rays which pass nearer the 
vertex of the prism-is shorter than that of those which pass at a 
greater distance from the vertex. Hence the angles of the re- 
fracted rays are not changed, but the sides of the triangles for 
the emergent rays ought to be in the one case greater, and in 
the other smaller. This difference ought to vanish if the rays 
