-88 M. Frauenhofer an the Refractive and Dispersive Power 
masses, as in the deep quarries beneath Paris. Why, then, 
should it surprise us, if a very violent commotion in the bowels 
of the earth make it shake in a radius of several hundreds of 
leagues ? In a word, earthquakes are only the propagation of 
a commotion across the mass of the earth, so much independent 
of subterraneous cavities, that it extends so much the farther 
that tile earth is more homogeneous *. 
Art. XVI. — On the Refractive and Dispersive Power of diffe- 
rent Species cf Glass , in reference to the improvement of 
Achromatic Telescopes , with an Account of the Lines or 
Streaks which cross the Spectrum * By Joseph Frauen- 
hofer of Munich. With a Plate. 
The calculation of an Achromatic Object-Glass, and, in ge- 
neral, that of every achromatic telescope, requires a precise know- 
ledge of the ratio of the Sines of incidence and refraction, and 
of the ratio of dispersion of the different kinds of glass which 
are used in the composition of telescopes. The methods hi- 
therto employed for finding these ratios, have given results dif- 
fering considerably from each other, in spite of the care and ac- 
curacy employed in the computation. W e ought, therefore, to 
expect inaccuracies which render the perfection of the object- 
glass doubtful. Experiments repeated during many years have 
led me to discover new methods of obtaining these ratios ; and I 
have therefore obeyed the wishes of several philosophers in pu- 
blishing these experiments, in the same order in which I made 
them, and wicli the modifications which the experiments them- 
selves obliged me to introduce. 
I began by determining the dispersion of a single kind of 
glass, from the size of the prismatic spectrum, formed by a 
prism of a given angle in a datk chamber, and at a given dis- 
tance, and from this I deduced the dispersion. The ends of 
the spectrum, however, were ill germinated, and a considerable 
uncertainty was attached to the results. 
* Translated from the Annales de Ckimie et Phi/siqtie , t. xxii. April 1823. 
-\- Translated from the French in Shumacher’s Astronomische Ahhandlungeftf 
zweites Heft, p. 13. Alton a, 1823. 
