different Species of Glass. 289 
In order to determine the ratio of the refraction and disper- 
sion of flint and crown glass, I made use of prisms of these two 
kinds of glass, having their respective angles small, and placed 
in opposite directions. These last were then successively chan- 
ged, till, on the one hand, the refrangibility, and, on the other, the 
refraction, was nothing. The ratio of the angles was then the 
inverse ratio of that of the refrangibility or the refraction. Se- 
veral prisms, however, thus put together in pairs gave different 
results, particularly for the dispersion. Hence, in order to de- 
termine the relative dispersion, I select larger prisms, having 
their refracting angles also greater, and placed in opposite direc- 
tions. The prism of crown-glass had an angle of from 60° to 70°. 
The angle of one of the prisms was changed till the dispersion 
was almost destroyed ; and the little that remained was then cor- 
rected, by changing the angle of incidence of the ray. Since 
in prisms with great angles, the light is totally reflected at the 
second surface, even by a small variation of the angle of inci- 
dence, I covered the two touching faces of the prisms with a 
strong refracting fluid, such as oil, and by this means the light 
was transmitted at almost all angles of incidence. I applied 
the two prisms before the object-glass of the telescope, and a re- 
peating theodolite, having placed them upon a horizontal plane, 
with a steel axis, round which it moved. The box in which 
the axis turned was firmly united with the telescope, as shewn 
in Fig. 1. of Plate VII. By this procedure I was enabled to 
measure exactly the angle of incidence at which the dispersion 
was destroyed. I first looked through the telescope across the 
prism, at a distant object, having its edges vertical and very 
distinct ; I then changed the angle of incidence, by turning 
the plane upon which the prisms rested, and the alidade of the 
theodolite, till the dispersion appeared to be very small, or ra- 
ther till the vertical edges of the object were most distinct. 
In order to measure the angle of incidence, I had put upon the 
turning plane a ruler, which carried two steel points, that exact- 
ly touched the first surface of the prisms. On this ruler was 
fixed a telescope, a little elevated, whose axis was perfectly pa- 
rallel to the two points of steel ; see Fig. 2. This telescope was 
fixed upon the ruler only by its two ends, so that through the 
interstice between the telescope and the ruler the light could 
VOL. ix. no. 18. OCT. 1823. 
T 
