REFRANGIBILITY of LIGHT. . 59 
with air; and from diminifhing thofe errors which arife from 
faults in the workmanhhip. 
Tue difadvantages under which reflecting telefcopes labour, 
arife from their requiring larger apertures to tranfmit the fame 
quantity of light; from being found to be more affected by 
imperfections of the atmofphere than refracting telefcopes, and 
being liable to tarnifh ; but {principally from imperfections in 
the workmanfhip of the object fpeculum hurting their perform- 
ance much more than equal imperfections in the objeé-glafs 
hurt refractors. 
Tue deviation of a ray from its intended courfe, occafioned 
by an imperfection in the figure of a reflecting fpeculum, is to 
its deviation, arifing from an equal imperfection ina lens, as 
four to one, when the ray pafles from glafs into air, and in the 
proportion of fix to one, when it paffes from air into glafs. At 
a medium, therefore, it may be ftated as five to one. It follows 
from hence, that fuppofing all other:caufes of imperfection re- 
moved. but this of workmanfhip, and that the metal of fpecu- 
lums were capable of as good a polifh as glafs, and of refleCting 
as much light as glafs tranfmits, ftill the perfeGtion of the 
images of objects formed by refraction would greatly exceed 
thofe by refleAion. 
Sucu is the cafe in the refraétions which take place in the 
confine of glafs and air. But in the refraétions made in the 
confine of glafs, and mediums of greater denfity than air, the 
difference is ftill much greater. 
Tue proportion of the fine of the angle of incidence to the 
fine of the angle of refraction of a ray in pafling out of one 
medium into another medium, is compofed of the proportion 
of the fine of the angle of incidence to the fine of the angle of 
refraction out of the firft medium into any third medium, and 
of the proportion of the fine of the angle of incidence to the 
fine of the angle of refraGtion, out of that third. medium into 
the fecond medium. 
H 2 TuHus, 
