TOR SOLAR OBSERVATIONS. Q7 
sity of the pencil eV may be varied either by varying the 
thickness of the plate, or by R 
changing the angle of inci- 
dence ; and if it cannot be ren- 
dered sufficiently feeble by any 
of these means, it may be ex- 
posed to other two reflections 
at c and d, when it will emerge 
in the direction e W, having its 
intensity very greatly reduced. 
In order that the light may be freely reflected at the points 
a, 6, c and d, the opaque plates C, D must be kept at a little 
distance from the surface of the glass, and all extraneous re- 
flexion must be removed, by covering their interior surfaces 
with a black pigment. This may be done most conveniently 
by making the plates C, D rest upon the glass only by their 
margins. The aperture at S, where the light is introduced, 
should be of an elliptical form, so as to admit a cylindrical 
pencil at an oblique incidence. 
Hitherto we have supposed, the reflecting surfaces to be 
sn contact with air, so that the reflecting force is allowed to 
exercise its maximum action upon the incident rays. But it 
is very easy to diminish the reflective power, in any ratio that 
we choose, by introducing between the opaque plates and the 
glass a cement either of a greater or a less refractive power 
than the glass. By this means we obtain a degree of reflexion 
corresponding to a refractive power equal to the quotient of 
the greater refractive power divided by the lesser; and the re- 
flexion will be made either from the surface of the cement, 
or from the glass, according as the one or the other exercises 
the most powerful action upon light. 
As it may sometimes be difficult to procure a thick plate of 
parallel glass, we may substitute in its place two common 
D2 plates, 
