POR SOLAR OBSERVATIONS. 27 



sity of the pencil cV may be varied either by varying the 

 thickness of the plate, or by 

 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\V y having its 

 intensity very greatly reduced. 



In order that the light may be freely reflected at the points 

 o, 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 

 in 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 



D 2 plates, 



