26 DESCRIPTION OF A NEW DARKENING GLASS 



The combination of coloured and smoked glasses which he 

 found most effectual for diminishing the heat of the sun's rays, 

 and at the same time preserving distinct vision, was extreme- 

 ly complicated ; and after every attempt to improve it, he 

 seems to have preferred another apparatus in which the light 

 was transmitted through a mass of diluted ink contained in a 

 trough bounded by plates of parallel glass. By this means he- 

 obtained an image of the sun as white as snow, and so very 

 distinct, that, with a mirror nine inches in diameter, and the 

 eye-piece open, he could observe the sun in the meridian for 

 any length of time, without injuring either his eye, or the 

 glasses of his eye-piece. 



Notwithstanding these advantages, this apparatus can never 

 be brought into general use, and can only be employed in in- 

 struments that are stationary. The necessity of frequently re- 

 newing the ink, and the difficulty of retaining it in the trough, 

 are evils which are not easily remedied. 



The darkening glass which we propose to substitute in place 

 of these contrivances, depends upon the diminution of light by 

 two or more reflexions within a thick plate of parallel glass. 

 The pencil, attenuated by reflexion, reaches the eye in the 

 state of white light, while the direct rays transmitted through 

 the plate are stopped by two pieces of metal properly disposed 

 upon the opposite surfaces of the parallel glass. 



The nature of this contrivance will be understood from the 

 following figure, where AB is a section of a piece of parallel 

 class, and C, D sections of two opaque plates placed on the upper 

 and under surfaces of this glass. A ray of light RS, incident ob- 

 liquely at S, will be transmitted in the direction a T, but is pre- 

 vented from reaching the eye by the opaque plate D. A portion 

 of this ray is reflected at a, and a second time at b> and emer- 

 ges in the direction c V in a very attenuated state. The inten- 



sity 



