OBSERVING WITH AND MANAGING ENGISCOPK. 205 



When observing faint, sombre, and dull opaque ob- 

 jects, especially if high powers are used, a vast increment 

 of apparent brightness is obtained, if all light is excluded 

 from the eye, save that of the visual pencil. Thus, it 

 will be found a great assistance if a large blackened 

 pasteboard shade, nearly a foot in diameter, is made to 

 fasten upon the eye-piece, and thus completely to ex- 

 clude the direct light of the candle, &c. ; or still better, 

 if a black hood, like those used by astronomers, is made 

 to cover up the face completely, but with an aperture for 

 the body of the instrument. Such an arrangement also 

 saves us the trouble of shutting one eye ; and the en- 

 largement produced in the aperture of the iris, by keep- 

 ing the eyes in the dark, enables the faintest rays to 

 aifect the retina. 



Condensed Artificial Light is procured by the action 

 of the double illuminator, f f; whenever it is used by 

 lamp or candle-light, the double convex must be screwed 

 on, still preserving the convex side of the piano, or bull's 

 eye, next the object. The split socket must be clamped 

 tight in such a position on the neck of the body, that it 

 shall present the face of the illuminator in a lateral 

 position relative to it, so that the rays of the candle, &c. 

 shall fall conveniently upon it ; its distance from which 

 should be about an inch and a half, and its distance from 

 the object should also be about an inch, which will give 

 the maximum of illumination, with a large spectrum, so 

 as to illume a considerable space. I think, whenever 



