144 



PRACTICAL TREATISE ON THE MICROSCOPE. 



4 



black cloth, and all the parts of this apparatus, as well as of 

 aU others through which light has to pass, should be covered 

 with some black pigment to absorb those rays of light which, 

 if reflected, would materially interfere with correct definition. 

 Camera Lucida. — This instrument, invented by Dr. Wol- 

 laston in 1807, is a most valuable addition to a microscope, 

 both for dehneating minute structures, and for obtaining with 

 a micrometer very accurate measurements. It consists of a 

 four-sided prism of glass, set in a brass frame or case, as 

 represented by fig. 99, and by means of a short tube capable 



of being applied to the 

 front part of either of 

 the eye-pieces, its cap 

 ^--^^- vv having been previously 



'^ ''*' ^ from one of whose in- 



struments fig. 99 is copied, 

 attaches the prism, by two 

 short supports, to a circu- 

 lar piece of brass at the 

 end of the tube ; on this 

 it can be slightly rotated, 

 whilst the prism itself can 

 also be turned up or down, 

 by means of two screws 

 with miUed heads ; so ar- 

 ranged, the camera may 

 be adapted to the eye- 

 piece, the microscope hav- 

 ing been previouslyplaced 

 in a horizontal position ; if 

 the hght be then reflected through the compound body, an 

 eye placed over the square hole in the frame of the prism wUl 

 see the image of any object on the stage upon a sheet of white 

 paper placed on the table immediately below it. But should 

 it happen that the whole of the field of view is not well 

 illuminated, then, either by revolving the circular plate or 

 turning the prism upon tlie screws, the desired object will 



Fig. 99. 



