The Microscope. 



103 



tions, this side stage is provided with two sliding ledges and a 

 spring as shown at TT. The milled head regulates the quan- 

 tity of light transmitted to the reflecting prism, winch is sent 

 from the small mirror F. 



To use the spectroscopic eye-piece it should be inserted into 

 the upper end of the tube, and the slit opened wide. The tube 

 containing the prisms should be so turned that the latter stands 

 upright — otherwise a distortion of the spectrum will be pro- 

 duced. Light should be reflected by the mirror of the micro- 

 scope into the body, and the slit gradually narrowed. It 

 should then be focussed upon by turning the large milled head. 

 If bright daylight should have been used, and the slit properly 

 brought into the focus of the eye lens, the spectrum will be seen 

 crossed by a great number of fine dark lines ; the more promi- 

 nent of which are known as Frannhofer lines. These are shown 

 on the plate. The one marked F is the extreme dark red, and is 

 seen with extreme difficulty unless the brighter yellow part of 

 the spectrum be shaded or covered in some way. The colors 

 corresponding to the various parts of the spectrum are indi- 

 cated in the figure, but it must be remembered that there is no 

 abrupt change from one color to another. The red, for exam- 

 ple, is very dark near the line A, but becomes progressively 

 more brilliant and lighter as we go towards C, from which posi- 

 tion it goes through the various shades of orange, nearly to the 



