THE DIRECT SPECTROSCOPE. 



125 



To combine both purposes, some form of direct vision- 

 prisms which can be applied to the body of the micro- 

 scope is required. Fig. 76 represents the arrangement of 

 direct vision-prisms, invented by A. Herschel. The line 

 B R' shows the path of a ray of light through the prisms, 

 where it would be seen that the emergent ray R' is parallel 

 .\nd coincident with the incident ray R. 



Kg. 7(3. 



Fig. 7<3. 



Another very compact combination is shown in fig. 76a. 

 Any number of these prisms (P p P) may be used, accord- 

 ing to the amount of dispersion required. They are 

 mounted in a similar way to a Nicols' prism, and are 

 applied directly over the eye-piece of the microscope. 

 The slit s s is placed in the focus of the first glass (P) if 

 a negative, or below the second glass if a positive eye- 

 piece be employed. One edge of the slit is moveable, 

 and, in using the instrument, the slit is first opened wide, 

 so that a clear view of the object is obtained. The part 

 of the object of which the spectrum is to be examined 

 is then made to coincide with the fixed edge of the slit, 

 and the moveable edge is screwed up, until a brilliant 

 coloured spectrum is produced. The absorption-bands 

 wilJ then be readily found by slightly altering the focus. 

 This contrivance answers perfectly for opaque objects, 



