CH. XV] 



PROJECTION OF SPECTRA 



633 



diffraction spectra are not as 

 bright as are the spectra obtained 

 with prisms and cannot well be 

 used except to demonstrate that 

 such spectra can be produced. 



Sometimes if a very high order 

 spectrum is to be shown the grating 

 can be held obliquely as in fig. 361. 



897. Use of reflection gratings. In 



case a reflection grating is to be used the 

 lantern is pointed directly away from the 

 screen and the reflecting grating held so as 

 to reflect the light back to the screen. The 

 objective is focused until the central image 

 is sharp on the screen and the spectra are 

 observed at both sides, or the grating may 

 be tipped so as to reflect the central image to 

 one side, when the spectra will appear in 

 the center of the screen. 



In this work it is essential in order that the 

 comparatively faint spectra can be seen, to have 

 the room perfectly darkened ; the arc house per- 

 fectly light-tight and the lantern well enclosed to 

 avoid stray light. 



Concave reflection gratings can be used by a 

 method similar to that for plane gratings, but in 

 order to have the central image sharply in focus 

 on the screen the objective must be closer to the 



FIG. 360. 



USE OF A GRATING WITH FINE LINES FOR THE 



PROJECTION OF SPECTRA. 

 5 Slit. 



Objective. 

 G Grating. 



* Primary image of the slit. 



1 V R Primary image of the slit (*'), and the first dif- 

 fraction image of the slit spread out into a spectrum (R V). 



Note that with a grating, the red is deviated more than the violet. Compare 

 with the prism, fig. 353. 



