646 ABBE DIFFRACTION THEORY [Cn. XV 



horizontal lines and fine vertical lines very close together. Dia- 

 phragms cut from black paper of various shapes, will give many 

 curious and beautiful appearances at I. A small diaphragm 

 placed so as to remove all but the central image of the pattern, or 

 one of the lateral images, will allow light to fall on the screen but 

 no detail can be seen. 



For lecture purposes, where one requires considerably more light 

 than for a small class demonstration, one can use a vertical slit 

 with a condenser as the source instead of the arc lamp (fig. 373). 

 See 891 and 900, figure 364. Use line gratings with the lines 



F IG - 373- DEMONSTRATION OF ABBE DIFFRACTION THEORY TO A LARC.K 



AUDIENCE, USING A SLIT. 

 Arc lamp. 

 Condenser. 

 Pinhole or Slit. 

 C t Condenser, preferably an achromatic combination. 

 G Grating. 



O Objective. The diffraction pattern is formed at the face of the objective. 

 Diaphragms are used at this point to modify the image. 

 7 Image of grating. 



vertical. The phenomena shown are not as interesting as when 

 using a point source. By using slits or rods to intercept part of 

 the diffraction pattern, the image on the screen can be made to 

 appear as if it were of a grating having finer lines than the grating 

 which is actually used. 



If this phenomenon is projected it will probably be desired also 

 to demonstrate it individually to a few of the observers. This may 

 be done by the use of a telescope t, (fig. 374). to observe the grating. 

 The eyepiece of the telescope should be of a high power. The 

 condenser C 2 focuses the image of the pinhole just in front of the 

 telescope objective. When the grating g, is in place, the diffraction 

 images will appear on both sides of the pinhole image. If the 

 grating is viewed by the telescope it will appear normal but if part 

 of the diffraction pattern is stopped out by diaphragms, the grating 



