Some Abhe Letters. 



223 



the lines on the simple grating is 0*0073 mm. (part A), and 

 0*0143 mm. (part B). The crossed gratings have two bands of 

 lines each, one with the distance • 0073 mm., the other band with 

 O'OllO mm. (2 : 3). Therefore each of those latter preparations 

 shows three different figures — quadratical and rectangular on the 

 one, rhombial and rhomboidivial on the other (figs. 4and 6). (The 



CL- 



/Ibbet's Letter- DEc.a^'^ 1876. 





.-^^v^W-^^ y^**^ 



O O a o o 



o 



-</W^<tQ^ 







o 

 o o 



.v.-v-,Aa«, (^'^■i^ JSuMfi-^ Fi&.S. 



Fi<J. to 



^^i1. 



silvered and ruled faces of the glasses are cemented together with 

 balsam). 



Those crossed gratings give a diffraction-spectrum in two 

 dimensions, and you will get a great variety of markings by 

 stoppng off various parts of it. You will easily command all 

 these effects if you remember what, in my theory, is the general 



