Rays of Great Wave-length in Rock-salt, Sylvite, fyc. 45 



the energj-spectra produced by rock-salt or sylvite, given 

 in figs. 1 and 2. While in these latter the breadth of the 

 interference-bands increases only slowly as the extreme infra- 

 red is reached, amounting finally to hardly more than double 

 the smallest value, the breadth of these bands varies in the 

 energy-spectrum, as given by the fluorite prism, from 

 5 minutes to more than %\ degrees. Corresponding to this 

 peculiar characteristic in the energy-spectrum of fluorite, the 

 quality of the dispersion in this mineral is quite different from 

 that of the materials previously considered. 



In the following Table, which contains these data, the indices 

 of refraction are given only to three decimal places. As a 

 result of the very considerable breadth of the interference- 

 bands, it is impossible to locate the position of the cha- 

 racteristic points with the precision attainable in other cases. 



Table IV. 



Eefracting Angle of the Fluorite Prism, $ = 59° 59J'. 



K = 8*070 fi ; a ± is 10th Order. 



Name. 



a. 



n. 



X. 



1 

 Name. 



a. 



n. 



X. 



H y ... 



32 5 



1-4398 



fi 



0-434 



b 5 



30 59 



1-4267 



1-466 



F 



31 52 



1-4372 



0-485 



«G 



55J 



1-4260 



1-613 



D 



36 



1-4340 



0-589 



K 



51 



1-4250 



1-792 







29 



1-4325 



0-656 



a„ 



46 



1-4240 



2-019 



«1 



19 



1-4307 



0-807 



b 7 



38 



1-4224 



2-303 



w 



17 



1-4303 



0-850 



a s 



29 



1-4205 



2-689 



«2 



14* 



1-4299 



0-896 



b 8 



13 



1-4174 



3225 



b 2 



12 



1-4294 



0950 



a 9 



29 46 



1-4117 



4-035 



«3 



10 



1-4290 



1009 



<?! 



29 



1-408 



4-62 



*3 



8 



1-4286 



1-076 



b 9 



4 



1-403 



5-38 



«4 



6 



1-4281 



1-152 



c 2 



28 30 



1-396 



646 



*4 



4 



1-4277 



1-240 



«io ••• 



27 5 



1-378 



8-07 



a o 



2 



1-4272 



1-345 



; 









The curve of dispersion, shown in fig. 4, c, exhibits more 

 graphically than this table the peculiar character of the dis- 

 persion. From, this it is seen that the dispersive power of 

 flrorite decreases ab far as \ = 2{jl and then gradually increases, 

 reaching at X = 8fj, a value only slightly inferior to the value 

 of the dispersion in the red. 



Compared with rock-salt and sylvite, the dispersion of 

 fluorite in the visible spectrum is exceedingly small and 

 unusually great in the infra-red ; so that this material is 

 peculiarly well adapted to the production of prismatic heat- 

 spectra, an advantage which is still further increased by the 

 ease with which it can be worked and by the permanence of 

 its surface in the air. 



Physical Laboratory, University of Berlin, 

 June 1892. 



