440 MOSSOTTi OX frauniiofer's reticular spectra. 



§ I. Value of the Refractive Index 0/ Frauniiofer's Prism No. 1.3* 

 in Function of the Length of the Waves. 



1. When Fraunhofer observed the solar spectrum formed by a 

 flint-glass prism, the angle of refraction of which was 26° 24' 30", 

 through the telescope of a theodolite, the prism being in the posi- 

 tion of the minimum deviation of the spectrum, he found that the 

 principal line D was refracted to the angle of 17°27'8"; and 

 when he measured the angles betueen the line D and the other 

 principal lines B, C, E, F, G, 11 (fig. 2, Plate II.), he obtained 



BD. CD. DE. Y>?. DG D«. 



12'20"-2; -9'4"-2; ll'50"-0; 22' 23";); 42'47"-8; Gl'5"-8. 



From a series of observations upon the solar spectrum formed 

 by a grating and merely observed with the aid of the telescope 

 of a theodolite, Fraunhofer also deduced the following mean 

 values for the length of the waves of the rays contiguous to 

 these principal lines, expressed in millionths of a millimetre : — 



B. C. T). E. F. G. II. 



688; G56; 589; 52G; 484; 429; 393 . .f 



Judging from these values, a grating in which the sum of a 

 dark and light interval would amount to about 0-0S8 millim. 

 (which was the mean of those used by Fraunhofer), would pre- 

 sent a spectrum in wliich the angular distances between the 

 line D and the others B, C, E, F, G, H, measured at the focus 

 of the telescope of the theodolite, would be expressed by 

 BD. DC. DE. DF. DG. Dll. 



-4' 15"; -2' 5 2"; 2' 43"; 4' 21"; 7' 3; 8'3G". 



In this spectrum, fig. 1, Plate II., which we shall call the nor- 

 mal, the intervals between the jirincipal lines vary in proportion 

 to the respective lengths of the waves of the contiguous rays ; and 

 if it be compared with the foregoing prismatic spectrum, it is 

 seen that in the latter the intervals BD, DC, &c., compared with 

 those of the former, diminish in extent, whilst the intervals DE, 

 DF, &c. towards the violet end, comparatively increase in ex- 

 tent. This ditl'^rence in extent depends upon the corresponding 

 rays, the waves of which are shorter, being refracted in an inverse 

 ratio, which is greater than the simple ratio, in which the lengths 

 of the waves diminish. 



* Gilbert's Annalen der P/ii/si/c, 1817. Miinckener DenkschiiJ'tcn filr 1814- 

 ]S1,5. 



t Dcnhschriflen der MuKcJuner Jcudcmie f. iy2-'5. 



