MOSSOTTI OX FRAUNHOFEr's RETICULAR SPECTRA. 443 



tion (1.) the values of ^ in the six spaces between the principal 

 lines of the spectrum, and obtained, 



BD. DC. DE. DF. DG. DH. 



-12'l9"-6; -8'57"-2; ll'56"-0; 22'4S"-8; 42'33"'6; 60'43"-l. 



The comparison of these values shows a sufficient agreement 

 with those mentioned above and given by observation. 



§ II. On the respective Intensities of Light in different parts of 

 the Prismatic and Retictdar Spectrum. 



4. As the prismatic spectrum is larger and of brighter and 

 more distinct colours, Fraunhofer was enabled to measure the 

 intensity of its light near the principal bands, when approximated 

 to and compared with the light of a lamp placed at various 

 distances. The results of his observations are contained in the 

 following; table : — 



The maximum of light assumed as unity falls between D and 

 E. From the nature of the maximum itself, it was difficult to 

 determine accurately the spot where it falls. Fraunhofer places it 

 between one fourth and one-third of the interval DE from D to E. 



The ordinates of the curve above the figure of the spectrum, 

 fig. 2, Plate II., represent the mean observed intensities of light 

 at the points of the spectrum situated beneath, corresponding to 

 the same abscissae. From the inspection of this cur\'«, we see that 

 the intensities of the light become comparatively further extended 

 towards the red than towards the violet end, which may depend 

 upon the index of refraction of the shorter undulations varying 

 more rapidly than in the inverse proportion of their length, and 

 the rays thus respectively being more condensed at the red end 

 and more diffused at the violet end. The proportion in which the 

 density of the rays in the varioiis ])arts of the prismatic spec- 

 trum alters, compared with that in wliich they are distributed in 



