30 



OPTICS. 



The light at A or the line A 1 Is 0.000 



0.032 

 0.094 

 0.64 

 1 00 

 0.48 

 0.17 

 0.031 

 0.0056 



ray lies between the blue and the indigo 

 spaces. 



Lines across the spectrum. One of 

 the most curious discoveries of modern 

 times is that of Fraunhofer, that the 

 spectrum is covered with dark and co- 

 loured lines parallel to one another, and 

 perpendicular to the length of the spec- 

 trum. In order to observe these lines, 

 it is necessary to use prisms perfectly 

 free from veins, to exclude all extrane- 

 ous light, and even to stop those rays 

 which form the coloured spaces which 

 we are not examining. It is necessary 

 also to use a telescope, and the light 

 must enter and emerge from the prism 

 at equal angles. In this way the follow- 

 lines will be seen. 



B B2 



C C3 



D D4. 



M MN. 



E E5. 



F F6. 



G G7 . 



H H 8 . 



I 19 .. 0.000 



M. Fraunhofer found that D M was 

 nearly $ or i of D E ; and hence the 

 most luminous rays of the spectrum 

 formed by the finest prisms, and when 

 all other light is excluded, in place of 

 being situated in the middle of the 

 spectrum, are nearer the red than the 

 violet end, in the proportion of 1 to 4. It 

 appears, also, that the mean refrangible 



At A a well-defined dark line within the red space 1 



At a a mass of 7 or 8, forming together a dark band 8 



At B there is a thick and distinct one, and one fainter 2 



Between B and C, the one at C being broad and black 9 



Between C and D 30 



At D two nearly of the same size, separated by a bright one . . 2 



Between D and E there are of different sizes 84 



The line at E consists of several, the middle one being strongest. 



Between E and b 24 



At b three very strong lines, with a fine clear one between two. . 3 



Between b and F, F being very strong 52 



Between F and G 1 85 



Between G and H, many being accumulated at G 190 



In all they amount to. . 590 

 The largest lines occupy a space of 5" to 10" "We have inserted only a few in the figiire. 



One of the most important practical several substances, the most important 



results of this curious discovery of M. of which are flint glass, crown glass, 



Fraunhofer is, that these lines are fixed water, and oil of turpentine. The 



points in the spectrum, or rather, that angle of the fiint glass prism was 26 C 



4-V*sv*T VirTT/- -ltvoT7c< -fVio c-rvYik T\s*cn4*/Wi in 9/1 ' QO" Q-nrl ifc? c?v\ari-fio rwoT7i4-TT Q TOO 



they have always the same position in 

 the coloured spaces in which they are 

 found. In flint glass, for example, the 

 distance B C (fig. 33.) is greater than 

 B C in crown glass, because the disper- 

 sion of the former is greater than the 

 latter. The index of refraction, there- 

 fore, for each coloured ray, may be 

 accurately determined by means of 

 these lines. Fraunhofer has done it for 



B 



1.627749 

 1.. 52 5832 

 1.330935 



1.629681 

 1.526849 

 1.331712 



1.635036 

 1.529587 

 1.333577 



24' 30", and its specific gravity 3.723. 

 The angle of the crown glass prism was 

 39 20' 35", and its specific gravity 

 2.535. The angle of the other prisms 

 was 58 5' 40"; and the specific gra- 

 vity of the oil of turpentine was 0.885. 



The following were the indices of refrac- 

 tion for the lines B,C, D, E, F,G, H, mfig. 

 33, corresponding with the red, orange, 

 yellow, green, blue, indigo, and violet. 



E F G H 



1.642024 1.64826 1.660285 1.671062 

 1.533005 1.536052 1.541657 1.546566 

 1.335851 1.337818 1.841293 1.344177 



Flint Glass.. 

 Crown Class 



Water 



Oil ofTur-1 



pentine J 



From these numbers M. Fraunhofer powers of the differently coloured rays 

 has computed the following numbers, in several combinations of the above 

 which are the ratios of the dispersive refracting substances. 



1.470496 1.471530 1.474434 1.478353 1.481736 1.488198 1.493874 



Flint Glass and Water 2.562 



Flint Glass and Crown Glass 1 .900 



Crown Glass and Water 1.349 



Oil of Turpentine and Water .... 1 .37 1 



Flint Glass and Oil of Turpentine . 1 .868 



Space BC. Space C D. 

 2.871 



1.956 

 1.468 

 1.557 

 1.844 



Space DE. 



3.073 

 2. OH 

 1.503 

 1.723 

 1.783 



Space E F. Space F G. Space G H, 



3.193 



2.047 

 1.560 

 1.732 

 1.843 



3.460 

 2.145 

 1.613 

 1.860 

 1.861 



5.726 

 2.195 

 1.697 

 1.963 

 1.899 



