OPTICS. 



In the following TABLE we have given 

 the dispersive powers of various bodies, 

 as determined by Dr. Brewster. The 

 first column contains the dispersive 

 power ; and the second, the difference of 

 the indices of refraction for the red 

 and violet rays, or the part of the whole 

 refraction to which the dispersion is 

 equal. Hence, ifwe add the half of the 

 numbers in the last column to the index 

 of refraction, we shall have the index ot 

 refraction for the extreme violet ray ; 

 and if we subtract it, we shall have the 

 index for the extreme red ray. We 

 may, therefore, obtain, by means of the 

 second column in the table, the length 

 of the spectra, formed by prisms of any 

 of the substances it contains, for any 

 refracting angle, for any position of the 

 prism, and for any SiSance of the 

 screen, upon which the spectrum is re- 

 ceived. In doing this, however, it must 

 be recollected, that the measures here 

 given are suited to the ordinary light 

 of the sky; and that when the sun's 

 image is used, and when great care is 

 taken to screen the middle rays of the 

 spectrum, the red and the violet are 

 found to extend to a greater distance 

 from the mean ray. 



TAWTF- of T>T<JPFW<;TVF 

 SPERSHE 



Dispersive 

 power. 



Chromateoflead, gr. refr. ext. 0.400 



- --- least refr. 0.262 



Real;?ar, melted .......... 0.260 



Oil o'f Cassia ........... 0.139 



Sulphur, after fusion ...... 0.^30 



Phosphorus ............ 0.128 



Sulphuret of Carbon ---- 0.115 



Balsam of Tolu .......... 0.103 



2 ri T? of ff ni .......... n'n 



Barbadoes Aloes ... ..... 0.08o 



Oil ot Bitter Almonds .... 0.079 



O or Anise Seeds ...... 0.077 



Balsam of Styrax ........ 0.067 



Guiacum .. ............ 0.066 



Oil of Cummin ......... 0.065 



Oil of Tobacco .......... 0.064 



Oil of Cloves .......... 0.062 



Sulphate of Lead ........ 0.060 



Oil of Sassafras .......... 0.069 



Muriate of Antimony, r< 



Tractive power.... 1.598 O.OoO 



" " ' ' ...... 



rays. 

 0.770 

 0.388 

 0.384 

 0.089 

 0.149 

 0.156 

 0.077 

 0.065 



n'n-8 

 0.05 



0.048 

 0.044 

 0.039 

 0.041 

 0.033 

 0.035 

 0.033 

 0.056 

 0.032 



0.036 



Disperse 

 powers. 



Oil of Fenffgreek ........ . 0.050 



Oil of Carraway Seeds ---- 0.049 



Flint Glass .............. 0.048 



Gum Thus .............. 0.048 



i of Jumper .......... 0.047 



Jg^JjJ ............ JJ 



Canada Bal'sam . ! ! ! '. '. .' .' ! ! !o45 



<^ a j eput oil ............ 0.044 



zircon, gr. refr.... .'.'..... 0.044 



Muriatic Acid .......... 0.043 



Oil of Turpentine ........ 0.042 



Balsam of Capivi ........ 0.041 



Calc. Spar, gr. refr ....... 0.040 



Sulphate of Iron ........ 0.039 



g' 3 ?^. .............. - 038 



l ot Olives ............ 0.038 



25 



rays. 

 0-021 

 0-024 

 0.029 

 0.028 

 0.022 



ool 

 o'.0il 



021 

 o!o45 

 0.016 

 0.020 

 0.021 

 0.027 

 0.019 



- 56 

 0.018 



It is o b v ious, from the preceding 

 results, that different bodies possess 



different powers of dispersing, or 

 J ,. ,,f j c-r *, 



of separating the coloured rays of hght. 



If * now proceed to a more minute 

 examination ot spectra of equal lengths, 

 by two bodies of very different dispersive 

 powers, such as oil of cassia and sul- 

 phuric acid, we shall observe a very 

 remarkable difference between them. 

 If A B> for example, (jig. 30.) is the 



spectrum, produced by a prism of oil of 

 - CD ^ ^^ by ^ 



acid, then we shaU 



Rock Salt ............ 0.053 



Oil of Pimento ......... 0.052 



Flint Glass .............. 0.052 



Oil of Angelica ........ 0.051 



Oil of Thme. . . . f i ...... 0.050 



0.029 

 0.006 

 0.026 

 0.025 

 0.02 1 



red > orange, and yellow, will occupy 

 less spaces, or will be more contracted 

 in the oil of cassia spectrum than in the 

 sulphuric acid one; while the most 

 refrangible colours, blue, indigo, and 



