12 i On the Optical Properties 



The least refracted image was a circular mass of white nehu- 

 lous light, condensed at its centre into a very faint image of the 

 candle, but without any strong prismatic tinge ; while the light 

 which had suffered the greatest refraction, formed a distinct and 

 highly coloured image. The great interval between the two 

 images; the achromatic nebulosity of the first, and the distinctness 

 and deep colours of the second image, formed altogether a s^ingiJar 

 pheenomenon, and, at the same time, afforded an ocular demtm- 

 slration of the existence of two dispersive powers in doubly re- 

 fracting cnjst a Is . 



The following measures of the refractive powers of the two 

 images were taken with the greatest care: 



Angle of the prism 60^ 2.V 



Angle of refraction for the 1st image . . 24*^ 8' 

 Angle of refraction for the 2d image . . SS^ 54' 

 Index of refraction for the 1st image . 1"3374 

 Index of refraction for the 2d image.. 1'5156 

 In order to confirm these results, I formed a new prism, and 

 obtained the following measures : 



Angle of the prism (i2^ 12' 



Angle of refraction for the 1st image . . 24" 48' 

 Angle of refraction for the 2d image . . 40' 39' 

 Index of refraction for the 1st image . . 1"3326 

 Index of refraction for the 2d image .. 1'5134 

 By taking a mean of tiiese results, which are extremely near 

 to each other, we obtain for the 



Least refractive power 1 '3350 



Greatest refractive power 1 '5 14.5 



Hence it follows, that the least refraction of nitrate of pot- 

 ash is almost exactly the same as that of water which is 1*3358, 

 — a result of such an extraordinary nature, that I felt it necessary 

 to confirm it by repeated observations. 



In measuring the dispersive power of this salt, we cannot ex- 

 pect the same accuracy of result on account of the great angle 

 of the prism. Owing to the nebulosity of the first image it is 

 impossible to measure its dispersive power ; but it evidently cor- 

 responds with its low power of refraction. In order to cor- 

 rect the dispersion of the second refraction, it requires a prism 

 of flint glass with an angle of nearly 60". With an angle of 

 66^, the dispersicjn is mwe than corrected ; but with an angle 

 of 56° the correction is not nearly completed. The dispersive 

 powers due to these different angles are contained in the fol- 

 lowing table : 



66" rO-06I3 



A 1 f*u a- » 1 "^06" r()-U6I3 



Angles of the fimt glass K^ Dispersive powers I 0-0573 



By 



