Light traverses Transparent Media, 337 



this case through the lower glass, for both sets of fringes. The 

 light I used was a red obtained by extracting the colouring 

 matter of alkanet root in oil of turpentine, and then transfer- 

 ring it, when filtered, by solution and quick evaporation, to 

 Canada balsam. This coloured balsam, placed between two 

 plane glasses and cemented round with sealing-wax, furnished 

 a convenient way of getting light of any required purity: that 

 which I used was such as to cause rather more than twenty 

 bright and twenty dark bands to be visible at once ; and yet 

 in this number there was no difficulty, with a little practice, in 

 determining with certainty which was the central one. 



The following are the measures I have taken, and the re- 

 sults which they give. 



On March 30th 1833, the angle between the glasses (?zy) 

 being 14° 1' 7", 



the distance ef was found to be * 29290 inch. 

 When the angle (?*» was 15° 0' 15", 



ef was found to be '34320. 

 For both the above ii, fig. 2. = -10126 



er — = 12-15 inches 

 rp — = 19*65 inches 

 whence ah — -1637661. 

 The thickness of the glass plates was found to be -1375 inch. 

 Taking the refractive index of the glass (jx) at 1*495, these 

 measures give 



d-d, -002812115 



D,-D " = -001743651 T 



The second one for th,e angle 15° 0' 15" I have not calcu- 

 lated. 



On the 6th of April, I obtained the following : 



Angle ?*>, fig. 1 = 18° 59' 17" 

 ef — = '540276 

 If fig. 2. = -104012 

 er — = 12*15 

 rp — = 19*9375 

 whence a b = -170678 



from which we find when ju. is taken = 1-495 



jfci - - 0051619 _ i. 67 04 

 D-D ~ -0030902 ~ L b7U4 ' 



On the 21st of April, I got the following measures: 

 Third Series, Vol. 3. No. 17. Nov. 1833. 2 X 



