] 12 Mr. Talbot's Facts relating to Optical Science. 



the name of which was similar to that of the tower and city 

 from whence the dispersion had taken place; whilst the in- 

 ference that the two places were identical, deduced from the 

 mere similarity of their names, is entirely rebutted by the fol- 

 lowing words of Scripture: " So the Lord scattered them 

 abroad from thence upon the face of all the earth : and they 

 left off to build the city*:" from which the only legitimate con- 

 clusion to be drawn is that the City and Tower of Babel were 

 left unfinished, and probably were altogether deserted. The 

 ground of the tradition which has thus improperly connected 

 these three places, may without difficulty be discovered in 

 that vanity with which the history of all nations (that of the 

 Jews, from the nature of its records, forming the solitary ex- 

 ception,) is replete. 



XXIV. Facts relating to Optical Science. No. I. Commu- 

 nicated by H. F. Talbot, Esq., M.P., F.R.S. 



1. Microscopic Cleavages in Talc or Mica. 

 TF a thin transparent film of talc or mica is held in the ex- 

 ■*■ terior flame of a candle, it speedily becomes white and 

 opake, like chalk. This opake portion is separated from 

 that which remains transparent by a sort of penumbra. Now 

 if we examine this penumbra with a powerful microscope, it 

 is seen to consist of myriads of minute fractures, each of 

 which exhibits the form of a cross. These microscopic crosses 

 are all similarly disposed, and pointing in the same direction. 

 This direction indicates, therefore, the line of least cohesion, 

 or of most easy cleavage, in the crystal, while the opposite arm 

 of the cross indicates another line of a similar nature. Two 

 opposite angles of the cross contain spaces which are a good 

 deal darker than the other two; which shows, if I am not 

 mistaken, that the plane of the cleavage is oblique to the eye. 

 This, however, disappears if the crystal is immersed in oil, be- 

 cause the refraction is thereby considerably diminished. It 

 would be desirable to ascertain the position of these cleavage 

 lines with respect to the axis of the crystal. 



2. Optical Properties of Chromium . 



Sulphate of chromium is a liquid which is green by day- 

 light and reddish by candle-light, as was first noticed, I be- 

 lieve, by Sir David Brewster. The cause of this peculiarity is 

 pleasingly exhibited by the following experiment. If a hollow 

 prism of a small refracting angle, as five or ten degrees, is 



* Gen. xi. 9. 



