504 



of 300 or 400 times, and no polishing can remove or modify them. 

 Sometimes 3000 of them may be counted in an inch, while in an- 

 other part of the same specimen they become much coarser ; and a 

 corresponding appearance may be detected in the substances which 

 have received the impression. The axis of extraordinary reflection 

 is always perpendicular to the direction of these grooves. Dr. Brew- 

 ster thinks that the general reflection by which the common image 

 is formed, is the effect of the repulsive force of the whole surface, 

 acting at such a distance, that its irregularities do not interfere with 

 the equality of the angles of incidence and reflection ; but that which 

 has escaped this reflection is subjected to the influence of the grooves, 

 the form of which, as he shows from the phenomena, must be curvi- 

 linear. The spectra do not resemble those which are produced by 

 ordinary inflection, and which are observed in circumstances mate- 

 rially different. Dr. Brewster has in vain attempted to obtain co- 

 loured impressions from the Labrador spar, and from several of the 

 metallic oxides : he finds that the crimson and green light of the 

 mother-of-pearl are dependent on its thickness, like the colours which 

 are seen in the common thin plates, but that they appear at much 

 greater thicknesses. 



The last peculiarity which the author has found in this interesting 

 substance, is the manner of its polarizing light. In crystallized bodies, 

 two portions of light are differently polarized with respect to the 

 direction of the plane of incidence ; but here the transmitted and 

 reflected light are polarized in the same direction. The polarization 

 is the most complete when the angle of incidence is about 60, and 

 when the thickness is about one fortieth of an inch ; the transmitted 

 light is in this case wholly polarized. If the plate is thicker, the 

 transmitted light is wholly polarized at a smaller angle of incidence ; 

 and this polarization remains unaltered when the superficial reflection 

 is destroyed by the contact of a substance of equal refractive density. 



Dr. Brewster concludes with observing, that the subject is far from 

 being exhausted ; and that if the investigation could be carried on 

 with the aid of analogous phenomena, we might confidently look for- 

 wards to some great change in the fundamental principles of physical 

 optics. 



An improved method of dividing Astronomical Circles and other Instru- 

 ments. By Capt. Henry Kater. Communicated by Thomas Young, 

 M.D. For. Sec. R.S. Read May 5, 1814. [Phil. Trans. 1814, 

 p. 419.] 



Although the author is most ready to admit the accuracy of Mr. 

 Troughton's method of dividing, which was described in our Trans- 

 actions for the year 1809, and by which the mural circle at Green- 

 wich has been divided, it appeared to him that some improvement 

 might be made in regard to simplicity and facility of execution. 



The general principle of the method here described by Capt. Kater 

 is, in fact, the same as that of the beam compass ; but his apparatus, 



