378 THIRD REPORT — 1833. 



in giving a plane surface and high poHsh. Comparing such 

 a surface with crown-glass, he obtained measures which 

 enabled him to calculate the light reflected for all requisite 

 incidences, from the known reflective power of crown-glass. 

 These show that glass of antimony follows the same law in the 

 reflection of light as low refracting substances, and that the 

 number of rays reflected of every 100 incident may be calcu- 



lated from the same formula, namely, y =. a x , , where 



r + —X 



y is the number of rays reflected of every 100, x the sine of 



incidence to radius (r), as 100 ; a, h, and c being constants, 



and having the following values for this substance, a=7"4, 



6=1 "25, and c = 9. The maximum polai'izing angle was 



found to be about Q^P, so that the refractive index of the 



specimen would be about 2*1 to 2'2. The undulatory formula 



by using this value of jx, shows that according to 





+ . . . 



the received principles of that theory, the reflection of glass of 



antimony at a perpendicular incidence should have been about 



13'33 rays of every 100 incident, whilst experiment shows that 



only 8'20 such rays are really reflected. 



On a Phcenomenon in the Interference of Light hitherto un- 

 deseribed. By R. Potter, Jun. 



When the image of a luminous point is viewed in a Newtonian 

 microscope, of which the mirror is of a spherical figure, and 

 the aperture large in proportion to the radius of the sphere, 

 there is a large circle of aberration visible. This appearance 

 is best studied when the point, where the rays reflected near 

 the edge of the mirror cross the axis, is in the focus of the eye- 

 glass. The circle of aberration then appears to be composed 

 of dark and bright rings, which are broad and distinct near 

 the edge, but from the mixture of colours become soon lost 

 in white light. 



To produce the luminous point, Mr. Potter used a small 

 mercurial globule, attached by some glutinous matter on a slender 

 wire. When the image of the sun given by this globule is 

 viewed in the microscope, the appearance of the rings formed 

 by interference is one of the most beautiful of this class of 

 phaenomena. It may be seen by candlelight, but to much less 

 advantage. Mr. Potter states, that on studying the theory of 

 this case of interference he found that, according to the undu- 

 latory theory, the outer ring should be brightest on its outside 

 edge, whilst, according to the theory of interference which he 



