PHILOSOPHICAL SOCIETY OF WASHINGTON. ^ 119 



In deducing these results visual and photograpliic data have 

 been intermingled as if they were homogeneous. Such a pro- 

 cedure is certainly open to objection, but it is not likely that it 

 introduced any error grave enough to impair the value of the 

 results as first approximations. 



The next communication was by the President of the Society 

 Mr. Simon Newcomb on 



THE PRINCIPLES OF TAXATION. 



This paper was reserved by the author. After remarks upon this 

 communication by Mr. Edward Atkinson, of Massachusetts, and 

 Mr. E. B. Elliott the Society adjourned. 



180th Meeting. April 24th, 1880. 



The President in the chair. 

 Forty-three members present. 



The order of exercises for the evening consisted in the following 

 communications: (1) by Mr. A. A. Michelson, U. S. N., on the 

 modifications to which light is subject in passing through a very 

 narrow slit ; (2) by Mr. Edgar Frisby remarks on the late solar 

 eclipse observed in California ; (3) by Mr. H. M. Paul on earth 

 tremors, as shown by asti"ouomical observation ; and (4) by Mr. E. 

 S. Holden remarks on Gould's Uranometria Argentina. 



Mr. Michelson's communication on 



the modifications suffered by light in passing through a 

 very narrow slit, 



was substantially as follows : 



When the sun is viewed through a narrow slit, the diffraction 

 fringes are observed. On making the slit narrower, the following 

 phenomena are presented. When the width of the slit is ,01™" 

 the light acquires a faint-bluish tint, and on viewing it with a 

 Nicol's prism, traces of polarization are evident. When the light 

 is faintest, the bluish tint is more marked. When the width of the 

 slit is .005"" the blue tint is more marked and the polarization is 

 quite distinct. When the light is very faint, the tint is deep blue. 

 When the width is .001"" the tint changes to violet and the polari- 

 zation is complete. As the light grows fainter, the violet tint 



