160 PROCEEDINGS OF THE AMERICAN ACADEMY 



sodia porophylloides ; Psathyrotes incisa ; Bartlettia scaposa, 

 a new genus of SenecionesBj dedicated to the U. S. Boundary 

 Commissioner under whose instructions this collection was 

 made ; Perezia Thurheri ; Stephanomeria Thurberi ; Jac- 

 qiiinia pungens ; and Pilostyles Thurheri, a parasitic flower 

 of the order Rafiiesiaceas, of which it is the only representa- 

 tive in North America. 



Professor Horsford exhibited an obstruction which had 

 been removed from a wooden pump-log at the Water-Cure 

 establishment of Brattleboro. Vt., formed of a compact mass 

 of small root-fibres, entirely closing the tube of the log. It 

 was three feet in length, and was developed from a slender 

 fibre of the root of a neighboring tree, which had penetrated 

 by a small crevice at a joint in the log. The log had been two 

 years in the ground. 



Dr. Walter Channing asked for an explanation of the fact, 

 that, in solar eclipses, the luminous spots upon the ground pro- 

 duced by the light which penetrated the foliage were always 

 of the same shape as the uneclipsed portion of the sun. 



Professor Lovering remarked in reply : — 



" This phenomenon has been frequently noticed. It excited at- 

 tention in the progress of the great eclipse of 1806, and is men- 

 tioned by Sir J. F. W. Herschel in reference to the eclipse of Sep- 

 tember 7, 1820. The explanation of it which the rectilinear motion 

 of light offers is simple and sufficient. If the aperture is a physical 

 point, a cone of rays will pass through it, having this point as its 

 apex. The section of this cone by a screen at right angles to its axis 

 will have the same figure as the body from which the light comes, and 

 its diameter will be equal to the diameter of the luminous body mul- 

 tiplied by the distance of the screen from the aperture, and divided 

 by the distance of the luminous body from the same point. Hence, 

 for the same body, it increases as the screen is placed at greater dis- 

 tances. If the luminous body is a physical point, and the aperture not, 

 a pencil of rays will pass through, which is a continuation of the cone 

 of which the luminous point is the apex, and the apertures the base. 

 The section of this pencil at right angles to its axis will have the 

 shape of the aperture, and its size will equal that of the aperture 



