402 LEV/SON. 



attained by the electromotor, bands of light are thus obtained 

 from certain minerals which afford perhaps a shorter afterglow 

 than uranium glass. In one or the other of these ways I have 

 obtained a band of green light from willemite, from Franklin, 

 N. J., and a band of crimson light from corundum, from near 

 Franklin, Macon Co., N. C. I have no doubt that other min- 

 erals affording too short an afterglow to be at all pronounced 

 with the cylinder revolved by a hand power motor, would be 

 effective with my light cylinder set in rotation at the high speed 

 of an electromotor. 



By further modification the apparatus may be used in two 

 other ways. The hollow pasteboard cylinder employed is 

 closed by a solid wooden block at the end which is fixed upon 

 the axle of the electric motor. The other end may be closed 

 with a paper cover, or left open ; in the former case I attach to 

 the inside of this cover ^ a spring forceps, by means of which an 

 object such as a diamond, a ruby, or a piece of willemite may 

 be held exactly in the center of the cyhnder. The cylinder is 

 provided with a side opening through which the light from a 

 lantern condenser may be focused upon the object in the center 

 of the cylinder when the opening is on the side away from the 

 observer, and through which the side of the object just pre- 

 viously illuminated, may be seen by the observer wholly 

 screened from any light whatever, when the opening is on the 

 side of the cylinder toward the observer. If the object be thus 

 examined in a totally dark room and affords no afterglow, noth- 

 ing whatever is seen ; but if it affords an afterglow, it becomes 

 visible owing to the persistence of vision, with a characteristic 

 colored light when the cylinder rotates with sufficient speed, 

 and its brilliancy increases as the speed of rotation of the 

 cylinder further increases. 



In the latter case a similar spring forceps supported upon a 

 suitable stand is introduced through the open end of the cylinder 

 to hold the object, which, therefore, does not partake of the 

 motion of the cylinder. The first form is adapted to both trans- 



* Modification adopted since the paper was read. Exhibited at the Annual Re- 

 ception of the Academy [Physics, No. 7], April 13, 14, 1897. 



