108 THE POLYTECHNIC INSTITUTION. 



it was a lecture-room arranged somewhat like a theatre. 

 There was a small stage at the farther side, with a curtain 

 drawn before it. In front of the stage was a narrow area, 

 which was open, and a desk, or small pulpit, with a table 

 by the side of it, for the lecturer, at one end, and something 

 that looked like a large and tall box on wheels at the other 

 end. Immediately back of this open area were the seats 

 for the audience, w T hich rose one above another by a rapid 

 ascent, so that every body could see. 



There were various exhibitions and performances during 

 the lecture, which continued for about half an hour. The 

 lecturer was the celebrated Professor Pepper, who is dis- 

 tinguished for his tact and skill in explaining and elucida- 

 ting philosophical principles, and making every thing clear. 

 He had an assistant with him, and the first thing to be done 

 was to darken the room, and then throw a beam of very 

 strong light from a kind of lantern that the assistant had 

 upon the table across the area or, rather, along the area 

 from one side of the room to the other before the specta- 

 tors. The beam made a round and very bright spot upon 

 the wall, but was not visible on its way through the air, 

 or scarcely visible, because there was nothing there to in- 

 tercept and reflect it to the eyes of the spectators. For 

 you will recollect that, as has been explained before, light 

 is not visible by itself as it passes through space before us, 

 but only so far as it is intercepted by some substance and 

 turned from its course, and so directed into the eye. 



There were, however, some few minute motes and parti- 

 cles of dust floating in the air of the room over the area, 

 which served this purpose of intercepting and reflecting 

 the light in some slight degree, so that the path of the 

 beam through the air was not wholly invisible. The as- 

 sistant, however, soon brought it very clearly into view by 

 using something which emitted a thick smoke or dust of 



