208 ANNUAL OF SCIENTIFIC DISCOVERY. 



only retained in darkness. I have hitherto been, unable to prevent the 

 action of diffused light, which by degrees destroys these images ; it is only, 

 so to speak, in a state of passage that the sensitive matter has the power of 

 reproducing colors. 



It will be seen, therefore, that the sensitive substance, whose method of 

 preparation is described in this work, enables us to obtain, not only -very 

 remarkable effects of color, but likewise results which are susceptible of 

 comparison, so as to observe the electrical effects due to the chemical 

 action of light. 



ON THE CONSTRUCTION OF LECTURE-ROOMS. 



Prof. Henry of the Smithsonian Institution, at the Washington meeting 

 of the American Association for the Promotion of Science, communicated 

 some views respecting the construction of lecture-rooms, and upon archi- 

 tecture, of which the following is an abstract : 



A lecture-room should be so built as to exclude the external light, for 

 it is not needed within, and prevent the waste of lights from our lamps. 

 There should be no unnecessary void space to waste the heat, and light, 

 and voice. Ventilation being properly cared for, the ceiling may be made 

 quite low. The audience should be as near as possible to the speaker, and 

 be as high as possible, consistent with good seeing on. the part of the 

 audience. The catoptric curve, if observed, gives every auditor a chance 

 to see as well as to hear. It requires that the seats should so rise as to 

 allow a direct line from the eye of each one to pass to the speaker, unob- 

 structed by the heads in the seat before. The resonance of the room must 

 be heeded. The room, large or small, will echo if naked and empty and 

 the walls are hard ; and the larger it is, the greater the danger of echoes. 

 Drape one or two sides of a room, and by -absorbing the sound you prevent 

 the resonance or echo, but do not drape the wall behind the speaker. He 

 wants that to assist his voice in reflecting the sound to the hearers ; for, 

 until you pass without " the limit of perceptibility," the reflection of the 

 soiind helps the hearer in gathering in the words spoken. A damp wall is 

 not so likely to give an echo as a dry one, nor a thin as a thick one. An 

 open door at the head of the speaker wastes his voice. The room should 

 be so arranged that the audience may be as nearly as possible before the 

 speaker. Wing walls behind him, cutting off the corners, are of service, 

 both for reflecting the sound and for the hanging of pictures, drawings, 

 maps, &c., so that they may be seen by all present. But this is the easiest 

 part of the subject ; how to arrange a room where the speakers are to 

 occupy different parts of a room, is far more difficult. 



Architecture should be looked upon more as a itsefid than a fine art. It 

 is degrading the Fine Arts to make them entirely subservient to iitility. 

 It is out of taste to make a statue of Apollo hold a candle, or a fine paint- 

 ing stand as a fire-board. But our houses are for use, and Architecture 

 is substantially one of the useful arts. In building, we should plan the 



