OF ARTS AND SCIENCES. 171 



area to be examined. When large pieces of quartz, the tempered 

 glass, or selenite figures, are examined under these circumstances, the 

 superiority of the illuminating arrangement is very decided, and al- 

 lows of the picture being displayed on a much larger scale than would 

 be otherwise practicable. 



"2. Since Newton's prime experiment on the prismatic dispersion 

 of white light, many contrivances have been used to produce the 

 synthetical counterpart to this grand result of experimental analysis. 

 The various methods of uniting the prismatic colors again, so as to 

 restore the white light, are enumerated by Moigno, in his Repertoire 

 (TOptique, Vol. IV. p. 1370, as given by Dove. 



" It is desirable to obtain the original pure prismatic tints, and then 

 unite the rays, if possible, by some process independent of refraction, 

 and involving no obscure process. Von Miinchow does this by giving 

 a reciprocating motion to the prism, and Steinheil, by turning a prism, 

 one side of which is ground or blackened so as to intercept the light, 

 rapidly round an axis parallel to the refracting edge. In either case, 

 as soon as the motion acquires a certain velocity, the colored spec- 

 trum vanishes, and a sti*eak of white light appears in its place. This 

 subjective mixture of the colors may be effected in a cheaper manner, 

 by any one in possession of a water-prism, as follows. The light is 

 introduced through one of the inclined glass sides of the prism, at 

 such an angle as to emerge from the upper surface of the water. If 

 the prism stands firmly upon a table, a spectrum will be projected 

 upon the ceiling of the room. But a moderate tap with the finger upon 

 the table will communicate a rapid vibration to the hollow prism, and 

 thence to the water contained in it, and the little change in the re- 

 fracting angle which ensues will make the spectrum dance backward 

 and forward in the direction of its length. As soon as the dance be- 

 gins, the spectrum, which hitherto had been of the usual colors, is 

 converted into a long streak of white light. 



" 3. The only remaining experiment to which I shall allude in this 

 communication carries us out of the province of Optics, and into that 

 of Acoustics. It is well known that, when a tube filled with air is 

 skilfully blown by the mouth at the embouchure, or a vibration in 

 the inclosed column of air is otherwise excited, it is capable of ren- 

 dering, without any fingering and without holes to finger, a certain 

 series of sounds, which are called harmonics. In the humming-top, 

 we may presume that the reaction is the same between the air inside 



