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or there might be a series of prime elevenths or thir- 

 teenths, the latter of which are sometimes heard in unac- 

 companied part-songs ; but such possible refinements may 

 for the present be very well dispensed with. If a key- 

 board can be devised which shall contain these four prin- 

 cipal series in perfect intonation, the problem will be 

 practically settled. These series, to recapitulate, are as 

 follows : 



1. Key-notes, deduced from perfect fifths, denoted by 

 a Koman capital, or in musical notation by a note of a 

 red color, as suggested by Prof. Poole. On the key- 

 board, a white digital may be used. 



2. Major thirds from key-notes, denoted by a Roman 

 lower case letter, or by a note of a yellow color, or a 

 black digital. 



3. Prime sevenths from key-notes, denoted by a Gothic 

 capital, or by a note or digital of a blue color. 



4. Major thirds from the tones of series 2, denoted by 

 an Italic lower case letter, or by a note or digital of an 

 orange color. 



The distinctive colors, red, yellow, blue, and orange, 

 were selected by Prof. Poole on account of an analogy 

 between the relative number of vibrations producing those 

 colors, and the relative number of vibrations producing 

 the third, the fifth, etc. Black and white were adopted 

 for the digitals because of the convenience of ebony and 

 ivory as materials. The use of the colored notation in 

 writing chords exhibits in a very curious and remarkable 

 way the most important principles of harmony ; but to 

 the discussion of this very interesting subject the present 

 paper cannot be extended. In order to have all these 

 four series complete in all the keys from C[y to F:ff, there 

 are required sixty-two notes to the octave. The only 

 method of bringing this great number of digitals within 



