264 OPTICAL PROJECTION 



to the plane-mirror M, which can be revolved on the perpen- 

 dicular pivot P. Thence it is reflected in the direction of the 

 screen at s, and it will be manifest that a slight revolution of 

 M opens out the scroll. 



This arrangement also enables us to combine any two sets 

 of vibrations in harmonic addition, as well as rectangularly. 

 Eectangularly an octave gives the well-known 8-figure ; but if 

 both reed-boxes are fixed perpendicularly and a scroll opened 

 out by rotating the mirror M, the harmonic combination is ex- 

 ceedingly instructive, as is also the optical representation of 

 any slight departure from unison, or any other intervals. 



RR 



FIG. 140. Plan of Apparatus 



Such an apparatus is far the best for the projection of 

 compound figures, being superior to the most expensive forks 

 in many respects. For its efficiency, it is also far the cheapest. 

 It possesses the following advantages, which are not found in 

 combination in any other apparatus so far as I know. 



(a] It projects with ease all compound figures and scrolls. 

 To project beats, all that is necessary is to fix the reed-box 

 EE (fig. 140), perpendicularly, the same as the other, insert 

 a second reed in unison with E, and having tuned (see g below) 

 rotate the mirror M, to give the scroll. When scrolls are not 

 wanted the mirror M is simply left unmoved. 



