APPENDIX. 925 



69 7b. Suitable source of Sound for experiments with 

 Sensitive Flames. Prof. W. F. Barrett. 



This is simply a loud ticking watch enclosed in a padded case with a 

 movable cover, and mounted on a sliding stand. 



69 7c. Practical application of Sensitive Flames. 



Prof. W. F. Barrett. 



By using a suitable burner a sensitive flame can be made to spread out 

 sideways into a fish-tail flame under the influence of sound. Under such con- 

 ditions the flame touches the compound metallic ribbon, which curves by 

 unequal expansion, closes an electric circuit, and rings an electric bell. An 

 arrangement of this kind could be adapted to detect burglars, or to act as a 

 self-recording phonoscope. First exhibited by Prof. Barrett at the Royal 

 Dublin Society, January 1868. 



787a. Electro-Diapasons, showing the composition of vibra- 

 tory movements by producing fixed acoustic figures. 



M. Mercadier, Paris. 



816j. Wollaston's original periscopic Camera Lucida. 



See Tilloch, xxvii. (1807), p. 343 ; Phil. Trans., 1812, p. 370. 



Wollaston Collection, Cavendish Laboratory ', Cambridge. 



816k. Wollaston's Camera Lucida adapted to Tele- 

 scope. 



Wollaston Collection, Cavendish Laboratory, Cambridge. 



857a. Automatic Motion for the Spectroscope. 



Walter Baity, Leeds. 



The apparatus consists of an axle and four parallel discs, the outer pair being 

 fixed, and the inner pair rigidly connected together, and capable of turning on 

 the axle. Between the inner discs are four arms, which also turn on the 

 axle. 



Taking the centre of the axle as origin, each disc has 4 slits, the equation 



to their middle lines being r = F(0), r = F (j[) r = ^^}> ? ' = E (')' The discs 



in each pair are placed with their slits parallel, but the inner discs are turned 

 over. The arms have straight slits radiating from the axle. A pin is passed 

 through the first slits of the outer discs, the 4th of the inner discs, and the 

 slit in one of the arms. The remaining arms are connected with the discs by 

 three other pins inserted in a similar manner. The first and last prisms are 

 fixed on the initial lines of the outer and inner pair of discs respectively, and 

 the four other prisms are carried by the arms. Motion is given by moving 

 the inner discs. The angles at which the slits cross are constant and differ 



a / 



least from right angles if F(0) = ae ', which is the form adopted in the 

 model. 



877a. Handy Folariscope with Nicol prisms to show rings 



in biaxial crystals. W. Previte Orion. 



This instrument was designed by the exhibitor and made for him by 

 Pastorelli. Its object is to utilize the Kicols of a small microscope so as 

 to show the effect of polarised light in a biaxial crystal ; and further, to do 

 this in a handy and manageable and inexpensive form. 



