APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 161 



intervals round the whole circle, so that whichever points 

 forwards, the others are backwards, and out of the way. 

 There are three doors with shuttered sight-holes of darkened 

 glass, between these nozzles, so that the arc can always be 

 examined either from the back in the optic axis, or from 

 either side. The foundation circle is supported on four 

 strong pillars, standing rather outside the body, for steadiness ; 

 and the whole body revolves easily and truly upon this circle, 



MICRO. FRONT 

 Fl(J. 87. Plan of Lantern 



and may be fixed at any angle, while a spring detent ensures 

 that each front ordinarily finds its exact position. Furnished 

 with the electric lamp and focussing- table next described, 

 this form of lantern is the most complete, convenient, and 

 powerful instrument for scientific demonstration with which I 

 am acquainted, and has been adopted by the Eoyal Institution. 

 A lantern made on this plan for the arc-light must be 

 pretty large on account of the heat, as the lamp may be in 



* M 



