APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 175 



mounting it like the glass prism shown in fig. 98, with a spin- 

 like rod or tube A B, which fits in the sockets of the pillar- 

 stands. Though such a prism does not give sufficient dis- 

 persion for many spectrum experiments, it is very convenient 

 for elementary ones. 



One or two focussing lenses should be mounted in the same 

 way ; but it is better, and shields stray light from the screen, 

 and thereby improves the projection, for each lens to be 

 mounted with a rim or shield of blackened tin round it, about 

 two inches wide. The size and focus of the lenses depend 

 upon the size of room, and screen distance. For class-rooms 

 a lens of 3 inches diameter and 8 to 10 inches focus will give 

 good projections. For a larger scale, two lenses 4 inches 

 diameter, and say 9 and 

 14 inches focus, will be 

 better. For good diffrac- g 

 lion effects, and acoustical 

 projections, a lens 6 inches 

 diameter and 14 to 18 FlG 98 



inches focus, will give su- 

 perior effects, the long focus giving more room to work. 



A plane mirror formed of a piece of good plate looking- 

 glass about 6x4 inches, should be mounted in the same way, 

 the spur projecting in the plane of the mirror from the middle 

 of one of the longer sides. For some experiments a larger 

 mirror may be useful. 



Occasionally a somewhat steadier mount than these sliding 

 sockets may be needed. In that case there is nothing better 

 than the usual telescopic stem with a tightening collar, screwed 

 into the centre of a very heavy foot, and with a socket of the 

 usual gauge drilled perpendicularly into the top end. This 

 socket should have a set-screw in one side, so that any 

 apparatus can be fixed fast in the socket. 



90. Stands and Manipulation. The mounting of a de- 

 monstration lantern requires some consideration, much more 



