DEMONSTRATIONS OF APPARATUS 215 



are habitually interspersed. Otherwise, the radiant must be 

 readily adjustable in order to obtain good results. The 

 practical importance of this matter in demonstrations cannot 

 be exaggerated. 



The inversion of the image is a third difficulty, but is 

 met by the use of an erecting prism. Each must decide for 

 himself how often this is necessary ; the only practical incon- 

 veniences about it are the space required in front of the objec- 

 tive, and the time needed for adjustment. The loss of light, 

 when properly adjusted at the smallest point of the pencil of 

 rays, is not probably more than 40 per cent., and this (for most 

 apparatus) leaves ample margin. 



108. Method and Scale of Experiment. Whilst, how- 

 ever, a vast number and variety of experiments, by reducing 

 the size of the apparatus until it comes within the field of 

 the condensers, can be demonstrated by projection, it is by 

 no means the purpose of this work to urge that this should 

 always be done. That is always a question for practical con- 

 sideration ; weighing on the one hand the saving in space 

 and weight of apparatus, and on the other ths comparative 

 effectiveness of the direct and projection methods. In many 

 experiments the direct method will remain the only one 

 available ; while in many more, where the method is optional, 

 it will depend upon particular circumstances which is to be 

 chosen. Let us take two illustrations from experiments 

 connected with the elasticity of bodies. 



Simple elasticity is easily illustrated in the lantern, by 

 adjusting upon the table, concave side upwards, a spare (or 

 cracked) meniscus lens from a condenser, in a cell or not 

 and dropping upon the centre, from the top of the field, a 

 well-tempered inch steel ball from a bicycle bearing. Or 

 *he concave bottom of a thick glass tumbler will answer instead 

 of the lens. The ball will rebound nearly to the height it 

 fell from, and the concavity will keep its motions within the 

 field ; and this small and simple apparatus will suffice. That 



