ANALOGY OF SOUND AND LIGHT, 151 



see most of the right-hand side of the lamp, and with the 

 left eye more of the left-hand side. These two images 

 are combined, and we see an object which we know to 

 be round. 



This is illustrated in a most interesting manner ty 

 the little optical instrument, the Stereoscope. It consists 

 either of two mirrors placed each at an angle of 45, or 

 of two semi-lenses turned with their curved sides towards 

 each other. To view its phenomena, two pictures are 

 obtained by the camera obscura on photographic paper 

 of any object in two positions, corresponding with the 

 conditions of viewing it with the two eyes. By the 

 mirrors or the. lenses these dissimilar pictures are com- 

 bined within the eye, and the vision of an actually solid 

 object is produced from the pictures represented on a 

 plane surface. Hence the name of the instrument; 

 which signifies, Solid 1 see. 



Analogy is often of great value in indicating the 

 direction in which to seek for a truth ; but analogical 

 evidence, unless where the resemblance is very striking, 

 should be received with caution. Mankind are so ready 

 to leap to conclusions without the labour necessary for 

 a faithful elucidation of the truth, that too often a few 

 points of resemblance are seized upon, and an inference 

 is drawn which is calculated to mislead. 



There is an idea that the phenomena of sound bear a 

 relation to those of light, that there exists a resem- 

 blance between the chromatic and the diatonic scales. 

 Sound, we know, is conveyed by the beating of material 

 particles the air upon the auditory membrane of the 

 ear, which have been set in motion by some distant dis- 

 turbance of the medium through which it passes. Light 

 has been supposed to act on the optic nerve in the same 

 manner. If we imagine colour to be the result of vibra- 

 tions of different velocities and lengths, we can under- 

 stand that under some of these tremors, first established 

 on the nerves, and through them conveyed to the brain, 



