igo Dr. Young's Experiments and Inquiries 



at the immediate surface of the body itself, and that, by mutual 

 reflections between these two surfaces, something like the 

 anguiform motion suspected by Newton may really take place; 

 and then the analogy to the colours of thin plates will be still 

 stronger. A mixture of vibrations, of all possible frequencies, 

 may easily destroy the peculiar nature of each, and concur in a 

 general effect of white light. The greatest difficulty in this sys- 

 tem is, to explain the different degree of refraction of differently 

 coloured light, and the separation of white light in refraction : 

 yet, considering how imperfect the theory of elastic fluids still 

 remains, it cannot be expected that every circumstance should 

 at once be clearly elucidated. It may hereafter be considered 

 how far the excellent experiments of Count Rumford, which 

 tend very greatly to weaken the evidence of the modern doc- 

 trine of heat, may be more or less favourable to one or the other 

 system of light and colours. It does not appear that any com- 

 parative experiments have been made on the inflection of light 

 by substances possessed of different refractive powers ; un- 

 doubtedly some very interesting conclusions might be expected 

 from the inquiry. 



XI. Of the Coalescence of musical Sounds. 



It is surprising that so great a mathematician as Dr. Smith 

 could have entertained for a moment, an idea that the vibrations 

 constituting different sounds should be able to cross each other 

 in all directions, without affecting the same individual particles 

 of air by their joint forces : undoubtedly they cross, without 

 disturbing each other's progress ; but this can be no otherwise 

 effected than by each particle's partaking of both motions. If 

 this assertion stood in need of any proof, it jnight be amply 



