ON LIGHT. 307 



we have explained the complete or very nearly com- 

 plete darkness of the central spot, and of a series of 

 rings correspondicg to thickness of i, 2, 3, or more 

 semi- wave lengths. At the intermediate thicknesses 

 (i.e., of i, 3, 5, &c., ^/tfr&r-wave-lengths) the exact re- 

 verse will happen the reflected rays will start together 

 in harmony and appear as a ray of double intensity, 

 thus explaining the intermediate bright rings. 



(90.) In the case of the rings produced between two 

 glasses of the same material, the intermediate film being 

 air, it is the reflexion from its first surface, not its second, 

 that is effected from a rarer medium ; so that it is at this 

 surface that the additional semi- undulation is gained by 

 the first reflected ray. In all other respects the reason- 

 ing is the same in both cases, and the explanation 

 equally complete in both. 



(91.) It will be perceived that we have not been 

 sparing of words in this explanation. The epigram- 

 matic style is ill-suited to clearness in the exposition 

 of a principle which it is essential to seize with perfect 

 distinctness, and in seizing which considerable difficulty 

 is commonly experienced. If any doubt or misgiving, 

 however, should still linger on the mind as to the appli- 

 cability of the analogy by which the loss of half an un- 

 dulation necessitated by the blackness of the central 

 spot has been explained, a simple but striking experi- 

 ment will suffice to dissipate it. Let a set of rings be 

 formed by interposing, between two glasses of very dif- 

 ferent refractive densities, a film of liquid intermediate 

 -in that respect as, for instance, oil of sassafras be- 



