COLOURS OF THIN PLATES. 133 



These determinations belong to the most luminous rays 

 of the spectrum, or those at the confines of the orange and 

 yellow. 



(146) The variation of the diameters of the rings (or of 

 the thicknesses of the plate of air at which they are exhibited) 

 with the colour of the light, may be observed by illuminating 

 the glasses with different portions of the spectrum in succes- 

 sion, or, yet more simply, by looking at the rings through 

 coloured glasses ; and it is found that the magnitude of the 

 rings is greater, the less the refrangibility of the light. This 

 being understood, it is easy to comprehend the cause of the 

 succession of colours in each ring, when white or compound 

 light is used. For the rings, in this case, are the aggregate 

 of the rings of different colours ; and these being of different 

 magnitudes, the compound ring will be variously coloured, 

 the more refrangible rays occupying the interior, and the 

 less refrangible the exterior parts of the Ting. It is easy 

 to see also, that all phenomena of colour must disappear 

 after a few successions, the rings of different colours, belong- 

 ing to different orders, being at length superposed. 



The variation of the rings (and therefore of the thick- 

 nesses) with the obliquity of the incident light may be observed 

 by depressing the eye. The rings are then seen to dilate 

 rapidly with the obliquity of the reflected pencil; the 

 thicknesses of the plate of air at which they are exhibited 

 being nearly as the secants of the angles of incidence or re- 

 flexion. 



The fourth and last law, which expresses the depend- 

 ence of the thickness at which any ring is formed upon the 

 refractive power of the plate, is easily verified by introducing 

 a drop of water between the glasses. The rings are then ob- 

 served to contract ; and if we compare their diameters in air 



