COLOURS OP THIN PLATES. 67 



plates in the instructive form in which their laws have since been 

 studied, namely, by placing two object-glasses in contact ; and he 

 found that any transparent fluid introduced between the lenses 

 furnished a succession of colours as well as air ; the colour, how- 

 ever, being more vivid, the more the refractive power of the plate 

 differed from that of the glasses within which it was inclosed. 



The attention of Newton was soon after directed to the same 

 subject ; and his investigations, which ended in the complete dis- 

 covery of the laws of the phenomena, will ever be considered as a 

 model of experimental inquiry. A convex lens of glass being laid 

 upon a plane surface of the same material, after the manner of 

 Hooke, the bands of the same colour are arranged round the point 

 of nearest approach in concentric circles ; and the diameters of 

 these circles will be obviously as the square roots of the thicknesses 

 of the plate of air at the points at which they are exhibited. In 

 order to investigate the relation between the colour and the thick- 

 ness, then, it was only necessary to measure the diameters of these 

 rings in the different species of simple light ; and taking similar 

 measurements when the other circumstances of the phenomena 

 were varied, Newton deduced their laws, as they depended on the 

 substance of the reflecting plate, and on the obliquity of the inci- 

 dent pencil. Newton observed, moreover, that there was a second 

 system of rings formed by transmission. The transmitted rings 

 were found to observe the same laws with this remarkable excep- 

 tion, that the colour transmitted at any particular thickness of the 

 plate was always complementary to that reflected at the same 

 thickness, so that in homogeneous light, the bright transmitted 

 ring is always found at the same distance from the centre as the 

 corresponding dark one of the reflected system. 



The observations of Mariotte,* Mazeas,f and Du Tour, have 

 added nothing essential to the laws discovered by Newton. Most 

 of these observations, in fact, related to the colours exhibited by 

 the plate of air inclosed between two plane glasses, and in circum- 

 stances, therefore, much less favourable to the analysis of the 

 phenomenon than those selected by Newton. Perhaps the most 

 interesting of the facts noticed by Mazeas are the effects produced 



* Traite de la Lumiere et des Couleurs. 

 f Memoires presentes, torn. ii. 

 Ibid., torn. iv. v. vi. 



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