OF NEWTON'S OPTICS. 



59 



glass, and reflected by the posterior sur- 

 face. The rays thus irregularly re- 

 fracted by the first surface fell upon the 

 second surface at various obliquities, 

 and were reflected by it regularly and ir- 

 regularly, so as to return through the 

 first surface at various obliquities. 



We must here call the recollection of 

 the reader to a property of thin plates 

 already mentioned. A plate of a cer- 

 tain thickness reflects or transmits a 

 certain colour, the rays of light being 

 perpendicular to it. The rays being still 

 perpendicular, every change of thickness 

 will produce a corresponding change of 

 hue in the reflected and transmitted lights. 

 The very same effects, the same shifting 

 of tints, which is thus produced by a 

 change of thickness, the rays remaining 

 perpendicular, may also be produced by 

 changing the direction of the rays, the 

 thickness of the plate remaining unva- 

 ried. Thus a plate of a given thickness, 

 at a certain obliquity, will transmit red 

 light, at another yellow, at another green, 

 and so on. Also, if a ray of a particu- 

 lar colour gradually increases its obli- 

 quity from the perpendicular direction, 

 it will be alternately reflected and trans- 

 mitted for many successions. 



The glass mirror used in the experi- 

 ment to which we now refer was, in 

 every 'part, of exactly the same thick- 

 ness ; therefore the rings of colour could 

 not, as in the case of the lenses, be 

 ascribed to a regular variation of thick- 

 ness. The light, however, reflected irre- 

 gularly by the silvered surface of this 

 speculum, encountered the first surface 

 after reflection at various obliquities. 

 Those rays which met the first surface 

 at obliquities proper for transmission, 

 were transmitted to the screen, where 

 they depicted their proper colour, and 

 those which fell upon it at other angles 

 were intercepted. Thus, suppose a ray 

 of homogeneous red light radiated from 

 the centre to the speculum, allj the re- 

 gularly reflected rays returned to the 

 centre. Those which being irregularly 

 reflected by the second surface met the 

 first surface at obliquities proper for 

 transmission, were propagated in cor- 

 responding directions to the screen, 

 and they produced a red tint. These 

 rays being regularly disposed in circles 

 round the axis of the speculum, de- 

 picted circles of red light on the screen, 

 and the rays returning to the first sur- 

 face at intermediate obliquities not 

 being in fits of transmission, were in- 

 tercepted, and thus caused the dark 



circles between the luminous red ones 

 on the paper. What we have here said 

 of red light may be equally applied to 

 homogeneous lights of other colours ; 

 and, hence, will easily be collected the 

 cause of the various coloured rings 

 produced when the light which emanates 

 from the centre of the speculum is com- 

 pound solar light, this effect being 

 nothing more than the result of the si- 

 multaneous exhibition of the rings of 

 each colour. 



(81.) Newton points out one diiference 

 between the effect of the speculum on 

 the light and that of the thin plates, de- 

 scribed in Chapter VII. In the latter 

 case the colours are produced by alter- 

 nate reflections and transmissions of 

 the light at the second surface of the 

 plate, after one passage through it ; 

 but here they pass fronf the first surface 

 to the second, and then return from the 

 second to the first, there being either 

 transmitted to the screen or reflected to 

 the silvered surface, according as they are 

 in fits of easy transmission or reflection. 



By measuring the diameters of the 

 rings of the different lights, and com- 

 paring this with the distance of the 

 screen from the speculum, Newton as- 

 certained the obliquities at which the 

 lights of different colours emerged from 

 the speculum, and thence derived the 

 angles at which they were incident on 

 the first surface after reflection at the 

 second. On comparing these with the 

 obliquities corresponding to the lights 

 of different colours deduced from the 

 theory established in Chapter VII., he 

 found a perfect accordance. 



(82.) In order, however, to put the mat- 

 ter to a more decisive test, he calculated 

 the effect which the thickness of the 

 glass which formed the speculum would 

 produce upon the diameters of the seve- 

 ral rings, and found that, according to 

 the theory of fits of easy reflection and 

 transmission, the squares of the diame- 

 ters of corresponding rings produced 

 by different mirrors should be in the in- 

 verse proportion of the thickness of the 

 mirrors. He, therefore, procured ano- 

 ther speculum, ground on both sides, to 

 the same sphere with the former. Its 

 thickness was $% parts of an inch, and 

 the diameters of the first three bright 

 rings, measured between the brightest 

 parts of their orbits, at the distance of 

 six feet from the glass, were 3, 4J, 5 

 inches. Now, the thickness of the other 

 glass being of an inch, was to the 

 thickness of this as i to ft, or as 62 to 



