OF NEWTON'S OPTICS. 



51 



put into fits of easy reflection and trans- 

 mission, and those which meet the se- 

 cond surface in a fit of easy transmission 

 will penetrate it, while those which meet 

 it in a fit of easy reflection will be re- 

 flected from it. 



(68.) The interval of the fits changes 

 with the refracting power of the thin 

 medium traversed by the light. The 

 relation of this interval to the refracting 

 power was detected by Newton, and 

 may be exhibited as follows : 



With a centre C (Jig. 49) and a radius 

 C A, describe a semicircle, and from any 



Fig. 49. 



point G, through the centre C, draw the 

 right line G C H. From H draw H m 

 parallel to B A, and meeting at m a line 

 E D drawn through the centre perpendi- 

 cular to B A. Let H m represent the 

 length of the interval of the fits when 

 the ray traverses a vacuum. To find 

 the length of a fit when the medium tra- 

 versed by the ray is water, draw C H in 

 that direction which a ray of light com- 

 ing in the direction GC would take in 

 passing from a vacuum, through the 

 surface B A, into water. Let this direc- 

 tion be C H', and from H' draw H'm' 

 parallel to B A. This line H' m 1 will 

 be the interval of the fits when the ray 

 passes through water. Again, if the 

 medium be glass, draw C H" in the 

 direction which the ray G C would take 

 in passing from a vacuum into glass, 

 and H" m" is the interval of the fits. 



Thus it appears that the greater the 

 refracting power of the medium tra- 

 versed by the light, the shorter will be 

 the interval of its fits. 



Newton discovered this law by intro- 

 ducing transparent liquids between the 



lenses, whose refracting powers were 

 greater than that of air. " Upon wetting 

 the object glasses at their edges the 

 water crept in slowly between them, and 

 the circles thereby became less and the 

 colours more faint, insomuch that, as 

 the water crept along, one-half of them, 

 at which it first arrived, would appear 

 broken off from the other half and con- 

 tracted into a less room. By measur- 

 ing them I found the proportion of their 

 diameters to the diameters of the like 

 circles made by the air, to be about seven, 

 to eight, and consequently, the intervals 

 of the glasses at like circles, caused by 

 the water and air, to be as three to 

 four*. Perhaps it may be a general 

 rule, that if any other medium more or 

 less dense than water be compressed be- 

 tween the glasses, their intervals, at the 

 rings caused thereby, will be to their 

 intervals caused by interjacent air, as the 

 sines are which measure the refraction 

 out of that medium into air." These 

 sines are the lines H m, H' m', H" m", 

 &c. in our explanation. 



(69. ) It is remarkable, that whatever be 

 the matter of which the then transparent 

 medium traversed by the light is com- 

 posed, the same colours will be reflected 

 in the same order. This arises from the 

 circumstance of the fits being regulated 

 by the same law in all substances, one 

 differing from another only in the length 

 of the interval of the fits, and whenever 

 the length of the fit for any one species 

 of homogeneous light undergoes any 

 change arising from a change in the re- 

 fracting power of the medium, the in- 

 tervals for all the other species of homo- 

 geneous light undergo a proportional 

 change. 



(70.) The scale for determining the 

 tints corresponding to different thick- 

 nesses of the transparent medium would, 

 accurately constructed, be a very exact 

 method, not only of ascertaining those 

 tints, but also the primary colours of 

 which they are composed. Newton has 

 given the following Table of the thick- 

 ness of air, water, and glass, which re- 

 flect tints corresponding to the several 

 series of rings described in (54). The 

 numbers express millionths of an inch. 



* The intervals of the glasses, being in the proper, 

 tion of the squares of the diameters of the rings, are 

 more accurately as 49 to 64 ; very nearly the same 

 ratio. 



E2 



