48 



A POPULAR ACCOUNT 



thickness of the air between the lenses 

 had so far increased as to be represented 

 by A 2. 



As the parallel to AY is moved to- 

 wards 3, the lines which are intercepted 

 between the upper curves and their 

 respective bases rapidly diminish and 

 disappear, while those which correspond 

 to the bases Y Y', and O O' attain their 

 maxima. Hence it appears, that by 

 this increase of thickness the reflection 

 of the bluish lights is subdued, and the 

 yellow and orange tints appear. When 

 the parallel arrives at 3, the line r a re- 

 presenting the red light attains its max- 

 imum, the blues and violets altogether 

 disappearing. Here the reflected tint 

 will be red. 



The first white ring which appeared 

 between the lenses was succeeded by a 

 yellow which passed through an orange 

 into a red one. The increased thickness 

 of the air between the lenses, in receding 

 from the centre, accounts for this suc- 

 cession of colours as explained above. 



As the parallel to A Y moves towards 

 4, 5, the lines representing the quantities 

 of the red lights reflected gradually 

 diminish and disappear, the indigo being 

 on the increase, and the violet at its 

 maximum. Hence the light reflected 

 at this thickness will have a violet hue. 

 Such is, in fact, the first ring of the 

 second series between the lenses (54). 

 As the parallel arrives at 5, the violet is 

 on the decrease, the blue attains its 

 maximum, and the reddish tints vanish. 

 The colour reflected will, therefore, be 

 blue, and corresponds to the blue ring 

 in the second series between the lenses. 



When the parallel to AY arrives at 6, 

 the lines representing the reds and violets 

 disappear, the blue and green are only 

 partially reflected, and the yellow is at its 

 maximum. The partial reflexion of the 

 blue and green mixed with the intense 

 yellow, produces a yellowish green. This 

 thickness of the air is that at which the 

 green ring is reflected in the second 

 series of rings between the lenses. After 

 the parallel passes 6, the yellow predo- 

 minates, the other lights being but 

 faintly reflected, as appears by inspect- 

 ing the curves. The yellowish green 

 last mentioned, therefore, gradually 

 changes into a bright yellow. This 

 corresponds to the yellow ring of the 

 *;econd series. 



As the parallel approaches 8, the red 

 Ijnd orange increase ; the greens, yel- 

 -, and blues nearly vanish ; the vio- 



ts are copiously reflected, but the 



stronger influence of the red and orange 

 gives the reflected light a glowing crim- 

 son hue. This corresponds to the last 

 ring of the second series between [the 

 lenses. 



At C, the red and orange are copi- 

 ously reflected ; the yellow, green, and 

 violet are not reflected in any perceptible 

 quantity, but the indigo is abundant and 

 the blue considerable. The result of the 

 composition of these lights is a rich 

 purple of a ruddy character, which cor- 

 responds to the first ring of the second 

 series between the lenses. By following 

 in this manner the parallel as it moves 

 from A Y, we shall be able to trace dis- 

 tinctly the lights reflected by plates of 

 air of every degree of thickness, and we 

 shall perfectly account for the succes- 

 sion of coloured rings, as far as they 

 are observable in the experiment with 

 the lenses. 



(62.) We have here constructed curves 

 to represent the quantities of reflected 

 light of seven distinct degrees of refrangi- 

 bility. But it must be remembered that 

 solar light consists of rays of every de- 

 gree of refrangibility between certain 

 extreme limits. To describe the phe- 

 nomena perfectly, there should there- 

 fore be an infinite number of other 

 curves between each pair of those 

 already exhibited, and with bases of 

 intermediate magnitude. Two such 

 curves having bases nearly equal, cor- 

 respond to lights which differ but little 

 in refrangibility, and which have no 

 perceptible difference in colour. When 

 the thickness of the plate of air becomes 

 considerable, the number of repetitions 

 of the bases of two curves, whose bases 

 have a very small difference, will be so 

 great, that the points of contact will be 

 separated by a considerable interval. 

 Accordingly, it must at length happen, 

 that when one ray is most copiously re- 

 flected, another of very nearly the same 

 colour will not be reflected at all. As 

 this is necessary to explain why a thick 

 plate of air will only reflect white light, 

 and that but faintly, we shall explain it 

 more fully. 



Let Y Y', instead of representing the 

 interval of the fits of the yellow light, 

 represent that of the fits of green light 

 differing a little in refrangibility and 

 not perceptibly in colour from the light 

 whose fits are at the interval G G'. 

 Suppose that the difference between 

 Y Y' and G G' is the fortieth part of 

 G G' ; after passing through a thickness 

 equal to twenty repetitions of G G', the 



