OF NEWTON'S OPTICS. 



45 



Difference. 

 

 5 

 5 

 3 



1* 



4 

 4 

 4 



number of these parts, in the interval 

 for each ray, is expressed in the second 

 column. 



Ten mlllionths 

 of an inch. 



Extreme rays 133 



Red rays 128 



Intermediate 123 



Orange rays 120 



Intermediate 117^ 



Yellow rays 113i_ 



Intermediate 109^ 



Green rays 105-!: 



Intermediate 101^ 



Blue rays 98 



Intermediate 94A 



Indigo rays 



Intermediate 



Violet 87 



Extreme rays 



That the magnitudes, which appear 

 in the third column of this Table, should 

 be subjects of accurate computation, 

 founded on measurements performed by 

 the hand of man, must, we conceive, be 

 matter of the greatest wonder and admi- 

 ration. An inch being divided into five 

 million of equal parts, a distance equal 

 to one of these parts is ascertained by 

 positive measurement, in the estimation 

 of the intervals of the fits of indigo rays 

 and those intermediate between them 

 and the violet! Magnitudes of such 

 minuteness far exceed even the powers 

 of imagination. We have, perhaps, a 

 distinct idea of the hundredth part of an 

 inch, by imagining ,the tenth of an 

 inch divided into ten equal parts. But 

 when we are required to conceive one 

 of these ten parts divided into fifty thou- 

 sand equal parts, imagination altogether 

 fails, and we cease to attach to the name 

 of such a magnitude any positive con- 

 ception. Nevertheless, this magnitude 

 is, as we have seen, capable of measure- 

 ment as accurately as any other, how- 

 ever gross and perceptible, so far does 

 the power of reason exceed that of the 

 imagination. 



(59.) The principles which we have 

 now explained will be found sufficient to 

 account for all the phenomena of co- 

 lours reflected, or transmitted by trans- 

 parent media. Let us suppose that two 

 glass surfaces are placed parallel at 

 the distance of 120 ten-millionth parts 

 of an inch, inclosing between them 

 a plate of air. If pure orange light fall 

 perpendicularly on the glass, it will pass 

 through, being transmitted freely by the 

 plate of air between the plates of glass. 

 For on passing from the first glass plate 

 into the air it is in a fit of easy trans- 



mission ; and since the distance between 

 the plates is equal to the interval between 

 the fits (see Table), it will be again in 

 a fit of easy transmission when it en- 

 counters the surface of the second plate 

 and will consequently pass through. If, 

 in this case, the eye be placed behind 

 the second plate, the coloured light will 

 be perceived to be transmitted. 



Now suppose the two plates to be 

 placed parallel as before, but only at 

 half the former distance asunder. The 

 orange light, on meeting the surface of 

 the second plate, will be in a fit of easy 

 reflection, and will consequently return 

 through the plate of air in the direction 

 from which it came. Having the same 

 space to move through, it will be in a 

 fit of easy transmission when it has 

 again reached the surface of the first 

 plate, and will consequently be trans- 

 mitted. If the eye be placed before the 

 first plate, it will be coloured over with 

 the orange light. 



Let us now suppose that the plates 

 are placed at the distance of 240 ten- 

 millionths of an inch asunder. This is 

 equal to two intervals of the fits of orange 

 light, and therefore that light being in a 

 fit of easy transmission when it encoun- 

 ters it, will be transmitted at the sur- 

 face of the second plate. An eye placed 

 behind the second plate will receive the 

 transmitted light, and the plate will take 

 an orange hue. 



In the middle of the interval between 

 120 and 240, that is, at the distance of 

 180 ten-millionths of an' inch from the 

 first plate, the ray will be in the middle 

 of the second interval of its fits, and will 

 therefore be in a fit of easy reflection. 

 If it encounter the second surface at this 

 distance, it will consequently be reflected. 

 When it has returned to the distance 

 120, it will again be in a fit of easy trans- 

 mission, which fit will return again after 

 passing through the distance 120, at 

 which point it will again meet the surface 

 of the first plate, and will be transmitted 

 through it. Thus an eye placed before 

 the first plate will perceive the orange 

 light reflected. 



Thus the space followed by the ray ] f 

 passing through the air being divide^ 

 into parts equal to 60 ten-millionths u 

 an inch, at each alternate point of divi 

 sion the orange ray will be in a fit o 

 easy transmission, and at the interme 

 diate points it will be in a fit of easj 

 reflection. In the one case, the orange 

 light transmitted will appear to an eye. 

 placed behind the second plate, while to 



