MODERN THEORIES OF HEAT AND LIGHT 



testing the light from "Mr. Coventry's exquisite mi- 

 crometers," which consist of lines scratched on glass at 

 measured intervals. These microscopic tests brought 

 the same results as the other experiments. The colors 

 were produced at certain definite and measurable 

 angles, and the theory of interference of undulations 

 explained them perfectly, while, as Young affirmed 

 with confidence, no other hypothesis hitherto ad- 

 vanced would explain them at all. Here are his 

 words : 



" Let there be in a given plane two reflecting points 

 very near each other, and let the plane be so situated 

 that the reflected image of a luminous object seen in it 

 may appear to coincide with the points ; then it is obvi- 

 ous that the length of the incident and reflected ray, 

 -taken together, is equal with respect to both points, 

 considering them as capable of reflecting in all direc- 

 tions. Let one of the points be now depressed below 

 the given plane; then the whole path of the light re- 

 flected from it will be lengthened by a line which is to 

 the depression of the point as twice the cosine of inci- 

 dence to the radius. 



" If, therefore, equal undulations of given dimensions 

 be reflected from two points, situated near enough to 

 appear to the eye but as one, whenever this line is equal 

 to half the breadth of a whole undulation the reflection 

 from the depressed point will so interfere with the re- 

 flection from the fixed point that the progressive mo- 

 tion of the one will coincide with the retrograde mo- 

 tion of the other, and they will both be destroyed ; but 

 when this line is equal to the whole breadth of an un- 

 dulation, the effect will be doubled, and when to a 



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