PROPORTIONS OF DIFFERENTIAL INTERFERENCES. 41 



Fourthly, That in the constant operations of composition and 

 decomposition, its active agency is ever ready for employment ; 

 existing, as it does, as a never -failing fund of matter, reduced, 

 for the purpose of its admirable office, to the minutest state 

 (as to its original constituents), that can possibly be conceived 

 to exist. 



Fifthly, That when from its treasury a sufficient quantum of 

 its material has become chemically combined with fixed bodies, 

 a further supply to repletion is afforded by emanation from that 

 luminous body, the centre of our system, or rather from the 

 effulgent atmosphere of the sun. 



The calculations made as to what may be termed the differ- 

 ential interferences of light, have been brought to an extreme 

 degree of precision by Dr. T. Young. They are stated at 

 .00000258th of an inch for the red ray, and .00000157th part 

 of an inch for the violet-colored end of the spectrum, while the 

 rays (compound and simple) occupying the intermediate space 

 are of gradations proportional, or nearly, by Newton's prism, as 

 follow : 



Red. Orange. Yellow. Green. Blue. Purple. Violet. 

 5 to 4J 4 3i 3 3J 3 



When two equal rays of red light fall on a sheet of white 

 paper in a dark room, they produce a red spot of double the 

 intensity of what each of them separately afford ; but to effect 

 this, the difference in the length of the two rays from the ema- 

 nating point to the paper must be .00000258th part of an 

 inch ; but the same effect will take place if the difference in 

 the lengths of the two rays are two, three, or four times, 

 &c., the quantity ; but if the difference is but one-half the 

 .00000258th part of an inch, or to its 1J, 2, 3J, &c., part, 

 the one light will extinguish the other, and produce absolute 

 darkness where the red spot had before appeared on the paper ; 

 while if the difference be but 1J, 2J, 3}, &c., of .00000258th 

 part of an inch, the red spot produced by the combined beams 

 will only equal one alone. 



