34 



OPTICS. 



served another system of circular co- The form of half of the system of 



loured rings, in which the colours were coloured rings is shown in fig. 36, and 



quite different from those seen by re- the letters in the following \able will 



flexion. The central spot, for example, point out the colours at any part of 



was white, and the colour transmitted the system. The relation of the re- 



at any. point was always the colour fleeted and the transmitted rays will be 



which, when combined with the colour understood from the following diagram 



reflected at that point, made white light, on the left-hand column, in which A B 



Fig. 36 



Fig. 37 



Red 



Yellow 



and C D represent the surfaces of the 

 lenses which touch at E. The names of 

 the colours engraved on the left hand 

 of the line A B, are those seen by re- 

 flexion, and those engraved on the right 

 hand of the line C D, are those seen by 

 transmission. 



The following table contains all the 

 results of Sir Isaac Newton's experi- 

 ments, and, if compared with figs. 36 

 and 37, it will give the reader the most 

 complete information respecting the 

 colours of thin plates. Some of the 

 leading results in it may be thus stated : 



1 . Air at and below a thickness of 



half a millionth of an inch ceases 

 to reflect light. At and above 

 a thickness of seventy-two mil- 

 lionths of an inch it reflects white, 

 or all the rays of the spectrum. 

 Between these two limits it re- 

 flects the various orders of co- 

 lours contained in the table. 



2. Water at and below a thickness 



of three-eighths of a millionth 

 of an inch ceases to reflect light. 

 At and above fifty-eight mil- 

 lionths of an inch it reflects 

 white, and between these two 

 limits it reflects the orders of 

 colours contained in the table. 



3. Glass at and below a thickness 



of one- third of a millionth of an 

 inch ceases to reflect light. At 

 and above a thickness of fifty 

 millionths of an inch it reflects 

 white, and between these limits 

 it reflects the orders of colours 

 contained in the table. 



