Astronomical and Nautical Collections. 127 



colour of the light, and that for the extreme red rays, for 

 example, it is [more than] half as great again as for the violet 

 rays situated at the other extremity of the spectrum. 



It may easily be imagined that the number of different 

 undulations is not limited to the seven principal ones which 

 are indicated in the table, and that there must be a multi* 

 tude of intermediate magnitudes, and others beyond the red 

 and the violet rays : for the ponderable particles, of which the 

 oscillations give rise to them, must be subjected to forces that 

 are infinitely varied, in the combustion or the incandescence 

 of the bodies which excite the motions of the ether: and it is 

 on the energy of these forces that the duration of each oscilla- 

 tion depends, and consequently the length of the undulation 

 produced by it. It is found that all the undulations com- 

 prehended [in the air] between the lengths .0000167 E. L 

 and .0000244, are visible ; that is, are capable of exciting 

 vibrations in the optic nerve : the rest are only sensible by 

 their heat, or by the chemical effects which they produce. 



It has been remarked, that when two systems of waves 

 differ half an undulation in their progress, two of the semi* 

 undulations must escape from interference ; that six must be 

 exempt when the difference amounts to three semiundula- 

 tions; and that, in general, the number of undulations exempt 

 from interference is equal to the number of lengths of a semi^ 

 undulation separating the corresponding points of the two 

 systems. While this number is very small in proportion to 

 that of the waves contained in each system, the motion must 

 be nearly destroyed, as in the case of the exemption of a 

 single undulation. But it may be imagined that, as we in- 

 crease the difference of the progress of the two pencils, the 

 undulations exempted from interference may become a mate- 

 rial portion of each group, and that it may finally become so 

 great as to separate the groups entirely from each other ; 

 and in this case the phenomena of interference would no 

 longer be observable. If, for example, the groups of undu- 

 lations consisted but of a thousand each, a difference of one- 

 twentieth of an inch in their routes would be much more 

 than sufficient to prevent the interference of the rays of all 

 kinds. 



