26 



Mr. J. Croll on the Excentricity 



First spectrum. 





Second spectrum. 



Great incidences. White. 



Great 



incidences. 



Blue. 



Pale red. 







Bluish. 



Red. 







Less blue. 



Purple. 







Bluish whit< 



Blue. 







White. 



Bluish. 







Pale red. 



Less blue. 







Red. 



Lesser incidences. White. 



Lesser incidences. 



j Purple. 



1 BlnP 



At small angles of incidence, about 42°, the bands become less 

 distinct and paler in colour, the white becoming yellow, and the 

 blue brownish. 



In the systems of grooves, whether on glass or on steel, em- 

 ployed in the preceding experiments, the part of the original 

 surface not removed by the grooves bears a very considerable 

 proportion to the part removed; but when the grooves occupy 

 a large part of the surface, and the intermediate parts a very 

 small one, a new set of phenomena are produced, which must 

 change in a remarkable manner all the bands of interference. 

 The execution, however, of such systems of grooves is very dif- 

 ficult. Sir John Barton, with all his experience, failed in pro- 

 ducing good specimens ; but even with those which he executed 

 for me, phenomena of a remarkable kind were exhibited not only 

 on the middle or colourless image, but upon all the paragenic 

 spectra, varying with the number of grooves, but still more 

 remarkably with the angle of incidence*. 



P.S. — The preceding experiments were made in 1823 and 

 1827, and those described in p. 24 were repeated in 1838. 

 Having lost or mislaid the glass gratings which I then employed, 

 1 am not able to compare the bands which they produced with a 

 more remarkable series which I have recently obtained with 

 new gratings, and which will be the subject of another commu- 

 nication. 



III. On the Excentricity of the Earth 3 s Orbit. 

 By James Croll-j-. 



THE following Table contains the values of the excentricity 

 of the earth's orbit and longitude of the perihelion for a 

 million of years past and a million of years to come. They have 

 been calculated for the purpose of arriving at some better know- 

 ledge regarding the general character of those secular changes 



* See Phil. Trans. 1829, p. 301. 

 t Communicated by the Author. 



