484 



c. In this case also the opposition of the laws is sufficiently well 

 marked ; the only divergence from opposition heing that due to the 

 minor minimum about the meridian of 1 9 h , due, it is believed, as 

 noted 7th c, to the partial combination of opposite laws in the 

 aphelion half-year. 



9th. It will be observed that the variations of the law with refer- 

 ence to the moon's declination for any given period of the year, con- 

 sists chiefly in the difference of the relative values of the maxima and 

 minima, the differences of epochs being small. Thus for perihelion, 

 the moon furthest north, the principal maximum occurs at the infe- 

 rior passage ; the moon on the equator going south, the two maxima 

 are nearly equal ; the moon furthest south, the maximum at the 

 superior passage is by far the greatest : on the equator going north, 

 the two maxima are again nearly equal ; and so on for other epochs. 



10th. The moon's action is chiefly, if not wholly, dependent on 

 the position of the sun, or (which is the same thing) on the position 

 of the earth relatively to the sun ; and the law of the lunar action at 

 the magnetic equator resembles in some points that for the solar 

 action at the same epochs. Thus about aphelion there is a minimum 

 of easterly (maximum of westerly) declination produced by the lunar 

 action, as well as by the solar action, for these two bodies near the 

 superior meridian; whereas about perihelion both actions for the 

 sun and moon near the superior meridian produce maxima of easterly 

 declination. A like analogy holds for near the epochs of sun- 

 rise and moonrise. 



III. Postscript to a Paper " On Compound Colours, and on the 

 Relations of the Colours of the Spectrum." By J. CLERK 

 MAXWELL, Esq. Communicated by Professor STOKES, 

 Sec. U.S. Received May 8, 1860. 



(Abstract.) 



Account of Experiments on the Spectrum as seen by the Colour-blind. 

 The instrument used in these observations was similar to that 

 already described. By reflecting the light back through the prisms 

 by means of a concave mirror, the instrument is rendered much 

 shorter and more portable, while the definition of the spectrum is 



