<58 



Solar and Planetary Evolution. 



schel, viewing the Milky-Way as it appears to us in the 

 heavens, conceived of the universe as a flat disk. The con- 

 ception of the late Mr. B. A. Proctor, however, which may 

 be regarded as his greatest scientific achievement, presents 

 a more probable idea of the form of the universe. (A pic- 



Bcale nf ^.i.T,,. 



10 20 90 4n 



4 1 1 1 1 



Fig. 12. A portion of the Moon's surface, showing lunar volcano, " Copernicus." 



ture illustrative of Mr. Proctor's conception is thrown upon 

 the screen.) Mr. Proctor dissented from Herschel's theory, 

 and regarded the Milky- Way as manifesting a spiral form, 

 similar to that exhibited by some of the nebulae. 



Various speculations have been made regarding the ex- 



