68 



Solar and Planetavi Erobitlon. 



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

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

 ce})tion of the late Mr. K. 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- 



rig. 12. A portion of tlie Moon's surface, showing lunar volcano, "Copernicus." 



ture illustrative of Mr. Troctor's concej)tion 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 l)y some of the nebulae. 



Various speculations have been made regarding the ex- 



