68 



Solar and Planetary Erolution. 



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

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

 ception of the late Mr. K. A. Troctor, however, which may 

 be regarded as his greatest scientific achievement, presents 

 a more ])robable idea of the form of the universe. (A pic- 



10 ? ' 

 Ecale of I ■■■''■■■■ 



10 20 30 ^i*^ 



Milss 



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



ture illustrative of Mr. Troctor'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 nebul*. 



Various speculations have been made regarding the ex- 



