FIG. 48. Spiral Nebula M 51 Canum Venaticorum. 



light about 2.5 times as intense. Thus an interval 

 of 5 magnitudes corresponds to an intensity ratio 

 of i to 100, while the most brilliant stars are about 

 100,000,000 times as intense as the faintest photo- 

 graphed with the 6o-inch telescope. 



