20 HULBURT AND BACON. 



approximately compensated by the time the innermost mirror float is 

 reached, as the two inner vessels are run by a liquid friction drive. . 



In practice, this apparatus has given very satisfactory results. The 

 images formed by a paraffin oil mirror are quite sharp and the focal 

 length and magnification are very easily regulated by changing the speed 

 of rotation by means of the • set of gear wheels shown in the figure. 

 With such an exceedingly simple apparatus we were able to secure good 

 views of the craters of the moon. 



Plate II shows the film of an incandescent light bulb as shown by a 

 mirror of 30 centimeters diameter, the rotation being such that the focal 

 length was about two meters and the magnification about four times. 

 It would seem that there would be neither great mechanical difficulty 

 nor great cost in constructing very large mirrors of this type, and as the 

 surface is shaped as a result of molecular forces, it is mechanically 

 perfect, the only possible disturbances being due to vibration or variation 

 in the rotation, both of which factors are practically eliminated by the 

 arrangement of the apparatus used by us. 



