MEMOIK XXX.] ON BURNING GLASSES AND MIRRORS. 



443 



Fig. 91. 



amount of light could be transmitted to enable one to 

 arrange the various details of the proposed experiment ; 

 and when everything was ready, the screen was re- 

 moved, and in the concentrated and brilliant focus the 

 action went on. I found that this simple contrivance 

 was an invaluable relief to the eyes. 



In Fig. 91, a a is the heliostat clock; Z>, its polar axis; 

 d d, a frame carrying the lens, <?, and 

 having an arrangement at /for support- 

 ing flasks, crucibles, or other apparatus. 

 This turns on a double joint at <?, so 

 that the lens may be directed to the 

 sun. 



In Fig. 92, a is a small flask receiving 

 the convertnncr rays, b, at their focus, 



O O t/ I 7 7 i 



The lens being five inches in di- 

 ameter, and the space covered by 

 the solar focal image, owing to 

 want of achromaticity and spherical aberration, one fifth 

 of an inch, the multiplying effect 

 would be 625 times, if the glass were 

 perfectly transparent, and there were 

 no loss by reflection from its sur- 

 faces. On a summer day of average 

 brightness, with the thermometer at 

 68 in the shade, and the bulb, not 

 being blackened, at 108 in the sun, 

 this lens could fuse copper instantly, the bead oxidizing 

 only superficially, and cutting readily after fusion. Black 

 oxide of copper in a little crucible of platinum foil melt- 

 ed into a slaty-looking substance at once. Wrought iron 

 did not melt alone; but if exposed on a charcoal support 

 in a globule of microcosmic salt, previously fused by the 

 lens, it gave a clear, round bead, which readily extended 

 when beaten upon an anvil. The globule of flux turned 



Fig. 92. 



