442 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



and polished for me by the late Mr. Fitz, whose skill 

 was shown in the large and excellent telescopic objec- 

 tives that he made. He mounted it on a suitable sup- 

 port ; it required, however, to be guided by the hand as 

 the sun moved. When the college building of the Med- 

 ical Department of the University of New York was de- 

 stroyed by fire in 1865, 1 had to regret the loss of this 

 instrument, with much other apparatus, and many docu- 

 ments that were of uu appreciable value to me. Mounted 

 as the lens was, its use was attended with considerable 

 risk to the eyes, on account of the excessive brilliancy 6f 

 the focus. Screens and dark spectacles were found to be 

 very unsatisfactory, and an illness which I consequently 

 contracted admonished me either to abandon the subject 

 or pursue it in some other way. 



The following experiments were made with a smaller 

 glass, consisting of a combination of two similar lenses, 

 their diameter being five inches and focal distance eight. 

 It was, in fact, the large lens of an old-fashioned lucerual 

 microscope, such as was made in London a century ago. 

 I had it fixed on a polar axis, as shown in Fig. 91, and 

 by the aid of a clock it could follow the motion of the 

 sun with such accuracy that, when once set in the morn- 

 ing, an object might be exposed in its focus, if desirable, 

 for a whole day. It had a contrivance on the frame car- 

 rying the lens for supporting small crucibles, glass mat- 

 rasses (Fig. 92), charcoal supports, etc., at the proper 

 point, which might be either at the focus or at any other 

 distance from the lens, as the circumstances of the exper- 

 iment required. Among these instruments were ther- 

 mometers, blackened or otherwise so arranged as to ex- 

 ercise any desired selective absorption. At the outset 

 of any experiment, the whole face of the lens could be 

 covered with a blackened pasteboard screen, with a hole 

 half an inch in diameter. Through this a sufficient 



