30 



ON THE CONSTRUCTION AND USE OF 



The iron wire ropi' required to be carefully made, so as to avoid rigidity. It 

 contains 2g miles of wire, ^-J ^ of an inch in diameter, and has 300 strands. Each 

 single wire will support 7 pounds It is, however, more flexible than a hempen 

 rope of the same size, owing to its loose twisting. 



At the lower end of the tube, at the distance of a foot, and crossing it at right 

 angles, held by three bars of iron i I' I", Fig. 'J9, is a circular table of oak c, which 



Kig. 29. 



Tin- MiiKir Sui)iK>rt. 



carries an India-nd)b(n' air sac r/, and upon this the mirror / is placed. The edge 

 support of the mirror is furnished by a semicircular baud of tin-plate «, lined inside 

 with cotton, and fastened at the ends by links of drain h, (// not seen) to two screws 

 cc!\ (/ and // are the win^ ropes, marked A and // in Fig. 28. 



Instead of the blanket support which Jlerschel found so advantageous, M. Fon- 

 cault has suggested this use of an air sat-. In his instrument there is a tulie going 

 up to the observer, by which he may adjust its degree of inflation. It requires 

 that there sliould l)e tliree bearings c r' c", in front of the mirror, against which it 

 may ]n-ess when tlie sac Ix'hind is inflated, otherwise the optical axis is altogether 

 too instable, and objects cannot be found. The arrangement certaiidy gives beau- 

 tiful di'flnition, bringing stars to a disk when the glass just floats, without touching- 

 its front bearings. Tlu- first sac that I made was composed of two circular sheets 

 of India-rubber cloth, joined around the edges. But this could not be used while 

 photographing, because the image was kept in a state of continuous oscillation if 

 there was a breeze, and even under more favorable circumstances took a hmg time 

 to come to rest. It was not advisable to blow tlie mirror hard up against its three 

 front bearings, in order to avoid the instability, for then every point in of an object 

 became triple. To the eye the oscillations were not offensive, because the swaying 

 image was sharp. 



Subsequently, however, an air chair cushion was procured, and as the surface was 

 flat instead of convex the difficulty became so much less, that the blanket support 

 was definitely abandoned. It is necessary that the mirror should ha\e free play in 



