A plain silvered glass mirror IL, inserted into a proper frame, is made to turn on two pivots, one of which in in rests on the cylinder 1VIM, while the other o o is inserted in the horizontal part of the branch HH. The straight line which passes through the centres of these pivots, must be exactly parallel to the silvered back of the mir ror, and at right angles to the axis of the telescope, and the black mark N, produced by a scratch upon the sil vered surface, must be bisected by the axis of the mirror.
Above the object end B of the tube is fixed a plate of metal, seen in the figure, and behind this plate is seen another square plate z z, on which are drawn the lines x x, y y, crossing each other at right angles. By means of a piece of brass fixed to the last of these plates, and traversing a square hole made in the other plate, the square plate may he moved up and down, and from right to left ; and it is kept in any position which is thus given to it, by a screw on the back of the fixed plate. The moveable square plate must be adjusted in such a man ner that the line x x may intersect the axis of the tele scope, and be parallel to the axis o in of the mirror. The position of the line yy must also be such, that its distance from the axis of the telescope is equal to the distance of the line IK from the same axis. When the plate z z is thus adjusted, the straight line y y will always be in the same plane with the line IK, whatever be the position of the mirror, and a line drawn from a point at N, where the axis of the mirror cuts IK, to the point where y y intersects x x, will be parallel to the axis of the telescope.
The spring QQ' is fixed at Q to the arm I Ili, and by a screw R working into its other extremity Q', the end II of the horizontal arm may be made to press the pivot o o upon the frame of the mirror.
The horizontal branch HH, which is represented se parately in Plate CV. Fig. 10, is surrounded with seve ral pieces. The piece d b and pivot o are fixed in an invariable manner. The pivot o o is inserted it•a square hole through the piece VV, and through the extremity of the arm HIT. The piece d b may be moved either be fore or behind, by turning the screw and the piece VV may be moved from right to left with the piece d b, by means of the screw S.
The apparatus being thus constructed, the next thing to be considered is the method of adjusting it. In order to effect this, the axis of the mirror must be perpendicu lar to the axis of the telescope ; the line drawn from a point near N, where the axis of the mirror cuts the line 1K, to the point of intersection of x x and y y, must be parallel to 1K, and the straight line y y must always be in the same plane with IK.
The mirror is first placed in such a manner that the line IK is at right angles to the axis of the telescope. By turning the screw T, the lower edge of the frame is made a tangent to the circular surface MM', which is parallel to the axis of the telescope. The screw T is then turned, in order to fix the piece d b in an invariable manner.
The axis of the telescope is next directed to a point on a plane surface, placed at a certain distance. This point must be situated in a vertical plane, perpendicular to the plane surface, and passing through the eye of the observer and the centre of the sun. A horizontal line being drawn through this point, a second point is taken, as far from the first as the centre of the mirror is distant from the axis of the object-glass. By unscrewing S, turning the telescope on its axis, and the mirror also about its own axis, the piece VV is moved backwards or forwards until the centre of the reflected image falls upon the second point. The square plate z z is then ad justed in such a manner that the shadow of the line IK falls on the line y y, and that the shadow of NN is bisect ed by the line x x. When this happens, the plate z is firmly fixed. hence it follows, that whenever this adjustment is made, and when the intersection of the cross wires in the telescope is directed to any point, the rays reflected by the mirror will be parallel to the axis of the telescope, and will always continue so while the sha dow of IK falls on yy, and while the shadow of NN is bisected by x x.
In making use of the mirror, the intersection of the cross wires must be first directed to any point of the ob. ject which is to be inflamed :—the telescope must next be turned round in the collars CC, C'C', till the shadow of the line 1K falls upon y y ; and, finally, the mirror must be turned about its own axis till the shadow of NN is bi sected by the line x x. The centre of the reflected image will consequently fall upon a point of the object as far distant From the point to which the intersection of the wires was directed, as the centre of the mirror is from the axis of the telescope. The image may obviously be preserved in this position as long as we choose, by keep ing the shadows of I K and N in the same position.