176 M. Steinheil's New Micrometer, for the Construction 



(a piece of very smooth ivory would be preferable), on which 

 a rectangular net-work has been cut by means of a dividing 

 engine, and which is then filed out so that nothing remains 

 but the small bars represented in the figure. These bars, of 

 the breadth of a division of the micrometer, must contract in 

 breadth towards the back part, in order to appear clearly de- 

 fined in every position of the eye. The divisions are cut 

 pretty deep, in order to be easily seen by a small portion of 

 light. The reading off is facilitated by a dot placed at every 

 fifth division. 



This plane, as well as the arm that supports it, must be 

 blackened (without polish), in order to prevent all reflection of 

 light. The arm (b) which carries the micrometer is a drawn 

 tube of brass, as it must be stiff and light. It is held in a 

 socket (c), which is screwed to the ring (d). The small ob- 

 ject-glass, which has its flint lens turned towards the micro- 

 meter plane, is fastened by its support (e) to the socket (c). 

 The support (e) must be blackened, and should be broad 

 enough to prevent any reflection of light (a') from coming 

 into the field of view. 



In making the adjustments, which must precede the use of 

 the instrument, some advantages have presented themselves 

 to me, which I shall here describe, in order to save others the 

 trouble of finding them out. 



First, the eye-piece is to be so placed, that the perfect di- 

 stinctness of the image be not in the centre of the field, but at 

 an equal distance from the border and the centre. For this 

 position the parallax of the cross wires in the eye-glass is first 

 to be destroyed, and they are to be placed parallel to the 

 motion of the axis of the telescope. 



The micrometer must next be fastened to the telescope, and 

 the micrometer plane must be placed parallel to the small ob- 

 ject-glass (for which operation a distant object may be used), 

 and the arm (b) must be slid backward and forward, until the 

 parallax between the cross wires of the eye-piece and the bars 

 of the micrometer plane is destroyed. 



In order to adjust approximately the position of the cross 

 wires, with regard to the micrometer plane, as is shown in 

 fig. A, it should be observed, that the image of the cross wires 

 may be distinctly seen in the same plane with the micrometer 

 plane if the eye is placed in the optical axis behind the micro- 

 meter. Keeping this image in view, we may turn the arm (b) to- 

 gether with the connecting ring (d) about their axes, and change 

 the position of (a 1 ) until the cross wires are respectively parallel 

 to and bisect the micrometer bars. The errors which may 

 then still remain may be easily corrected by repeating the same 



operation 



