174- M. Steinheil's New Micrometer, for the Construction 



duced at our leisure, and the stars afterwards inserted among 

 the others in their proper places. 



Amongst the micrometers hitherto used, none answers this 

 purpose; for those in which the field of view is illuminated, 

 weaken the effect of the telescope too much ; and if illumi- 

 .nated lines are employed, either these, or the faint stars when 

 approaching near them, will disappear. This led me to the 

 idea that the difficulty might be removed, if it were possible 

 to illuminate at pleasure thin divided cross bars in the dark 

 field of view. 



The beautiful discovery of Professor Gauss (Astr. Nachr. 

 No. 43), that the diaphragm of a telescope may be seen by 

 means of another, fixed in its axis, afforded me the means 

 of effecting my object. With this view I fastened upon the ob- 

 ject-glass of the telescope a second smaller object-glass, and 

 then fixed in its focus a micrometer plate, so entirely perforated 

 as to have nothing left but the above-mentioned cross bars. 

 Parallel rays issuing both from the micrometer bars and a very 

 distant object, situated in the optical axis of the telescope, will 

 reach the large object-glass ; and the images of both these ob- 

 jects will necessarily appear clearly over each other in the 

 field of the telescope. 



In order to render the image of the micrometer visible on 

 the dark ground of the heavens, the light, which I used for 

 noting down my observations, was found sufficient without 

 any other apparatus, and I was enabled to vary the brightness 

 of the image, by changing the position of the light with respect 

 to the optical axis of the telescope. On trying this method, 

 however, it did not quite answer my expectation ; for it was still 

 difficult to determine the position of very faint points of light, 

 because they still disappeared in the vicinity of the bars, when 

 illuminated ever so faintly. The very faint stars, therefore, 

 which a telescope shows in the dark part of the field of view, 

 will absolutely not bear any light in their vicinity. If their 

 position with regard to other observed stars is to be ascertained, 

 it is necessary to make them coincide with a dark object whose 

 position with regard to the micrometer is known. For this 

 purpose it will be sufficient (as Plate I. fig. A and A' show) to 

 fasten in the eye-glass thick cross wires, or to give to the dia- 

 phragm a square form instead of a round one ; by which means 

 greater distances may be measured, even in case one could 

 not distinguish either the cross wires, or the bounds of the 

 field of view, from the dark appearance of the heavens. 



We may, however, determine the coincidence of the stars 

 (as in a circular micrometer) from their disappearance. If the 

 star that is to be observed has been thus noted, the telescope 



is 



