accurate Survey of the United States of America. 409 



ends a compound microscope is placed, which stands upon a 

 separate support, and therefore does not change its place when 

 the bar is moved or taken away. When this microscope is 

 placed over the spider's web, the place of the end of the bar 

 is determined by it ; the bar then can be taken away, and the 

 other end of it can be made to coincide with the point where 

 the first had been before seen to coincide with the cross strokes 

 of the microscope, which in the mean time has retained its po- 

 sition independently. The microscope has the following ar- 

 rangement: the object-glass consists of two half lenses of dif- 

 ferent foci, one of which makes, in the focus of the eye-glass, 

 an image of the spider's web of the bar, and the other an image 

 of two rectangular crossing black lines, drawn upon an ivoiy 

 plate, which is fastened to the microscope : this arrangement 

 can be elevated and lowered, and moved in two horizontal di- 

 rections at right angles to one another. In the use, the stand 

 being first properly placed, the microscope is brought to that 

 elevation in which the spider's web thread is distinctly visible, 

 then it is moved until this thread appears exactly to cut the 

 cross upon the ivory plate ; the bar is then removed and ad- 

 vanced one length forwards, the end of it is next brought into 

 the proper position by the mechanism of the bar, and it is 

 moved by it until the spider's web of this other end coincides 

 again by an optical contact with the cross on the ivory plate. 

 Of these microscopes there are three with all their arrange- 

 ments; the last ones always remain standing during the next 

 subsequent operation, that in case of any accident the work 

 might be begun again from them. The bar itself is a junction of 

 four pieces, each of two meters in length, held together by iron 

 clamps ; the inclination of this bar to the horizon is measured 

 by a sector, nearly as in Delambre's appfiratus. When the 

 work is interrupted during the night, the last position of the 

 bar and the microscopes remain undisturbed in their position 

 until morning. The arrangement of the boxes in which the 

 bars are contained and the mechanism of the movements ap- 

 pear to me very well planned. 



From what little I have quoted, it may be easily seen, that 

 the paper of Mr. Hassler deserves the attention of those who 

 take an interest in the mechanical arrangements necessary in 

 practical astronomy and geodesy. It is to be lamented, that 

 such a complete apparatus as that now on hand in America, 

 has not been applied according to its intention and by its au- 

 tl'">"- r. W. Bessel. 



N.S. Vol. 6. No. 36. Dec. 1829. 3 G LXIII. Oh 



