40 MICHIGAN ACADEMY OF SCIENCE. 



the telescope tube, so that its weight came entirely on the three collima- 

 ting screws. A new cell was made, therefore, with a bell flange, fitting 

 tightly in the tube and having a better spring to work against the object 

 glass. I have wondered whether the curious flexures shown by some 

 meridian circles might not be produced in this way, by the bending of the 

 collinating screws on account of the object glass cell resting on them. 



After the instrument was remounted it was necessary to take up some 

 work that should test it. Also, the reductions could not be too heavy, for 

 it would not do to allow them to fall behind. Finally it was decided to 

 begin again observations of zenith distances of Polaris with the idea of 

 determining the aberration constant, since a number of the values found 

 according to the method of Kiistner differed considerablv. 



'& 



Jf. — Description of Meridian Circle. 



This instrument was constructed by Pistor and Martins in 1854. It is 

 of brass with steel pivots. It is unsymmetrical with respect to the cube, 

 the cone carrying the object glass being 4 feet \Y2 inches long, while the 

 length of the eye end cone is 3 feet and % inches. The total focal length 

 is approximately 8 feet 3i^ inches. A lead ring is placed inside the tube, 

 near the micrometer, to balance the greater weight of the object glass end. 

 The length of the axis is 3 feet 5% inches. The diameter of the objective' 

 is 6.3 inches. The object glass and eye ends of the telescope cannot be 

 interchanged. 



The instrument is mounted between sandstone piers as described in Art. 

 3. The piers are not covered with wood or felt. The substructure is brick. 



The dimensions of the observing room are. north and south. 2(5 feet 3 

 inches; east and west, 17 feet 8% inches; height, 13 feet 31^ inches; width 

 of opening, 2 feet 6% inches. The room is too small. There is hardly 

 sufficient space for reflected observations, and there is no proper arrange- 

 ment for ventilation so that the tem]ierature inside shall follow quickly 

 the changes of the outside air. Moreover, being built on as a wing to the 

 main building, the heating and cooling of this might produce refractions 

 different from those of the tables. 



5.— The Ohject Glass. 



This is rather poor, contafning tree-like formations. The rays of light 

 are not brought sharply to one focus. Still the images are pretty fair, 

 and round all the way across the field. 



I). — The Micro meter. 



The micrometer has a righl ascension screw, willi the Repsold device 

 for automatic registration of transits. The value of one revolution is ap- 

 proximately 3.640s. There is no zenith distance screw, and I may have 

 made a mistake in not having it. Uut I had become suspicious of some 

 of the complicated arrangements at the eye-ends of the large meridian 

 circles, and determined to make the zenith distance pointings with the 

 tangent screw of the telescope. 



WWh this micrometer can be used only a bright field. The light is 

 thrown down the axis of the instrument, and reflected by a large mirror in 

 the cube, the mirror being pierced in the center by a circular opening in 

 order to let through the cone of rays from the object glass. The amount 



