108 Prof. J. Emerson-Reynolds on a New Form of 



with the teeth of the wheel on the rod r-, and to act upon the 

 teeth directly so as to cause the rod carrying the index, A, to 

 rotate easily. This strip is bent to a curve whose radius is 

 equal to the distance from the axis of the pillar, P, of the 

 instrument to the middle of the toothed wheel. The strip is 

 attached to a stout arm ; and this is in turn screwed to the 

 slightly projecting end of the heavy plate, D, which carries, 

 and of course moves with, the observing-telescope, the motion 

 being communicated to the latter by turning the milled head 

 m. As the observing-telescope moves over the graduated arc 

 <7, the index A moves in front of the graduated plate B, but in 

 the opposite direction; for the motion of D is communicated 

 to the rod r by means of the serrated slip c. When the fittings 

 are well made, the movement of the index A is steady and 

 corresponds in both directions with those of D. By the simple 

 means described, a very slight motion of the observing-telescope 

 produces a comparatively considerable displacement of the 

 index A. 



In my instrument, the telescope and the index move in 

 opposite directions. Any objection on this score can be re- 

 moved ; for it is only necessary to point out that the motions 

 may be made to coincide in direction by placing c under in- 

 stead of over the toothed wheel. 



Graduation of the Glass Plate.— It is very desirable that the 

 graduations on the plate and on the arc of the instrument 

 should agree ; the best mode of securing this is to graduate 

 the plate with the aid of the arc. For this purpose the tele- 

 scope is moved into such a position that the rays less refrangible 

 than the red potassium-line shall occupy the field of view ; 

 the zero of the vernier is then made to coincide with the nearest 

 convenient degree marked on the arc. The rod r is then firmly 

 grasped and the index A brought down to a horizontal posi- 

 tion, and a fine dot made on the plate under the point by 

 means of a pen dipped in " black japan." This point is taken 

 as the zero of the scale. Each half-degree is marked off' in 

 a similar manner until the semicircle is graduated. The two 

 scales are again compared at different points, and the opal- 

 glass plate removed ; each large division, corresponding to 

 half a degree, is then subdivided into 10 * equal parts. Finally 

 the semicircle is numbered from zero up to 200 : each division 

 of the scale therefore corresponds to 3' of the arc g. In my 

 spectroscope the angular motion of the observing-telescope is 

 magnified 25 times, and the width of each division of the glass 

 scale is 2 J millimetres, so that the readings are easily made 

 in a feeble light without straining the eyes of the observer. 

 * In the woodcut onh' five subdivisions are shown. 



