216 Mr Sang on an Improvement in the 



placing it in the chuck, well centred, turned a portion of the end 

 conical : to this cone I fitted a small piece of brass with a flat 

 face, on which to cement the crystal or prism whose inclination 

 is wanted. By means of the adjusting screws, four of which work 

 on one end, and four on the other end of the square bar the faces 

 of the crystal can be rendered parallel to the axis of the lathe. 

 The first approximation can be readily obtained by causing the 

 reflected image of one side of the shear, agree with the direct 

 image of the other. To obtain the final and accurate adjust- 

 ment, the following process is adopted. On the opposite wall 

 of the room is fastened a paper scale with divisions marked so 

 strongly as to be perceived readily from the lathe ; and near 

 the floor on the same wall is placed a small black circular mark. 

 The image of the scale is then brought to coincide with the cir- 

 cular spot, and the division which culminates it is noted. Turn- 

 ing the lathe head half round, the reflection of the spot is 

 brought in contact with the scale, and if the same division be 

 again read off no farther adjustment of that face of the crystal 

 is needed ; if any difference exist, one-half of it is to be correct- 

 ed by means of the screws in the chuck. To save time in after 

 adjustments, there is then placed a distinguishing mark at the 

 mean of the two readings, and this mark is ever afterwards 

 compared with that at the bottom of the wall ; at least if the 

 position of the lathe be not changed. If the two marks be 

 placed at the same distance from the axis of the lathe, the two 

 readings on the graduated limb will not differ from each other 

 exactly by 180 ; but by that 4. or twice the parallax aris- 

 ing from the face of the crystal not passing actually along the 

 axis of the spindle. Half the sum of the two readings then will 

 be freed from the influence of parallax. The analogous half 

 sum obtained from the other face of the crystal will also be freed 

 from parallax, and thus the difference between the two half 

 sums will give exactly the inclination wanted. 



To repeat the measurements on another part of the limb, we 

 have only to turn the piece of brass round on the cone to which 

 it is fitted : but as the axis of the cone is not coincident with 

 that of the spindle, unless by chance, a new adjustment of the 

 crystal is then needed. The repetition is, however, scarcely call- 



