APPENDIX. 



493 



the wheel, the milled edge of which is shown at c. A circular 

 opening is made through the cover at d, immediately under 



Fig. 267. 



the object-glass, and through which the preparations may" be 

 examined as they alternately present themselves at this point ; 

 this opening is covered with a circular piece of thin glass, to 

 protect the ceUs beneath from dust; h h la a, buU's-eye, 

 mounted on a dove-tailed sliding piece, i, seen on a larger 

 scale at fig. 267, i showing the sliding piece. The body, j, 

 and the limb, k, are made in the usual way, the latter being 

 attached to a stage, /, with right angled rack-work fittings, 

 and so constructed that by means of thej milled-head pinions, 

 m n, the object-glass may be made to traverse the entire space 

 formed by the opening, d, and thus to bring the whole of the 

 object beneath it into view. 



Fig. 268 shows another form of Mr. Hett's instrument; 

 the body, j, limb, k, and rack-work fittings, I, m, n, are 

 similarly arranged to those in the instrument previously 



