APPENDIX. 349 



means of the milled heads (K), and with a very delicate and fine 

 adjustment by the milled head (L). This milled head works 

 against a lever, which moves a slide independent of the rack- 

 movement, and gives an adjustment at once certain and 

 decided. 



The compound stage is of an entirely new construction : the 

 object is most frequently merely placed upon it, but, if neces- 

 sary, it can be clamped by carefully bringing down the spring- 

 piece (M), the ledge will slide up or down, and the object may 

 be pushed sideways; this arrangement forms the coarse adjust- 

 ment. Finer movements in vertical and horizontal directions 

 are effected by means of two milled heads (N" and O), the screws 

 attached to which are kept up to their \vork by opposing springs 

 so as to avoid all strain or loss of time. The whole stage 

 revolves in a circular ring by the milled head (P), or this can be 

 drawn out, and then it turns rapidly by merely applying the fin- 

 gers to the two ivory studs (Q Q) fastened to the top plate, 

 which is divided into degrees to register the amount of revolu- 

 tion. The stage is attached to the limb on a pivot, and can be 

 rotated to any angle, which angle is recorded on the divided 

 plate (R), or can be turned completely over, so that the object 

 can be viewed by light of any obliquity without any interfer- 

 ence from the thickness of the stage. 



Beneath and attached to the stage is an iris diaphragm (S), 

 which can be altogether removed, as shown in the illustration, 

 from its dove-tailed fitting, so as not to interfere during the 

 rotation of the stage. The variations in the aperture of this 

 diaphragm are made by a pinion working into a racked arc and 

 adjusted by the milled head (T). 



Beneath the stage are two triangular bars (U Y), the one 

 revolving round and the ocher rigid in the optical axis of the 

 instrument. On the former the sub-stage (G), carrying all the 

 apparatus hereafter described for illumination and polarization, 

 fits, and is racked up and down by the milled head(\V); the 

 mirror also, if desired, slides on the same bar ; the revolving 

 motion to this bar is given by the milled head (X), and the 

 amount of angular movement is recorded on the circle (Y), 

 whilst the whole of this part of the stand is raised and lowered 



