390 THE AMERICAN MONTHLY [Dec. 



ment by the prism sleeve for any change of place or position, 

 they no longer — as under the old construction is the case — sub- 

 ject the prism, its fastening, and the micrometer screw to undue 

 strain, especially as a horizontal plate (m) projects from the 

 prism sleeve and expands underneath the arm of the tube car- 

 rier, preventing the fingers from coming in contact with any of 

 the parts above. 



With the help of the illustrations the details of the new con- 

 struction will be understood. The block of the joint, the stage 

 and the socket (Fig. 1 and 2), for the reception of the prism 

 sleeve are made of one casting. At the bottom of the socket and 

 within the sleeve a chamber is seen for the spiral spring which 

 forces the prism i upward against the micrometer screw. The 

 latter is placed in the center of a disk s (Fig. 1 and 3), which is 

 supported on three hollow pillars a, a, a, standing on the upper 

 end of the sleeve, and three screws z (Fig. 1. and 3), serve to 

 combine disk (Fig. 3), pillars and sleeve into one firm structure, 

 from and through the top of which the micrometer screw can 

 with exactness control the ascending prism i. In order to unite 

 the prism with the tube carrier, the arm of, the latter flattens 

 out into a thick, round; horizontal plate p, the central portion 

 of which receives the prism by means of a screw thread, whilst 

 its circumference projects downward, in the form of a ring r, over 

 the prism sleeve. In order to gain passage-ways for the pillars 

 a, a, a, the plate p is provided with three openings o, o, o, of such 

 ample width that any contact of the pillars with plate p is made 

 impossible. 



All those parts of the old construction which have given sat- 

 isfaction have been retained, only such factors having been 

 eliminated as exert a deteriorating influence on the fine 

 adjustment. 



For the sake of differentiation we insert plates Fig. 8 and 9, 

 taken from the catalogues of Zeiss of Jena, and Leitz, of Wetzlar, 

 showing the construction of the fine adjustment as used by these 

 firms. 



The Horseshoe-Foot. — The advantages offered by this foot- 

 form are not fully appreciated and turned to use. The charac- 

 ter of this foot must not be confounded with that of a tripod. 

 This latter would have three points to rest upon "which, when 



