646 



SUMMARY OE CURRENT RESEARCHES RELATING TO 



brass collar turned to fit the lower part of the nicol tube is soldered ou, 

 as shown in fig. 83. A shows a section through the collar, the nicol 

 tube being here indicated by dotted lines ; B is a view from below ; C 

 a view of the inside of the collar as it would appear if straightened out ; 

 and D, the outside of the collar similarly unrolled. A groove is cut ou 

 the inside to receive the head of a screw projecting from the side of the 

 nicol tube b b' (B and C). On the opposite side of the tube a lever c 

 moves in the slot a' a" (B), which is of such length that the distance 

 between centres of the lever, in the position a a", is just 90°. The 

 screw head and the lever bar thus form the bearings to carry the nicol 

 tube. As the lever is moved from a' to a", the screw-head b slides in 

 the groove from b to b ' . The nicol tube may be removed or inserted 

 easily by slightly raising and rotating the tube until the lever bar passes 

 over the projection at a (D). 



4. Additions to the Hirschivald Stage. — Fig. 84 shows two scales en- 



Fig. 84. 



graved upon the two parts of the Hirschwald stage. A mark on the 

 sliding portion indicates the distance through which the plate has been 

 moved. Horizontal movement is registered by a small scratch made 

 with a diamond point on the lower margin of the thin section. Any 

 mineral whose position is once registered may again be located by 

 resetting the stage to the former reading. It is necessary to read both 

 the horizontal and the vertical scales. 



Watson and Sons' "Royal" Microscope.* — This model has been 

 subjected to revision in several of its structural details, and to enable 

 these to be more clearly understood, illustrations are appended of the 

 principal parts. Reference to the fig. (85) of the Microscope itself will 

 show that it has mechanical movements to stage, a compound substage 

 with screws to centre and rackwork to focus, and a mechanical draw-tube 

 carrying large-sized eye-pieces. 



The principal feature in the construction is one which has hitherto 

 prevailed only in this firm's " Van Heurck " instrument. Usually the 

 stage is attached to the lower part of the limb by screwing, but in this 

 instrument a closer aud more rigid union is effected. It will be seen by 

 reference to fig. 86 that the limb is continued downwards, so as to form 



* Catalogue, 1910-11, pp. 46-9 (3 figs.). 



