844 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The chief advantages of the instrument consist* : — (1) In the 

 facilities for turning an object in its own plane between fixed 

 crossed nicols, the rotation being concentric with the axis of 

 vision ; (2) In the ability to read off accurately the angle through 

 which the object may be turned in a horizontal plane by means 

 of the graduation round the circular stage ; (3) In the facility with 

 which the polarizer and analyzer can be displaced and replaced, 

 and the means by which the exact position of the principal sections 

 of the polarizer and analyzer can be noted ; (4) Where the total ex- 

 tinction of light by means of crossed nicols interferes with the 

 researches on any mineral, means are provided for facilitating ob- 

 servation under such circumstances. 



The peculiarities in the construction of the Microscope consist in 

 the tube which carries the eye-piece and objective b (Fig. 150), being 

 as it were suspended within an outer tube p, its only attachment 

 being at the top at I. A block h is fixed between the inner and outer 

 tubes to prevent any rotation during focal adjustment. The coarse 

 adjustment is effected by hand, the thumb and forefinger sliding the 

 inner tube up and down by pressure on the disk /, other fingers being 

 applied to the top I, of the fixed tube. The fine adjustment consists 

 of a micrometer screw, shown at g, graduated in 500 divisions, 

 each being equal to 1 /*. The unattached portion of the inner 

 tube is steadied in the outer one by means of a spring f and three 

 little screws x (see side figure, a section through M — N), set hori- 

 zontally and capped with scraps of parchment, which are more or less 

 compressed as the adjustment is made. The arm of the Microscope 

 carries two screws with milled heads, one of which is shown at n, and 

 both at n and m in the side figure. These are set at right angles to one 

 another, and serve to centre the tube. The eye- piece carries two cob- 

 webs, which intersect at right angles in the centre of the field. To the 

 outside tube of the eye-piece a small peg a is fixed, which slides into 

 a corresponding slot in the top of the inner movable tube of the 

 Microscope. This arrangement prevents any rotation of the eye- 

 pieces, and so keeps the cobwebs in a fixed position. An analyzer «, 

 fitting in a brass cap, slides over the top of the eye-piece. The 

 bottom of the cap is surrounded by a bevelled flange, which is 

 graduated to 5°. An index mark on the plate /, serves to record the 

 angle through which the prism is rotated. The stage of the Micro- 

 scope is circular, and a circular plate T is arranged so as to revolve 

 horizontally on it. This plate is graduated on its margin, and an 

 index to record the amount of the revolution is attached to the front 

 of the fixed stage at i. It also has a spring clip q, and a Wright's 

 indicator (two scales at right angles). Beneath the stage is set an 

 easily displaceable polarizer r, consisting of a Nicol prism, which 

 revolves within its external tube by means of the lower disk, which is 

 graduated to 10°, and has its index marked on the fixed outer tube. 

 This polarizer does not turn when the stage plate is rotated, but 



* See Kutley's < Study of Bocks' (8vo, London, 1879) p. 5i. 



t In the figure, in order to show the spring, it is brought too far round by 45°. 



