EYE-PIECE MICROMETER. GONIOMETER. 97 



little as 1-1 00, 000th of an inch can be measured with a near 

 approach to exactness. Even this exactness may be increased by 

 the application of the diagonal scale (Fig. 52) devised by M. 

 Hartnack. The vertical lines are crossed by two parallel lines, at 

 a distance from each other of five divisions of the vertical scale ; 

 and the parallelogram thus formed is crossed by a diagonal. It is 

 obvious from this construction that the lengths of the lower seg- 

 ments of the 50 vertical lines, cut off by the diagonal, will pro- 



Fig. 52. 



Hartnack's Eye-piece Micrometer. 



gressively increase from "1 to 5*0; so that when it is desired to 

 obtain an exact measurement of an object between these limits, it 

 is only requisite to find out that one whose length precisely coin- 

 cides with the diameter to be taken, which it will then give in 

 tenths of the value of the vertical divisions, whatever these may be. 

 Thus, at a, the length of the segment will be 1 *8 ; at b it will be 3*4. 

 — Micrometric measurements may also be made with the Camera 

 Lucida, in the manner to be presently described (§ 72). — What- 

 ever method be adopted, if the measurement be made in the Eye- 

 piece and not on the stage, it will be necessary to make allowance 

 for the adjustment of the Object-glass to the thickness of the 

 glass that covers the object, since its magnifying power is con- 

 siderably affected by the separation of the front pair of lenses 

 from those behind it (§ 113). It will be found convenient to com- 

 pensate for this alteration, by altering the Draw-tube in such a 

 manner as to neutralize the effect produced by the adjustment of 

 the Objective ; thus giving one uniform value to the divisions of the 

 Eye-piece scale, whatever may be the thickness of the covering 

 glass : the amount of the alteration required for each degree must 

 of course be determined by a series of measurements with the 

 Stage -micrometer. 



69. Goniometer. — "When the Microscope is employed in researches 

 on minute Crystals, a means of measuring their angles is provided 

 by the adaptation of a Goniometer to the eye-piece. The simplest 

 form (contrived by Schmidt and made by Ross), which answers 

 sufficiently well for all ordinary purposes, essentially consists 

 merely of a ' positive ' eye-piece, with a single cobweb-thread 

 stretched diametrically across it in a circular frame capable of 

 rotation ; the edges of this frame are graduated in degrees, and a 

 Vernier is also attached to the index, whereby fractional parts of 



H 



