ACCESSORY APPARATUS. 95 



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 segments of the 50 vertical lines, cut off by 

 the diagonal, will progressively 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 the segment whose length precisely coincides 

 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 #, the 

 length of the segment will be 1.8; at b it will be 3.4. Whatever method 

 be adopted, if the measure- 

 ment be made in the Eye- 

 piece and not on the stage, 

 it will be necessary to make 

 allowance for the adjust- 

 ment of the Object-glass 

 to the thickness of the glass 

 that covers the object, 

 since its magnifying power 



is considerably affected by Hartnack's Eye-piece Micrometer. 



the separation of the front 



pair of lenses from those behind it ( 17). It will be found convenient 

 to compensate 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 deter- 

 mined by a series of measurements with the Stage-micrometer. Micro- 

 metric measurements may also be made with the Camera Lucida, in the 

 manner to be presently described, or with Dr. Beale's neutral tint 

 reflector ( 94). 



92. Goniometer. When the Microscope is employed in researches on 

 minute Crystals, their angles may be measured by adapting a Goniometer 

 to the Eye-piece; but as all First-Class Microscopes are now provided 

 with rotating Stages graduated at their edges, with the addition of a 

 Vernier-scale if desired, the measurement may be more conveniently 

 made by giving rotation to the object. An Eye-piece is required whose 

 field is traversed diametrically by a fixed line (either a filament stretched 

 across it, or a line ruled on glass), and is turned so as to bring this line 

 into coincidence with one of the lines forming the angle to be measured, 

 when the Stage is at zero; the stage is then rotated until the fixed line 

 coincides with the other line of the angle, and the amount of movement 

 is read off on the scale. If a higher degree of precision be required than 

 either of these methods is fitted to afford, the Double Refracting Gonio- 

 meter, invented by Dr. Leeson, may be substituted. 1 



93. Diaphragm Eye-piece. It is often useful to cut off the light sur- 

 rounding the object or part of the object to be examined; for the sake 

 alike of avoiding glare that is injurious to the eye, and of rendering the 

 features of the object more distinct. This may be accomplished on the 

 plan of Mr. Slack, by the introduction, just above the ordinary ' stop/ of 



1 For a description of this instrument, see Dr. Leeson 's description of it in Part 

 xxxiii. of the "Proceedings of the Chemical Society," and Mr. Richard Beck's 

 " Treatise on the Microscope," p. 65. 



