124 ACCESSORY APPARATUS. 



ever 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, 

 and should be recorded on the table of the micrometric values of 

 the several objectives. 



47. Goniometer. When the Microscope is employed in re- 

 searches 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, essen- 

 tially consists merely of a "positive" eye-piece, with a single 

 cobweb-thread stretched across it diametrically in a circular frame 

 capable of rotation; the edges of this frame are graduated in de- 

 grees, and a vernier is also attached to the index, whereby frac- 

 tional parts of degrees may be read off. By rotating the frame 

 carrying the thread, so that it shall lie successively in the direc- 

 tions of the two sides of the crystal, the angle which they form 

 is at once measured by the difference of the degrees to which the 

 index points on the two occasions. For the cobweb-thread, a 

 glass plate, ruled with parallel lines at about the l-50th of an 

 inch asunder, may be advantageously substituted ; since it is not 

 then necessary to bring the crystal into such a position as to lie 

 along the diametrical thread, but its angle may be measured by 

 means of any one of the lines to which it happens to be nearest. 

 If a higher degree of precision be required than this instrument 

 is fitted to afford, the Double-refracting G-oniometer, invented by 

 Dr. Leeson, may be substituted ; for a description of which (too 

 long to be introduced here) the reader is referred to Dr. L.'s ac- 

 count in the "Proceedings of the Chemical Society," Partxxxiii, 

 and to Mr. Quekett's "Practical Treatise on the Microscope." 

 (See Appendix for a description of a Micrometer and Goniometer, 

 by Prof. J. L. Smith.) 



48. Indicator. When the Microscope is used for the purpose 

 of demonstrating to others such objects as may not be at once 

 distinguished by the uninitiated eye, it is very useful to introduce 

 into the eye-piece, just over the diaphragm, a small steel hand 

 pointing to nearly the centre of the field ; to whose extremity 

 the particular portion of the image which the observer is intended 

 to look at, is to be brought by moving the object. The hand 

 may be so attached, as to be readily turned back when not re- 

 quired ; leaving the field of the eye-piece quite free. This little 

 contrivance, which was devised by Mr. J. Quekett, is appropri- 

 ately termed by him the indicator. 



49. Camera Lucida. Various contrivances may be adapted to 

 the eye-piece, in order to enable the observer to see the image 

 projected upon a surface whereon he may trace its outlines. The 

 one most generally employed is the Camera Lucida prism con- 

 trived by Dr. Wollaston for the general purposes of delineation ; 

 this being fitted on the front of the eye-piece, in place of the 



