98 APPENDAGES TO THE MICROSCOPE. 



degrees may be read off. By rotating the frame carrying the 

 thread, so that it shall lie successively in the directions 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. In the Large Microscopes 

 of Mr. Ross and of Messrs. Powell and Lealand (Plates iv., vi.) 

 the same purpose is answered by the rotation of the Stage, the 

 angles being read-off on the graduated circle. — If a higher degree 

 of precision be required than either of these methods is fitted to 

 afford, the Double-refracting Goniometer, invented by Dr. Leeson, 

 may be substituted.* 



70. Diaphragm Eye-piece. — It is often useful to cut off the light 

 surrounding 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 four small shutters, worked by as 

 many milled-heads projecting slightly beyond the flange of the eye- 

 piece. By combining the movements of these shutters in various 

 ways, it is easy to form a series of symmetrical apertures, bounded 

 by straight lines, and of any dimensions required. As remarked 

 by its inventor, this Diaphragm Eye-piece may also be used to 

 isolate one out of many objects that may be on the same slide, and 

 thus to show that object alone to persons who might not otherwise 

 distinguish it. — For this last purpose the Indicator of Mr. 

 Quekett may also be used ; which is a small steel hand placed just 

 over the diaphragm, so as to point to nearly the centre of the field, 

 whilst it may be turned back when not required, leaving the field 

 of view quite free. The particular object or portion of the object 

 to which it is desired to direct attention, being brought to the 

 extremity of the hand, is thus at once 'indicated' to any other 

 observer. 



71. 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 'cap' 

 by which it is usually surmounted. The Microscope being placed 

 in a horizontal position, as shown in Fig. 53, the rays which pass 



* 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. 



