570 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and Oah, and being equal, their bases enlarge by equal quantities. 

 It would not be the same if the paper was in any other position. 



If we now replace the eye-piece and objective, the enlargement 

 which the drawing will have will be produced entirely by the optical 

 system formed by the eye-piece and objective, and that will be true 

 whatever is the length given to the tube of the Microscope, 



As we see, this process consists in referring the microscopic image 

 to paper which is no longer placed at the distance of distinct vision as 

 in the old method, nor on the table which carries the Microscope, as 

 in the process above indicated — positions which all require corrections 

 — but at a distance from the eye equal to the distance of the eye from 

 the object. 



How is this position to be determined ? The surest way is to find 

 it experimentally by varying the position of the paper until the 

 drawing and the object are of equal size (the eye-piece and objective 

 being removed). If to refer the image to the paper we utilize the 

 phenomenon of double sight (double vue), the paper must evidently be 

 placed at the height of the stage, assuming that the sight is exactly 

 similar in the two eyes, which is not always the case. If we employ 

 the camera lucida, the paper must necessarily be placed higher than 

 the stage, because the double reflection and the refraction which the 

 visual rays undergo in the camera produce a diminution in the direct 

 distance between the eye and the paper. With the new camera lucida 

 of Nachet, for instance, the loss being about 3 cm., the paper must be 

 placed 3 cm. above the stage of the Microscope. For greater exact- 

 ness the paper must, of course, be raised on one side, as previously 

 described. 



Simple and Cheap Eye-piece Micrometer.*— Mr. W. M. Bale 



points out how a simple but efficient eye-piece micrometer for 

 ordinary work may be constructed without any expense, except of a 

 little time and patience. The material is fine silk, a single fibre of 

 which is not perceptibly thicker than a cobweb, and is far less 

 difiicult to manipulate. It may be ravelled out of a ribbon ; a corded 

 ribbon, in which the transverse threads are straight, and woven 

 over by two series of longitudinal threads, is better than an ordi- 

 nary one, in which the threads of the warp and woof have acquired 

 a series of " kinks " which are difficult to get rid of. The best 

 eye-piece for the purpose is a C or D. The method of procedure is 

 as follows : — 



Unscrew the field-lens, and at two opposite points on the under 

 side of the diaphragm (not close to the edge) apply minute spots 

 of rather stiif bali^am. Cut a piece of the silk fibre about as long 

 as the diameter of the eye-piece, and with the forceps place one 

 end of it in contact with one of the spots of balsam, to which it will 

 adhere, after which the other end is similarly attached to the opposite 

 spot of balsam. With a pointed but blunt penknife the two ends are 

 pressed well into the balsam and drawn apart till the line is " taut," 



* Southern Science Kecord, iii, (1883) pp. 13-lG. 



