176 



HARD Wl CKE 'S S C1ENCE - G OS SIP. 



In using the camera lucida two things have to be 

 considered : the dista nee of the prism or the reflector 

 from the object, and its distance from the paper. 

 (In my own instrument the edge of the prism with B 

 ocular, and i-in. Ross objective, is exactly 12 inches 

 from the object ; this distance with the paper 10 

 inches from the edge of the prism gives a magnifica- 

 tion of 369 diameters.) As the length of the body 

 and the magnifying power of the objectives and 

 ocular are variable, it is best to construct a scale for 

 the purpo se of ascertaining in the first instance 

 the amplification employed ; the most convenient 

 is the following : Rule a line 10 inches in length, a, 

 (fig. 117), and from the centre of this rule another 



from 300 to 500 diameters by using the A B and C 

 oculars, and adjusting the amplification by means 

 of the draw tube or the elevation of the microscope. 

 A memorandum may be kept of the ocular used, the 

 length of tube, &c, but I always measure off a "Ol 

 with micrometer, which should of course represent 

 I, 2, 3, 4, or 5 inches, according to the amplification 

 employed. 



The eye-piece micrometer, in its simplest form, con- 

 sists of a disc of glass upon which a series of equi- 

 distant lines are engraved; this is placed on the 

 diaphragm in the ocular, the lines being magnified 

 by the eye-lens. The distance of these lines is not 

 important, but, whatever their distance, they must be 



Fig. 117. Diagram to 



illustrate Micrometer 



Measurements. 



C3 

 PL. 



o 

 ■t 



H 



Bj 



line, b, of the same length at right angles to it ; care- 

 fully divide these lines into inches and tenths, then 

 rule lines commencing at a to the point c parallel to 

 the vertical line a ; rule nine other lines 1 inch 

 apart : each of these divisions represents one hundred 

 diameters. The inner margins of the lines b c should 

 be divided into tenths, and we shall then have a 

 series of diameters increasing by tenths 100, 1 10, 

 120, &c. This space between the lines a and e 

 should be ruled as accurately as possible with lines 

 "i'o of an inch apart ; this will give measurement to 

 the •0001 of an inch. I always prefer using such 

 diameters as are easily divided, viz., 100, 200, 300, 

 400, &c. With my i objective I obtain a range of 



made to coincide with the divisions on the stage 

 micrometer ; when this has been done the object is 

 placed on the stage, the number of divisions occupied 

 by it gives the diameter in parts of an inch or milli- 

 metre, according to the divisions of the stage microme- 

 ter. This method, although somewhat less trouble- 

 some than that just described, is not so accurate, and 

 if the object is somewhat opaque (e.g., Aidacodiscus 

 formosus), the divisions on the eye-piece micrometer 

 are very indistinct. 



For very delicate measurements eye-piece microme- 

 ters with fine screw adjustments are used, but for 

 ordinary measurements the camera lucida and stage 

 micrometer will be found sufficiently accurate. 



