286 



ACCESSORY APPARATUS 



the r.iv is reflected in the eye by r<H itn,-?/ reflection in the direction 

 (l f. I K. ;ind :i p.-n-i transmitted to ,)' on the face A C of the right- 

 angled prism. Of the latter :i portion is also reflected to K by 

 ordinary reflection ai J'. Tlie hypotenuse fe.ce A C is cut at such 

 an angle thai the relied ion from J' coincides with that from J at 

 t he eye point K , 1 hns utilising the secondary reflection to strengthen 

 the luminosity of the image. The angle G is arranged so that the 

 extreme marginal ray II' from the field of the B eye-piece strikes 

 upon I)G at a point just beyond the angle of total reflection, the 

 diffract ion Lands at the limiting angle being faintly discernible at 

 this edge of the field. This angle gives the greatest amount of 

 liidit hv iinJiiiiii-i/ reflection, short of total reflection. 



In use, the microscope should be inclined at an angle of 45. and 

 the image tbcussed through the eye-piece as usual; the camera is 

 then placed in position on the eye-piece, and pushed down until the 

 image of the object is fully and well seen. The drawing paper 

 must be fixed upon a table on a level with the stage immediately 



FIG. 226. Schroder's 



rainrra llN/ida. 



l''n;. 227. Diagram explaining Schriider's camera lucida. 



under the camera. The observer will then see the microscopical 

 image project eil on 1 he paper, and the fingers carrying the pencil 

 point will lie clearly in view, the ir/t<>t<.: pupil of the eve being 

 available for both images, the diaphragm on the instrument bein^ 

 considerably larger than the pupil. The eye may be removed as 

 often as required, and. if all is allowed to remain wit limit alteration. 

 I lie drawing may be left and recommenced without the slightest shift- 

 ing of t he image. 



If a \ertical position of the microscope be needful, this may be 

 done by inclining the table and drawing paper to an angle of 4?> 

 either in front or at the side of the microscope. For accurate 

 drauing. in all a/.imutlis, the drawing paper should of course coin 

 'ide \\ ith the plane of the optical image. When the paper is in its 

 proper position, the limiting circle of the field of the microscope 

 be projected a- a true circle, but if ot her\\ ise it will appeal- 

 It is recommended that a circle about the sixe of the 

 dra\\ n upon t lie paper, and its coincidence with the projected 

 field compared. 



