286 



ACCESSORY APPARATUS 



the ray is reflected to the eye by ordinary reflection in the direction 

 of J K, and a part transmitted to 3' on the face A C of the right- 

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

 ordinary reflection at J'. The hypotenuse face A C is cut at such 

 an angle that the reflection from J' coincides with that from J at 

 the eye-point K, thus utilising the secondary reflection to strengthen 

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

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

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

 diffraction bands at the limiting angle being faintly discernible at 

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

 light by ordinary reflection, short of total reflection. 



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

 the image focussed 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 

 camera lucida. 



FIG. 227. Diagram explaining Schroder's camera lucida. 



under the camera. The observer will then see the microscopical 

 image projected on the paper, and the fingers carrying the pencil 

 point will be clearly in view, the whole pupil of the eye being 

 available for both images, the diaphragm on the instrument being 

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

 often as required, and, if all is allowed to remain without alteration, 

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

 ing of the image. 



If a vertical position of the microscope be needful, this may be 

 done by inclining the table and drawing paper to an angle of 45 

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

 drawing, in all azimuths, the drawing paper should of course coin- 

 cide with the plane of the optical image. When the paper is in its 

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

 will be projected as a true circle, but if otherwise it will appear 

 elliptical. It is recommended that a circle about the size of the 

 field be drawn upon the paper, and its coincidence with the projected 

 field compared. 



