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right angles to the axis of projected cone, or until 

 the field appears round. A convenient method of 

 accomplishing this, with the further advantage of 

 being able to elevate the drawing board and thus 

 produce a variation in the size of drawing, is the 

 drawing table made by Bausch & Lomb Optical Co. 

 Care must also be used in attaching the camera 

 lucida to the tube, so that the opening in the 

 prism shall be in the optical axis. This can be 

 accomplished by the centering screws and observ- 

 ing whether any of the field is cut off. If the field 

 appears smaller with the prism than by directly 

 looking into the eyepiece, the prism is too close, or 

 too distant from the eye lens and should be prop- 

 erly adjusted. 



No particular .skill is required in drawing as it 

 is simply a question of copying, but the lines 

 should be light so that any irregularities may be 

 corrected after the work with the camera lucida is 

 completed. To determine the standard distance 

 of 10 inches, measure from the optical axis to the 

 axis of mirror and from this point to drawing 

 paper. 



If it is desired to determine the amount of 

 enlargement of the image on paper, this can be 

 done by replacing the object by a stage micrometer 

 and drawing its spaces over the object. If the 

 divisions or spaces are 0.01 mm. and it were found 

 that Jo spaces covered the object and the actual 



