314 AV. M. BALE OX THE MEASUREMENT OF MAGNIFYING POWERS. 



make the drawing on a concave plaque of such concavity that all 

 parts of its surface would be equidistant from the optic axis of 

 the eye-piece. 



Of course camera-drawings are made at all sorts of distances 

 from the eye-piece, in order to obtain particular magnifications ; 

 and when they are made at other than the 10-inch distance, the 

 foregoing rule will obviously not apply. But in all cases when 

 using the camera the simplest way of indicating the magnification 

 is to mark off on the paper a few divisions of the micrometer 

 scale, placing the paper so that the part where the divisions are 

 to be marked is immediately under the camera. 



