of the image at one side and indicate each line of the 

 micrometer on the drawing until the outline at the 

 opposite side is reached. The computation is made by 

 dividing the diameter of the drawing by the value of a 

 division of micrometer multiplied by the number of 

 divisions which are projected on the drawing. Thus 

 if the ruling on the micrometer is o.oi mm. and 10 

 divisions cover the outline of the drawing, while the 

 actual measurement of the drawing is 30.0 mm. the 

 formula would be3o-^-(i-ioox 10) = 300. 



If a standard of 10 inches is maintained in all 

 drawings and the amount of magnification with certain 

 objectives and eyepieces be previously determined by 

 means of the micrometer, a standard is established for 

 each and further measurement will not be required. 

 If, however, variations from this standard distance are 

 made, the actual magnification should always be 

 determined. 



The magnification is sensitive to slight variations 

 in the distance. The standard distance of 10 inches 

 is determined by measuring from the optical axis of 

 the microscope to the axis of the mirror of the camera 

 lucida and from this to the drawing surface. 



Drawing Table. It will be noticed that great 

 stress has been laid upon the fact of keeping the draw- 

 ing surface at right angles to the axis of the projecting 

 cone of rays and that the drawing board must be tilted 



