New Methods in Microscope Work. By E. M. Nelson. 147 



Project a stage micrometer on to a screen at any convenient dis- 

 tance, measured from some part of the lens mount ; determine the 

 magnifying power, and call it M. Move the screen nearer the lens, 

 and again measure its distance from the same point on the lens 

 mount, and call the difference between these distances d ; determine 

 the new magnifying power, and call it m ; then, the focal length is 

 given by the formula 



/- d 

 J M-w* 



During the operation the position of the lens must on no account 

 1)6 shifted. The distance the screen has been moved d, may with 

 advantage be made 10 or 100, and then the result can be found in 

 a reciprocal table, without calculation. This formula is a very con- 

 venient one, and the necessary measurements may be easily made 

 when a photomicrographic outfit is available. 



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