MAGNIFICATION 77 



should be drawn up, showing the magnification pro- 

 duced by every possible combination of lenses at this 

 standard image distance, the tube length given by the 

 maker being always maintained. When using the ob- 

 jective alone the image should be formed 10", or 250mm., 

 from the shoulder of the objective, and when an eye- 

 piece is used the camera extension should be 10", or 250 

 mm., measured from the top of the tube on which the 

 eyepiece rests. The magnifications given in the table 

 of objectives and eyepieces, page 8, approximate 

 to those found under these conditions. Such a table 

 is very useful, as the magnification produced in any 

 photomicrograph can be instantly determined, with 

 quite sufficient accuracy for most purposes, without 

 the trouble of actual measurement, if the baseboard of 

 the camera is furnished with a scale to measure the 

 image distance. 



Camera Extension and Magnification. The effect pro- 

 duced by any variation in camera extension, measured 

 from the objective or eyepiece according to circum- 

 stances, and the result of alterations in the tube length 

 from the standard adopted by the maker of the objective 

 are determined as follows without any great error. 

 The factors obtained being used to multiply the magni- 

 fications given in the table : 



Factor for camera extension 



extension in inches extension in mm. 



oi- 



lO 250 



actual tube length 



Factor for tube length ^ , , ; 77- 



standard tube length 



Magnification and Tube Length. The camera extension 

 may be varied as desired to influence the magnification, 

 but the tube length must only be altered with caution. 

 With high powers any alteration from the standard, unless 



