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Note on an Optical Rule. 

 By Edward M. Nelson, F.R.M.S. . 



(Bead December 20th, 1895.) 



The rule is made of box, is 20 inches long and square in 

 section. On one face there is a scale of inches and tenths, and 

 on the opposite side centimetres and millimetres. On one of 

 the sides at right angles to these is a scale of dioptrics marked 

 D, and on the remaining side opposite to it is a new scale of 

 powers marked P. All the scales read from the same end and 

 are ruled on both edges of the rod. This is important, because 

 any two contiguous scales can be read and compared together ; 

 thus dioptrics can be converted into inches by the inspection of 

 one edge of the rod, and into centimetres by viewing the other, 

 and vice versa. On the opposite face of the rod the scale of 

 powers can be treated in a similar manner. The following are 

 some examples of the use of the rule : — 



(1.) "When the focus of a lens is measured on the P side of 

 the rule, its magnifying power, when the eye is held at the back 

 principal focus, is indicated. By adding one to this figure the 

 power, when the eye is held close to the lens, is found ; but if 

 one be subtracted from it, the enlargement of an image on a 

 screen distant 10 inches from the lens is given. Example: A 

 lens measures four on the P side of the rule ; this will be its 

 magnifying power when the eye is held at the back principal 

 focus of the lens. Its power, when the eye is held close to the 

 lens, will therefore be five, and an image projected on a screen 

 distant 10 inches from the lens will be enlarged three times. 



(2.) When the power of any lens is known, its focal length 

 can be determined by inspection, either in inches or mm. 

 Thus a lens of two-power has a focal length of five inches or 

 127 mm. 



(3.) The focal length of a diverging lens can be easily found 

 by overpowering it with a converging lens and measuring the 

 power of the combination ; this power, less the power of the 



