38 G. WEST ROYSTONPIGOTT ON OBJECT GLASSES. 



cii^rocal of the focal length (eight is the reciprocal of ^) by one 



hundred. 



The standard rule by which nominal " inches," " quarters," 



*' eighths," '' sixteenths," and " twentieths " are constructed is 



therefore most properly taken, so that with a C eye-piece of one 



inch focal length and the stop of the eye-piece being exactly ten 



Jjiches from the stage, their respective magnifying powers shall be : — 



(Objectives) ... Incli Quarter Eighth Sixteenth Twentieth 

 (Powers) 100 400 800 1,600 2,000 



I have fomid Nobert's lines to form very beautiful stage micro- 

 meters ; but as they are fractions of the Paris line, observations 

 with them require laborious reduction to the English standard. 

 But I wish to acknowledge here the kindness of Mr. Baker, the 

 optician, in placing at my disposal Jackson's own beautifully ruled 

 micrometer lines, 2,000 to the inch. With the aid of this, and a 

 micrometer in the stop of the eye-piece, I found the powe?' of 

 Powell and Lealand's new l, with a one inch Kellner of Browning's 

 make and searcher (with a fine definition), to be 5,250 diameters. 



Without searcher and one-inch eye- piece : — 



nearly 



Andrew Ross . . . 1851 . "quarter" . power, 540 = ^th. 



Wray 1870 . one-fifth . power, 540* 



Jiesume. 



I. — The focal length of a lens is one-fourth the least distance 

 between image and object at which it can be distinctly formed. 



II. — If a distance of ten inches between object and image be 

 taken (to simplify the calculation), and the amplification measured 

 for a division, then in the case of small lenses the focal length is 

 found by dividing ten by the divisor increased by two. 



III. — The magnifying power of an object glass for any length 

 of tube can be ascertained by using an eye-lens of one-inch focal 

 length, with or without a field lens, by measuring the amplification 

 of a stage micrometer upon another placed in the stop of the eye- 

 piece, and then multiplying it by ten. 



IV. — Different eye-pieces being compared by the Camera Lucida, 

 or marked in focal length by the maker, all other powers are im- 

 mediately ascertained by the simple rule of proportion. | 



* The actual focal length = 10 -i- (54 + 2 + J:,) = 10 ^ (56' 0185) ; or a 

 power of 540 represents a focal length about 2-lOOths of an inch less than a 

 true ^. 



t A half -inch eye-piece will of course be twice the power of the inch, and so 

 forth. 



