new Definition of Focal Length, Sfc. 141 



These photographs, being printed off the same negative 

 are exactly alike, and in use are turned with the film-side 

 inwards towards the lenses. 



If these photographs are made to occupy the positions of 

 object and image, a comparison of the divisions actually on 

 the plate examined with those of the image of the other scale 

 received upon it will readily give the magnification. 



Groups of exactly ten divisions, at ten divisions' interval, 

 are also indicated by dark bars for the convenience of reading. 

 An anatomical magnifier standing behind the scale serves to 

 adjust to coincidence the planes of the actual and the image- 

 scale with great accuracy ; and it is also obvious that for any 

 one position of the sliding-pieces the magnifier may be applied 

 at both ends successively, each observation being thus checked 

 by another. 



To measure the movement of any sliding-piece a brass ruler 

 is screwed to it at right angles to the motion ; a slip of tele- 

 graph exchange-paper is fixed by drawing-pins to cork pieces 

 at either end of the bank and the rulers pass immediately over 

 this slip, so that lines can be drawn on the paper by a drawing- 

 pen or pencil at any position, and the range of motion accu- 

 rately measured. 



To employ the instrument for convex lens-combinations, 

 such as photographic lenses and projectors, several methods can 

 be adopted. The one scale and the combination being fixed, 

 search may be made for the image, which is adjusted accurately 

 to the other scale, and a mark made on the paper at the edge 

 at the corresponding ruler, and the magnification determined 

 and checked, if necessary, at the other end. 



The first scale may be then moved any distance, and the new 

 position of the image found and adjusted as before by moving 

 the second scale. A new mark is made, and the new magnifi- 

 cation found and checked. 



If in and n are the two magnifications and / the distance 

 between the marks, 



-*--/ 



m—n " 



Or the two scales may remain fixed and the combination may 

 be moved until accurate adjustment is obtained in the two 

 possible positions ; the distance between those two positions 

 being obtained by marks made on the paper along the edge 

 of the ruler attached to the combination. 



In this case the magnification which is the inverse in one 

 of the positions to what it is in the other may be ascertained 



