280 Peirce — Permeabilities and Reluctivities for Steel. 



K and L, and it became necessary to make the scale distance 

 nearly four meters. Under these circumstances it was prac- 

 tically impossible with a very large reading telescope of the 

 very best make (Clark's) to get a bright image of the scale suf- 

 ficiently magnified to render ballistic determinations easy, so 

 we had recourse to the simple device'" represented by 

 figures 1 and 2, and this has given great satisfaction. In front 



Fig. 4. 



of the reasonably plane mirror (M) of the galvanometer, 

 instead of the usual cover glass, was placed a convex spectacle 

 lens (A) of about four meters focal length. At a distance in 

 front of A equal to its focal length was a horizontal scale (S) 

 mounted in the usual manner on a thin strip of wood at least 

 twice as wide as the scale itself. Through the middle of this 

 strip above the scale was bored a round hole (H) rather more 

 than 20 millimeters in diameter, and just behind the scale a fine 

 vertical wire or silk fiber (W) was stretched across the opening 

 to serve as a cross hair. Behind H, and at a distance suited to 

 its focal length and to the eye of the observer, was placed 

 another spectacle lens (B) to serve as an eyepiece. This had a 

 focal length of about 20 centimeters. A peephole (P) on the 

 common axis of H, A, and B, so placed that B's aperture 

 appeared wholly filled with a large clear uncolored image of 

 the scale with the crosswire running vertically across it, com- 

 pleted the arrangement. If different persons used the device 

 the position of B had to be changed to suit the eyes of the 



* Peirce, Proc. American Academy Arts and Sciences, xlii, 1906. 



