174 PROCEEDINGS OF THE AMERICAN ACADEMY. 



length, and to the eye of the observer, is placed another spectacle lens 

 (B) to serve as an eyepiece. The centre of H should lie approximately 

 in the common axis of A and B. A peephole (P) on this same line 

 is placed at such a distance behind B, — which may conveniently have 

 a focal length of 12 or 15 centimeters — that B's aperture shall appear 

 wholly filled with a large, clear, uncolored image of the scale, with the 

 wire W running vertically across it. The proper distance between A 

 and S is determined by making the parallax of W across the lines of 

 the scale, when the eye is moved sidewise across the peephole, to 

 disappear : when this distance has once been found, the scale is firmly 

 clamped in position. The distances between S and B and between B 

 and P may be adjusted by every person to please himself, though it is 

 usually possible to fasten P securely in a suitable position and to move 

 B only to fit the eyes of different observers. It is important to notice 

 that if the distance between the eye-lens B and the peephole P is not 

 properly chosen, only a small round portion of the image of the scale 

 will be seen, not nearly large enough to fill the aperture of B, and this 

 will appear to move about in a distressing manner, when the eye at the 

 peephole is moved slightly. When the simple adjustments have been 

 properly made, however, the eye need not be pressed close to the peep- 

 hole, for the deflections of the galvanometer can be accurately and 

 easily read whether the eye is one inch or four or five inches behind 

 P. It is not necessary to close the unused eye, for the field should be 

 extremely bright and clear ; the observer is, of course, looking at a 

 life-size image of the scale at a distance PH from the peephole, and it 

 is easy to get any desirable magnification of this image by a proper 

 choice of the eye- lens. Ordinary astigmatism on the part of the 

 observer's eye can be partially corrected by a suitable spectacle lens 

 at B. One slight drawback to the use of this reading device, where 

 space is limited, is the fact that the eye will generally need to be 25 or 

 30 centimeters behind the scale. When it is desirable to do so, the 

 eye-lens, B, may be mounted in a tube which slips over the tube which 

 carries the peephole. 



Figure 1 shows one form of the arrangement just described, mounted, 

 for students' use, with a mirror amperemeter on a wooden frame. 

 Several adjustments are here provided for, so that difterent instruments 

 may be used on this frame. Figure 2 shows the scale and eyepiece as 

 mounted on a brass rod, by Mr. J. Coulson, and Figure 8 represents a 

 convenient way of using the device with a wall galvanometer. 



The Jkfferson Physical Larouatory," 

 Hauvaku University. 



