462 



EEPORT 1 880. 



direction the mirror A is moved so as to alter the direction in which rays are 

 reflected to the distant object, and the angle at which part impinge on E and are 

 reflected through the lens K, the image visible in tue glass moves in the same 

 direction. Several attempts to produce this result were made bv the use of mirrors 

 and pi'isms, before the lens K was introduced, but they all failed. It was easy to 

 make the image of the sun cover the object when the two occupied the centre of 

 the field of view, but directly the mirror wa? inclined so as to direct the rays not 

 strictly parallel to the axis of the field-glass, the apparent image diverged generally 

 in the same direction along one co-ordinate, and in the opposite along one at right 

 angles to it, so that nowhere, but in one line across the field, did the image lie in 

 the desired position. The mirrors E and F are adjusted parallel once for all, by 

 noticing the position on a screen of the small spot of light reflected from the front 

 of F as the light passes from E to K. The mirrors are moved by the adjusting- 

 screws till this spot has, to the bright reflection from the mirror A, the same 

 relative position that the centre of mirror F has to the mirror A. 



In actual use the field-glass is first fixed in position pointing to the object, 

 either by holding steadily in the hand, or better by a clamp attached, by which it 

 can be screwed into a tree or post, or fixed in the muzzle of a rifle. The instru- 

 ment is turned on the barrel of the glass till the sun is in the plane passing through 

 the two axes of the instrument, and the mirror A is turned till the bright image of 



the sun is seen on the screen II, through a hole left for the purpose in the side of 

 the tube. On looking through the glass the sun's image is seen, and by then slightly 

 rotating the instrument or moving the mirror, is made to cover the object. The 

 mirror A is connected not directly to the body of the instrument, but to a lever B, 

 on which it works stiflly, so as to retain any position in which it is placed. Lever 

 B works easily and has a limited range of motion, to one end of which it is pressed 

 by a spring ; slight pressure with the finger moves it and its attached mirror, so as 

 to throw the liglit on and off" the object in a succession of long and short flashes 

 by which letters and words may be indicated. 



Mr. F. Galton said that the steadiness of aim required would be so great that 

 it would be impossible with the instrument exhibited to gi^'e signals with precision 

 without using a stand. lie proposed that a convex or concave lens, of about 30 

 feet focus, should be attached near E, in the path of the rays from the mirror to 

 the distant object, so as to disperse them over a field about three times the apparent 

 diameter of the sun. This would render free-handed signalling practicable, though 

 it would diminish the brightness of the flashes, and the instrument without the 

 lens could be used to attract attention. 



The above instrument answers well for all positions of the sun except when 

 very low bihind the observer's back. For this case another mirror is provided by 

 which the light is reflected on to the mirror A. 



