August, 1887.] ELLIOTT SOCIETY. 175 



With the constrnction of the other hehotrope contrived by Ganss, 1 am not 

 acquainted, nor do L know the form of Steinheil's, except that neither was 

 portable. 



The hehotropes of Francis Galton and of WiUiam Hallows Miller, ( the min- 

 eralogist,) are very portable, and depend upon the same ojitical principle as 

 that used in Gauss's sextant-heliotrope, though they differ widely in the meth- 

 ods by which that principle is carried into effect. The ojjtical principle re- 

 quired, or problem to be solved is this : to cause two rays proceeding from the 

 sun, finally to travel in opposite directions, along two lines parallel and very 

 near to each other. Mr. Galton effects this by causing some of the rays pro- 

 ceeding from the reflecting mirror to pass through one half of a lens, by which 

 an image of the sun is formed on a surface placed at the focus of the lens ; 

 this image is then viewed by the eye of the observer behind the same lens, and 

 the direction of the motion of the light in passing from the second half of the 

 lens to the eye is exactly opposite and parallel to that of the light in passing 

 from the mirror to the first half of the lens ; and these last rays are part of 

 those intended to be sent from the mirror to the given distant point. If then 

 we so manipulate the instrument that the rays proceeding from that image of 

 the sun through the lens to the eye seem to proceed from the given distant 

 point, then will the rays reflected by the mirror, proceed accurately to that 

 point. 



My construction of this instrument is very simple, a strip of wood 10 or 12 

 inches long, | -inch wide, and | inch thick, carries at one end a wooden peg 

 with flat surface to receive the sun's image. A common lens, ( of the kind 

 used for cheap spectacles, ) of 6 or 8 inches focus, is carried by a small brass 

 clamp nailed to the strip, at a distance from the peg, equal to the focal length 

 of the lens. Behind tLe lens is a small bit of mirror glass clamped in a piece 

 of wood, which is attached to the longer strip by a single screw about which it 

 is movable friction -tight, as on a pivot. To manipulate it: point the long 

 strip towards the given distant point, supposed to be in the horizon, turn the 

 strip about its longitudinal axis, until its upper surface, if prolonged, would 

 pass through the sun ; then move the mirror around its pivot until the sun's 

 rays pass through one half of the lens and form its image on the peg ; then ap- 

 plv the eye to the other half of the lens, and view the image with one half of 

 the pupil of the eye whilst the other half of the pupil views the given distant 

 point ; make the image apparently cover that point, and it will then receive the 

 rays reflected from the mirror. By the use of the smaller of the two instru- 

 ments before you, I have received flashes from my son, at the summit of a 

 mountain nearly three miles distant ; the focal length of lens is 7 inches, and 

 area of mirror less than 6 square inches. With the larger instrument now on 

 the table, lens of 15 inches focus, and 28 square inches area of mirror, held 

 in the hand, T have sent flashes from Pendleton, S. C, which were easily seen 

 at the house of Mr. S. P. Raveuel, near Highlands, N. C, a distance of 25 or 

 30 miles. As the mirror is oblique to the reflected rays the visual area is always 

 less than the actual area of the mirror, 



Mr. Miller's heliotrope requires that three pieces of plate glass ( clean sex- 



