Dec, 1868.] elliott society. 63 



very simply and readily by tlie following expedient, based on the optical prin- 

 cijjle that an incident ray and the corresponding reflected one coincide wlien 

 they are normal to the 7'ejlecting surface. A bit of a good glass mirror with par- 

 allel surfaces, and a few inches square, having two Unes traced on it with a 

 writing diamond perpendicular to each other, is placed on the plane of projec- 

 tion, and moved under the given point, representing the place of observation, 

 until by the eye placed above that point and close to it its image is seen to co- 

 incide with the intersection of the lines on the glass. Evidently, this intersec- 

 tion is then the projection required on the plane of the mirror, and the corres- 

 ponding point on the plane of the instrument is easily determined, if desired, 

 by the use of the intersecting lines as lines of reference. In my instrument 

 there is no necessity for determining this point on the plane of the instrument, 

 as the time of disappearance is found by moving the point representing tae 

 place of observation, and the disc representing the moon, to successive corres- 

 ponding positions belonging to successive minutes of time, uncil t/ie image oft/ie 

 'point is occulted by tlie disc ; the time of reappearance is determined in like 

 manner. Instead of the pointed extremity of the rod, may be used a minute 

 perforation in a thin sheet of metal or of pasteboard, through which may be 

 viewed its image in the mirrors. If instead of either of these, were used a 

 polished bead of glass or metal, smaller than the pupil of the eye, the light re- 

 hected from it would be seen in the mirror as a star-iike point, which by the 

 principles of the projection employed, would be tne representatives of the star 

 to be occulted, and by the interposition of the disc representing the moon, 

 would actually disappear and again reappear, as in an occultation. The use of 

 the minute perforation gives the best resuis, as the eye cannot then fail to be in 

 the proper position above the point indicating the place of observation. 



i use ihe same method for adjusting in position a plane which in my instru- 

 ment represents the plane of the parallel of latitude of the place of observa- 

 tion ; this revolves about an axis, to which ought to be parallel the plane of the 

 instrument or the chart, and this parallelism I test by finding, by reliexion, 

 the projections of a certain point in three or more widely distant inclined posi- 

 tions of the plane ; these projections ought to lie in a right line. This plane 

 ought also to form with the plane of the instrument, or plane of projection, an 

 angle equal to the complement of the decUuation of the star, and any line in it, 

 perpendicular to the axis of rotationj being taken as radius, will have its pro- 

 jection equal to the sine of the declination. Having a scale of equal parts, for 

 sines, drawn on the plane of the instrument, corresponding to a certain line in 

 the movable plane as radius, I take from a table of natural sines the proper 

 length for the given dechnation, and placing the intersection of the cross hues 

 on the mhror, to coincide with the proper point in the scale for sines, i rotate 

 the plane about its axis, until the image of the extremity of the radius adopted 

 coincides with the center of the cross ; the plane will then have che proper angle, 

 without requiring an arc divided into degrees and pans as in Mr. Hill's instru- 

 ment. 



Mr. HUl mentions several methods of adjusting the varying relation between 

 the moon's hourly motion and semi-diameter, and decides finally upon a fixed 



