AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
65 
plane, that the reflected pencil may pass close by the side of the chimney. It is ob- 
vious that the angle contained between the incident and reflected pencils may be 
diminished to any required extent by increasing the focal length of the mirror, as the 
lamp will then be placed at a greater distance from it, and by the same means the 
portion of- light incident on the mirror is diminished, but the illumination of the 
image remains constant, its area alone being altered by varying the focal length of 
the mirror. Mirrors of 5, 6, 8|, 10, 12, and 15 inches focal length have been tried ; 
and in conjunction with the cylindrical lenses which have been used, the mirrors of 
8^ and 10 inches focal length and 3 inches aperture, appear to have produced the 
best effects. It may be here remarked, that the lenses used have been either cylin- 
ders, or portions of cylinders cemented together, filled with water ; but with solid 
glass lenses, and more particularly with achromatic combinations, could such be 
obtained, it is probable that a better effect would be produced with mirrors of from 
12 to 16 inches focus, and 3^ or 4 inches aperture*. 
In the adjustment of the lamp, it is necessary that the plane of incidence and re- 
flexion should be perpendicular to a vertical plane passing through the slit and the 
centre of the mirror. This adjustment may be most conveniently effected by placing 
a piece of wire horizontally across the slit and the reflected pencil, and having fixed 
the lamp at such an azimuth that the brightest portion of the pencil may be incident 
on the centre of the mirror, by moving the lamp vertically until the wire appears to 
coincide with its image at the centre of the mirror. By this adjustment, the image of 
the slit, being a portion of a caustic surface, is condensed in the direction of a tangent 
plane, and the confusion of the rays forming the luminous point is a minimum : the 
effect may be observed by comparing the line, fig. 3, Plate VII., in which this adjustment 
was intentionally disregarded, with fig 4, in which it was attended to, all other cir- 
cumstances remaining precisely the same. It may be here observed, that the line 
being formed by a portion of a caustic surface condensed, the distinctness of its edge 
is independent of the distance at which the image is formed, and consequently of the 
* The register, fig. 8, Plate IX., has been obtained by means of a mirror of 5 inches aperture, and 20 inches 
focal length ; the reflected pencil being refracted through a combination of two plano-convex cylindrical lenses, 
one of which has a radius of 2 inches, and the other of 1 inch, the aperture of each being about 60 degrees, 
the interval between them about inch, and the more convex nearer to the paper. These lenses have been 
very beautifully executed by M. Lerebours. The mirror has been by successive trials carefully rendered 
elliptical, the conjugate foci being at the distances of about 2 feet, and 12 feet 4 inches from its surface. The 
latter distance was selected as suiting the locality of the Royal Observatory for which the apparatus was con- 
structed. It may not be irrelevant to mention here the means by which the elliptical figure of the mirror w r as 
obtained. After being brought nearly to the proper figure, a screen was placed in front of it consisting of narrow 
concentric annuli of card, moveable on a wire, so that either of them could be placed perpendicular to the 
surface of the mirror, w'hile the rest remained parallel to it, the lamp being placed at the distance of the nearer 
focus. By means of this screen the incident pencil might be received upon any annular portion of the mirror 
separately, and after many successive trials, the figure of the mirror was altered until an image was found to 
be formed by each annulus at the required distance. It is obvious that on the perfection of the figure of the 
mirror, the sharpness of the edge of the line will in a great measure depend. — May 1847. 
MDCCCXLVII. 
K 
