periment, are placed at different distances from the field of the photometer, the shadows of the two equal cylinders, unassisted by their projecting wings, will of course be of unequal widths. To bring their widths to be equal was the sole object of the contrivance we have been describing.

l, m, n (Fig. 1, Plate I.), are the three strong feet which support the photometer, and also a round table on which one end of each of the long narrow tables rests that support the sliding carriages which carry the lights. In each of these feet there is a screw (represented more distinctly in the Fig. 2, Plate II.) by means of which the stand or pillar which supports the photometer may be brought into a position exactly vertical.

A ground plan of a part of one of the long and narrow tables (that on the right hand) is represented in this figure; and a part also of one of the carriages which carry the lights is seen at o, p, q, r. The top of the pulley is also seen, and the line which passing over it draws the carriage on which the light stands. The place occupied by one end (that next to the photometer) of the other long table is represented by the dotted lines t, u, w, x. The place of the strong pin which, passing through a hole made to receive it, near the end of the table, is represented (in a ground plan) at s. These pins are shown very distinctly at s, s, in the Fig. 2, Plate II.

1, 2, 3, Figs. 1 and 2, are three strong braces which assist in supporting the pillar, on the top of which the photometer is placed.

T in the Figs. 1 and 2 is a strong circular table on which one end of each of the long narrow tables is

supported. This circular table, through the centre of which the pillar of the photometer passes, is supported on a strong flange or shoulder in the pillar which is made for it to rest upon.

The box of the photometer is fixed to its stand or pillar by means of a ball and socket. In the Fig. 2, this box is represented shut up by three sliding wooden doors, a, b, and c. Through the door-way on the left at a, and through that on the right at 6, light is admitted into the photometer; and that in the middle, at c, is opened in order to observe the shadows cast on the field of the instrument.

The places occupied by these three sliding-doors in the ground plan of the photometer (see Fig. 1) are as follows. The first (a) fills the opening from a to k; the second (6) that from ƒ to g; and the third (c) that from i to h.

Plate III., Fig. 3. This figure represents a plan, or rather the outlines of a bird's-eye view of the whole of the apparatus, drawn to a small scale.

a is the box of the photometer, which is represented as being closed above with its lid or wooden cover. b and c are the two sliding carriages on which the lights are placed, which are the subjects of the experi

There is a movable stage or platform belonging to each of these carriages, which, by means which will presently be described, can be placed higher or lower. It is upon these platforms, and not on the bottoms of the carriages, that the lights are placed; and, as they are movable upwards and downwards, the lights to be compared can easily be placed exactly at the same height, which is always necessary. Each of the pieces of board which form these platforms has

three holes through it, in which three cylindrical pillars pass, which stand on the bottom of the carriage, and are firmly fixed in it. The platform is attached to these three pillars at any height above the bottom of the carriage, by means of small horizontal screws, which can be made to press against the pillars. These screws are fixed in large hollow knobs of wood which are fixed to the platform, just over the holes, in such a manner that each pillar passes through the axis of one of these knobs.

One of these knobs, together with the end of the screw by which it is fastened to the pillar, is represented in the bird's-eye view of the carriage b, and another in that c, Fig. 3. The reason why the other two knobs belonging to each of these carriages are not seen is this: they are hid by a flat narrow piece of wood (represented in the figure) which, passing from the top of one of the two front pillars of the carriage to the other, serves to make those pillars more steady. A front view of the three knobs belonging to each of the carriages may be seen in the next figure.

d and e are the winches by means of which the sliding carriages, b and c, are occasionally brought nearer to and carried farther from the field of the photometer. The strong wires stretched along upon each side of each of the long tables on which the carriages slide are represented in this figure, as also the cord stretched along the middle of each table, and passing over pulleys at each end of it, and round the cylinder of the winch, which serves for drawing the carriage backwards and forwards.

The two ends of this cord are united under the table, forming of the whole a kind of band, which is kept at