In the description of the transit circle we saw how the Astronomer-Royal had contrived to throw light into the axis of the telescope, so that the wires were either rendered visible in a bright field, or, the field being kept dark, the wires were visible as bright lines in a dark field. That is the difference between a bright field, and a dark field of illumination. Now a bright field of illumination in the case of equatorials is managed by an arrangement as follows.—A A, Fig. 149, is a section of the tube of the telescope. Near the eyepiece is a small lamp, D, swung on pins on either side which rest on a circular piece of brass swinging on a pin at C, and a short piece of tube at E, through which the light passes into the telescope and falls on a small diagonal reflector, F. This reflects the rays downwards into the eyepiece. When the telescope is moved into any position the lamp swings like a mariner’s compass on its gimbals, and still throws its light into the tube, and the light mixes up with that coming from the star, but spreads all over the field of view instead of coming to a point, so that the star is seen on a bright field, and the wires as black lines. Now if the star which is observed is a very faint one, we defeat our own object, for the light coming from the lamp puts out the faint star. [Illustration: FIG. 150.—Cooke’s illuminating lamp. ] We have seen how the illumination of the wires, instead of the field, is carried out in the Greenwich transit. The same method can be adopted in the case of equatorials, the light from the diagonal reflector being thrown on other diagonal reflectors or prisms on either side the wires in the micrometer so as to illuminate them. Messrs. Cooke and Sons have devised a lamp of very great ingenuity, Fig. 150. It is a lamp which does for the equatorial, in any position, exactly what the fixed lamp does for the transit circle. It is impossible to put it out of order by moving the telescope. There is a prism at P reflecting the light into the telescope tube, and at whatever different angle of inclination, or whatever may be the size of the telescope on which this lamp is placed, it is obvious that the lamp never ceases to throw its light into the reflector inside the telescope; and any amount of light, or any colour of light required can be obtained by turning the disc containing glass of different colours or the other having differently sized apertures, in order to admit more or less light, or give the light any colour. In both these arrangements the lamp is hung on the side of the telescope, while Mr. Grubb prefers to hang it at the end of the declination axis, as shown in Figs. 139 and 140. The function of the lamp then is to illuminate the wires of the micrometer eyepiece, of which more presently; but Mr. George Bidder places the micrometer itself outside the tube of the telescope, the light of a lamp being thrown on the wires.