4 Scientific Proceedings, Royal Dublin Society. 



meter. The tube which carries the micrometer is cut up with 

 slots, so that it forms a " cage " into which the beam of light can 

 enter at any part of its periphery, excepting only for the slight 

 obstruction of the bars of the cage. Inside this cage is another 

 piece of tube carrying a diagonal reflector, m, m, with an elliptical 

 hole large enough to allow the whole pencil of rays for objective 

 to pass. By a simple mechanical arrangement the inside tube 

 carrying the diagonal reflector, m, m, and the outside tube carry- 

 ing the reflector, R, are made to revolve simultaneously by the 

 action of one pinion head. Thus, the reflector, R, and the reflector, 

 m, m, can always be kept in such a position as to receive the 

 light from the prism, P, which is thus reflected from R to m, m, 

 and from m, m, to the four reflectors, r, r, surrounding the wire 

 frame, as in Plate 1, figure 2. 



A fifth plan is shown, Plate II., figure 3, as used by Mr. Grubb 

 for some of his larger instruments. In this plan the lamp is hung 

 at end of Dec. axis, and sends a beam through that axis. At a 

 certain point of that axis is placed a low power condensing lens, b, 

 having a small opaque disc, o, attached to its centre. In the tele- 

 scope tube opposite Dec. axis is fixed a prism, R, ground on 

 one of its faces, concave, and having a convex lens cemented on 

 to it thus forming an achromatic lens as well as a reflector. 

 This reflector directs the rays in a diagonal direction down to the 

 draw tube of the telescope, and when the curves of this prism 

 and lens are properly proportioned it forms an image of the con- 

 denser, b, and its opaque spot, o, on the upper end of the draw 

 tube, P P. The image is, therefore, of the form of a ring of light. 

 At P P is placed a portion of an excentric lens, with a hole in the 

 middle to allow the cone of rays from objective to pass. This 

 lens serves a double purpose. It alters the direction of the beam 

 so that it may travel down between two concentric tubes to the 

 little prisms, p p, round the wires, and it also condenses the beam 

 so that none need be lost, but all be caught by those small prisms, 

 p p. From these prisms, p p, the light is sent across the wires as 

 in the former cases. 



But few objections can be urged against the last three systems 

 here described, except that they are all, more or less, difficult to 

 X]i\; I-iii.! W13-1 adjusted, and, at the best, only affbrd 

 lumination for the fainter class of objects. 



