Powell's small bull's-eye illuminator. 77 



its slip, transferred to the disc, and cemented thereon, as shown in 

 the figure. A rough edge to the disc greatly facilitates its handling. 



Fig. 2 is a tubular setting or holder, 1" long and f" diameter 

 inside measure ; the top cut out so as to leave 3 equidistant \ " 

 uprights, having small lips upon which the disc (fig. 1) is to rest. 

 The tips of the uprights are slightly inclined inwards to spring 

 lightly against, and steady, the disc, to overcome the suction of an 

 immersion objective. The uprights should be as slender as possible, 

 consistent with the needful strength, that as little obstruction as may 

 be shall be presented to the passage of the light beneath the disc. 



Fig. 3 is a brass plate 3" by 1", having a tube -§-" long fixed into 

 central hole. The tube to well fit fig. 2, the latter resting on the 

 flange, and rotating easily and smoothly, but without any " shake." 

 The small milled heads screw the plate to the principal stage of the 

 microscope (two holes being tapped in the stage-plate to receive them) , 

 that the disc may be subject to the concentric rotating and other 

 mechanical motions ; so that the whole mount may be brought suc- 

 cessively under the objective. A ny other mode of firmly fixing the 

 plate to stage may be substituted for the milled heads, which are 

 suggested merely as an easy but secure method of fastening. 



Fig. 4 is a brass rod to support the " bull's-eye " — a plano-convex 

 lens of about %' diameter and ^" focus. The rod is furnished with 

 ball and socket joint (to give universal motion), to be inserted in the 

 usual fitting in the head of the carrier (Ross model), or in the crane 

 arm (Jackson model). The rod is curved to advance the "bull's- 

 eye" approximately into position, which is afterwards brought close 

 to the disc by the ball and socket joint. The shape of the curve 

 will depend upon the form of the microscope to which the rod may 

 be fitted. The cell of the "bull's-eye" is attached to the rod by a 

 tongue-joint that it may be "kept square" whatever the inclination 

 of the rod. The rod must be sufficiently long to allow the "bull's- 

 eye" to be conveniently manipulated ; it is divided in the figure to 

 show the necessary milled stud at the tail-end. The pin of the joint 

 should fit into a square hole, thus confining all motion to the joint 

 itself, and preventing any tendency to " crawl." The socket should 

 be carefully packed to obviate all jumping motion in the joint, the 

 ball moving freely. 



Fig. 5 is a brass plate, 3" by 1|", with central aperture f " 

 diameter, turned out so as to leave a very thin and narrow flange, 

 as shaded in the figure. This aperture is to receive and support 



