DARK-GROUND ILLUMINATION. 81 



is to be scribed from the centre (Fig. 2) will now be more or less 

 clear. If there were too many sheaves and the slots were too 

 near together at the centre, they would break into one another 

 and spoil the whole thing. This point should be well considered 

 before starting to make the apparatus, though, of course, the 

 greater the number of sheaves the rounder the spot will be. 



]f there are many sheaves, do not scribe the arc quite so near 

 the centre the smaller the number of sheaves the nearer to the 

 centre may the arc be scribed. The spot will always expand to a 

 maximum, but with a very small number of sheaves the circle 

 is not very true. It would be as well to make paper models, 

 to see exactly what happens at varying distances from the 

 centre. 



Having prepared nil the necessary pieces, the whole may now 

 be put together, starting from any hole. Lay a sheave on the 

 plate, with the pivot-pin in the hole of the lower plate (Fig. 1). 

 Nos. 2 and 3 will go on easily, but No. 4 will have to be put in 

 by lifting up B of No. 1 and so on, adjusting each under No. 1 

 till the whole are in their places: this can be done witli the aid 

 of a sharp needle. When all are in place, take the slot-plate 

 (Fig. 5) and drop it very carefully over the whole, getting as 

 many pins into their proper slots as possible, and placing the rest 

 by the use of the needle. This done, pass a pin through the centre, 

 and the working of the apparatus may be tested. No force must 

 be used, for it should open and close very easily. The most 

 troublesome part of the work is now over. 



The next thing requiring consideration is how the apparatus 

 is to be mounted, whether on a stem or on a stop fitting into the 

 spot-carrier. Cut a circle of 0*04: in. brass to fit the spot-carrier ; 

 scribe a circle in the centre of the size of the template Fig. 1. 

 Drill a small hole in the centre, and prick off and drill all the 

 other holes ; then, with a fret-saw, cut away the metal as shown 

 in Fig. 6, leaving two bars to support the centre-piece. This 

 will now act as the lower plate ; the sheaves are put in, the 

 slotted plate on the top, and the thing is done. All that is now 

 required is a lever of proper length to move the upper plate and 

 expand the sheaves. One small matter, however, must not be 

 forgotten — viz. a " steacly-pin " notch on the edge of the large 

 stop, and a pin in the shoulder of the spot-carrier to prevent the 

 whole from turning bodily when moving the lever. 



Journ. Q. M. C, Series II.— No. 60. 6 



