362 SUMMARY OF CUKRENT RESEARCHES. 



an observing telescope with cross wires and rack-aud-pinion focusing 

 adjustment. It is of such precision that the various wave-lengths can 

 be easily determined by its use. The observing telescope is moved by a 

 micrometer screw which measm-es e sin Q or the sine of the angle of 

 rotation multiplied by the grating space in Angstrom units. 



Expanding Spot for Dark Ground Illumination.* — W, R. Traviss 

 describes in detail the construction of an ingenious expanding spot for 

 dark ground illumination. The principle of the mechanism is the con- 

 verse of that of the iris diaphragm, that is to say, the thin metal sheaves 

 are so pivoted that instead of producing by their movement a circular 

 opening of adjustable diameter, they produce an expanding disk. 



The apparatus is built up on an ordinary " spot " such as is supplied 

 by opticians, fitting into the ''spot" carrier or swing arm of their 

 condensers. 



Round the spot are drilled a number of pivot holes, as near the edge 

 as jjossible, one for each sheaf, the number being only limited by the 

 skill of the workman, though 10 or 12 have been found in practice to 

 give a sufficiently rounded disk (plate XV. fig. fi). 



The sheaves are actuated by minute pins which fit each into a slot or 

 groove on an upper or moving plate (plate XV. fig. 5), centred on the 

 " spot," to which a lever arm is attached for the purpose. As this slotted 

 plate turns the radial pins are forced to move along the slots or grooves 

 and so the sheaves are uniformly and gradually expanded at the will of 

 the operator. 



The mechanism, though exceedingly simple, depends for its efficiency 

 on the acciu'ate working of the individual sheaves. The method of 

 securing this is as follows : — 



A circular disk of metal, about 0'005 in. thick is taken and a concen- 

 tric circle inscribed in it at the same distance from the edge as the centres 

 of the pivot holes (plate XV. fig. 6). Starting from any point on this 

 inner circle an arc is inscribed with a radius such that it passes a little 

 above the centre (plate XV. fig. 6) [The part C E of the diameter A C 

 represents the length of the slot (plate XV. fig. 5) and it will l)e obvious 

 that the number of slots regulates the length of C E, for, as the number 

 increases, so does the risk of their breaking into one another at the 

 centre. These slots may be usefully replaced by radial grooves in a 

 thicker piece of metal ; this greatly increases the strength and rigidity 

 of the apparatus.] 



With centre B, the pivot at which the arc cuts the inscribed circle, 

 and the same radius, another arc is inscribed and the sector between B 

 and A is removed, leaving enough metal round the points B and A for 

 the pivot and radial pinholes respectively, as shown in the shaded 

 portions of fig. 3, plate XV. The resulting disk with sector removed as 

 described constitutes a sheaf. 



The number of sheaves required having been prepared, one for each 

 pivot hole (plate XV. fig. 6) and a minute pin fixed in each, one up and 

 one down as in plate XV. fig. 4 [the pins shown are intended to have their 

 ionger parts cut off flush with the sheaf, leaving the projecting points of 



* English Mechanic, Ixxxiv. (1907) pp. 596-7 ; see Journ. Quekett Micr. Club, 

 X. (1907) pp. 77-82 (6 figs.). 



