I I 



NOTE ON AN EXPANDING STOP FOR DARK-GROUND 



ILLUMINATION. 



By W. R. Traviss. 



(Read December 2Ut, 1906.) 



Workers with low powers, who use dark-ground illumination, 

 know that the best effects are obtained by the use of a set of 

 " spots" or a graduated cone, by which the size of the "spot" can be 

 adjusted until that which gives the desired result is obtained. When 

 changing powers this causes a waste of time. Diaphragm aperture 

 can be controlled to a nicety by the well-known Iris diaphragm, 

 and an expanding spot working on the same principle would 

 complete the apparatus. I propose to explain in this paper as 

 simply as possible how an amateur may make such an expanding 

 spot for himself. A lathe is not necessary, the following tools 

 and materials being all that are required — a small table vice, a 

 pair of spring dividers, a pair of sharp nippers, a few so-called 

 Swiss " needle " files, a broach No. 62, and " Eagle " drills No. 7 ; 

 two dozen or so of ordinary tine toilet-pins, some hard roll sheet 

 brass about O04 in. thick, and some " sheave metal " about 

 •005 in. thick. The files, which should be 10 cm. flat-pointed, 

 can be obtained from the dealers in watchmaker's tools, and the 

 brass from the well-known merchants in Clerkenwell. 



I shall explain the geometry of the apparatus on a large scale, 

 leaving the worker to choose a size for himself, simply remarking 

 that a disc O'-i in. in diameter is about the size which will suit the 

 ordinary Abbe condenser. 



Let Fig. 1 represent a circle, within which a concentric circle 

 is drawn as close to the edge as may be, so that the holes to be 

 drilled on this inner circle shall not break through. The first 

 thing to be done is to divide this circle into ten or twelve equal 

 parts — the larger the number of divisions the rounder will be the 

 expanding spot. There are limits, however, to the number of 

 divisions, as will be seen later. Two or more of these circles 

 should be made from the 0*04 sheet brass, for use as templates 

 or as a stand-by. 



Now, taking a drill of a size that will make a hole which will 



