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XYII.— THE PHOTOMETRY OF A BULL’S-EYE LENS 
FOR ILLUMINATING MICROSCOPIC OBJECTS. 
By Conrad Beck, C.B.E., F.R.M.S. 
{Bead April 19 , 1922 .) 
Five Text-Figures. 
The bull’s-eye first used to increase the illumination of opaque 
objects has since been employed in connexion with substage con- 
densers and dark-ground illuminators. 
The term bull’s-eye is here understood to mean a powerful convex 
lens used for the purpose of condensing light to a focus, or for 
collecting it from a point source and rendering it parallel. Those 
used with the microscope are generally between 2J in. and 3J in. 
in focal length, and of a diameter only slightly less than their 
focal length. 
The method by which they concentrate light upon an object 
cannot be properly explained by the ordinary optical text-book 
diagrams which illustrate the passage of light through a lens. 
Such diagrams show the light as a point, and illustrate what 
happens to the light which emerges from that point as it passes 
through the lens ; but a light, except in the case of a distant star, is 
never a point, but an area of a definite and often a considerable 
size. 
Fig. 1 shows the ordinary text-book illustrations in which 0 is 
a point source of light, and S is a screen illuminated by the light 
at (a) without a bull’s-eye ; at ( b ), where the bull’s-eye is so placed 
as to give out so-called parallel light ; at ( c ) and ( d ) in the two 
positions where it can be placed in order to focus the light to a 
point on the screen. Such diagrams are misleading. It would 
appear that the condition at ( c ) would produce most light, because 
as compared with ( d ) it collects a larger angle of light from the 
source and brings it all to a point on the screen, but this is not 
correct. 
The size of the source of light must be considered, and if a new 
set of diagrams are drawn in which not only the light from the 
central point but that from a point at each extreme edge of the 
source is shown, the size of the picture of the source of light which 
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