244 
Transactions of the Society. 
rays in each bundle of course converge, to form an areal image of 
the object in the plane of the eye-piece diaphragm.) The rays 
now meet the middle or field lens and are rendered convergent by 
it as they pass toward the eye lens. (The latter causes a magnified 
image of the areal image produced in the plane of the diaphragm 
to be received by the eye in the usual manner.) An eye-piece of 
approximately six diameters magnification constructed on the 
above plan was found to have a field 50 p.c. larger in diameter 
(and therefore of 125 p.c. larger area) than that of the Huyghens 
type eye-piece of similar power. Further, it was parfocal with the 
Huyghens type, and was equally free from chromatic and spherical 
aberration and had a rather flatter field. 
The new eye-piece had the following optical constants : — 
Collimator 
Field 
Eye Ramsden 
Lens 
Diaphragm Lens 
Lens Disc 
Diameter 
20 mm. 
18 mm. 20 mm. 
10 mm. 
Focal length .. 
12 D 
: 22 D 
31 D 
Intervals 
40 mm.-> : -4- 5 mm. 25 mm. 
12 mm.->: 
Apparent diameter of field (at 25 cm. distance) = 18*4 cm. 
Calculation shows that other eye-pieces designed on the same 
plan should have fields roughly 50 p.c. larger in diameter than 
those of the usual eye-pieces of similar power. 
Compensating eye-pieces of similar design can be readily made 
by making the eye lens achromatic on Abbe’s principle. An 
eikonometer with large field has been designed on a similar 
principle. 
