56 BELL. 



long focus. When so mounted the field is over 40°, beautifully flat 

 and orthoscopic with the finest of definition clear up to the edge. 

 The focus of the field lens is so long that no ghost is noticeable, while 

 without the field lens the slightly reduced field is still of high quality. 

 The eye lenses being large allow a relief practically equal to the focal 

 length, and sometimes more. It is a combination of which good astro- 

 nomical use might be made in finders. These two oculars, # 9 and 

 # 10, are so much superior optically to the ordinary forms that it is a 

 pity they are not more used. 



Finally we come to the group of so called negative oculars, all sub- 

 stantially ghost free, and modifications of the Huyghenian t}qpe. 

 The freedom from visible ghosts results from the relatively consider- 

 able separation of the field and eye lens whereby the 2nd order ghost 

 from the former falls much beyond the focus of the latter and merely 

 scatters a little light over the field. No. 1 1 is the ordinary Huyghen- 

 ian with ratio between the focal lengths of field and eye lens of 3:1 to 

 2:1, gi^■ing fields generally of 40° to 45° a little distorted toward the 

 edges, with pretty efficient pseudo-achromatism and generally good 

 definition. No. 12 is the Airy form with the lenses in the ratio of 3:1 

 as before, but with the field lens a rather strongly curved meniscus, 

 and the eye lens crossed. There is a distinct gain in marginal field 

 and a generally slightly better performance at wide angles. 



No. 13 is the Mittenzwey eyepiece, manufactured by Steinheil and 

 others. It is (juite similar to the Airy except that it is often nuide on 

 the 2:1 ratio, the front meniscus is less curved, and the eye lens plano- 

 convex. I can say from experience that these eyepieces are most 

 admirable. The definition is good and they give a preposterously big 

 field, near 50°. This is perhaps the most practical form of eyepiece 

 where the maximum possible field is required, the optical properties 

 being excellent and the meniscus curves being within the bounds of 

 easy construction. 



No. 14 is the solid Huyghenian eyepiece of the late R. B. ToUes 

 which he frequently made both for telescopes and microscopes. It is 

 practically a Huyghenian eyepiece with curvature ratios of 1|-: 1 

 between field and eye lens, the eye end being of course convex. A 

 groove stop, in diameter about |- the focal length, is cut around the 

 long lens at about ^ its length from the field end, practically in the 

 focus of the eye end, of which the radius is substantially half the focal 

 length. This ocular seems to have passed out of use with the death 

 of its original constructor, but it possesses remarkable qualities in 

 that it is c^uite free of reflections, gives a wide and beautifully sharp 



