GHOSTS AND OCULARS. 55 



and orthoscopic. It is a form which when properly designed makes 

 one of the most useful of eyepieces, beautiful in definition and remark- 

 ably free of ghosts. 



A modification, ^5, is the so-called monocentric eyepiece of Stein- 

 heil, in which ad^■antage is taken of thickness of one of the compo- 

 nents to secure better flatness of field and also orthoscopy. Thick- 

 ness gives an additional disposable factor for this puipose in— r Avith 



respect to the last surface. This is a device practically very useful, 

 and employed both in other triplets and in photographic lenses. The 

 monocentric eyepiece has a field of about 32°, remarkably flat and 

 orthoscopic, and like other forms of triplet has a fairly good eye 

 distance. All these oculars are practically free of ghosts. 



The positi\e doulilets are commonly troul)lesome as respects ghosts. 

 As already mentioned, the Ramsden,^^ 6, even at its common separa- 

 tion gives a large hazy ghost as well as some residual color. It has an 

 ordinary field of about 35°. Its achromatic form, #7, has a slightly 

 better aperture, is more nearly orthoscopic and is quite free of color. 

 The ghost is much less troublesome, for the reason already mentioned, 

 that it lies further out of focus. 



The ordinary Kellner eyepiece,^ 8, is the worst of those in common 

 use with respect to ghosts, although otherwise valuable, since the field 

 runs to as high as 45° and the orthoscopy and color are both excellent. 

 The trouble comes from the field lens being so nearly in the focus of 

 the eye lens. Sometimes, indeed, the eye lens focus lies within rather 

 than outside the plane surface of the field lens. Just what the original 

 form of Kellner was is somewhat dubious, but probably it had a double 

 convex field lens which would not, however, improve it in the matter 

 of ghosts. 



Two other forms of positive doubtlets are Avorth mentioning, as 

 they are not only unusually free of ghosts but possess singularly valu- 

 able properties. One of them is the so-called orthoscopic ocular of 

 Zeiss and Steinheil j^ 9, consisting of a thick triple achromat with a 

 plano-con\ex eye lens almost touching it. The field is about 40°, is 

 quite free of visible ghosts for obAious reasons, and is quite flat and 

 orthoscopic, with exquisite definition and a fairly good eye distance. 

 The other form, ^ 10, is a type of long relief ocular much used in artil- 

 lery sights. It consists of two simple achromats with practically 

 equi-convex crowns placed close together with the crowns almost 

 touching. It is used either with or without a field lens of relatiAcly 



