\_CH.I .MICROSCOPE AND ACCESSORIES 27 



Goniometer-Okular. An ocular with goniometer for measuring the angles of 

 minute crystals. High Ocular, sometimes called a deep ocular. One that 

 magnifies the real image considerably, i. c. , 10 to 20 fold. Huygenian Ocular, 

 Huygens' O., Campani's O., Airy's O.; Fr. Oculaire d'Huygens, o. de Cam- 

 pani; Ger. Huygens'sches Okular, Campanisches Okular, see \ 45. Index 

 Ocular; Ger, Spitzen-O. An ocular with a minute pointer or two pointers at 

 the level of the real image. The points are movable and serve for indicators 

 and also, although not satisfactorily, for micrometry. A'cllner's Ocular, see 

 orthoscopic ocular Low ocular, also called shallow ocular. An ocular 

 which magnifies the real image only moderately, i. e., 2 to 8 fold. 

 Micrometer or micrometric Ocular; Fr. Oculaire micrometrique ou a 

 micrometre; Ger. Mikrometer-Okular, Mess Okular Beneches O., Jack- 

 son m. o., see $48. Microscopic Ocular; Fr. Oculaire microscopique ; 

 Ger. mikroskopisches Okular. An ocular for the microscope instead 

 of one for a telescope. Negative Ocular, see \ 42. Nelson's screw- 

 micrometer ocular. A modification of the Ramsden's screw or cob-web 

 micrometer in which positive compensating oculars may be used. Orthoscopic 

 Oculars; also called Kellner's Ocular; Fr. Oculaire orthoscopique; Ger. Kel- 

 ner'sches oder orthoskopisches Okular. An ocular with an eye-lens like one 

 of the combinations of an objective (Figs. 27, 29) and a double convex field- 

 lens. The field-lens is in the focus of the eye-lens and there is no diaphragm 

 present. The field is large and flat. Par-focal Oculars, a series of oculars so 

 arranged that the microscope remains in focus when the oculars are inter- 

 changed (Pennock, Micr. Bulletin, vol. iii, p. 9. 31, 1886). Pe riscopic Ocular; 

 Fr. Oculaire periscopique ; Ger. periskopisches Okular. A positive ocular 

 devised by Gundlach. It consists of a double convex field-lens and a triplet 

 eye-lens. It gives a large, flat field. Positive Ocular, see $ 43. Projection 

 Ocular; Fr. Oculaire de projection; Ger. Projections-Okular, see 47. 

 Ramsden's Ocular ; Fr. Oculaire de Ramsden ; Ger. Ramsden'sches Okular. 

 A positive ocular devised by Ramsden. It consists of two plano-convex lenses 

 placed close together with the convex surfaces facing each other. Only the 

 central part of the field is clear. Searching Ocular ; Fr. Oculaire d'orienta- 

 tion ; Ger. Sucher-Okular, see \ 46. Shalloiv Ocular, see low ocular. Solid 

 Ocular, holosteric O. ; Fr. Oculaire holostere ; Ger. holosterisches Okular, 

 Vollglass-Okular. A negative eye-piece devised by Tolles. It consists of a 

 solid piece of glass with a moderate curvature at one end for a field-lens, and 

 the other end with a much greater curvature for an eye- lens. For a dia- 

 phram, a groove is cut at a proper level and filled with black pigment. It is 

 especially excellent where a high ocular is desired. Spectral or spectroscopic 

 Ocular ; Fr. Oculaire spectroscopique ; Ger. Spectral-Okular, see Microspec- 

 troscope, Ch. VI. Stauroscopic Ocular ; Fr. Oculaire Stauroscopique ; Ger. 

 Stauroskop-Okular. An ocular with a Bertrand's quartz plate for mineralog- 

 cal purposes Working Ocular; Fr. Oculaire de travail; Ger. Arbeits- 

 Okular, see $ 46. 



\ 45. Huygenian Ocular. A negative ocular designed by Huygens for 

 the telescope, but adapted also to the microscope. It is the one now most 

 commonly employed. It consists of a field-lens or collective (Fig. 36. ), aid- 



