9 MICROSCOPE AND ACCESSORIES. 



\ 22. Huygenian Ocular. — A negative ocular designed by Huygens for the tel- 

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

 employed. It consists of a field-leus or collective (Pi. I, Fig. 6), aiding the ob- 

 jective in forming the real image, and an eye-lens which magnifies the real image. 

 While the field-lens aids the objective in the formation of the real, inverted im- 

 age, and increases the field of view ; it also combines with the eye-lens in ren- 

 dering the image achromatic (| 35). 



\ 23. Compensating Oculars. — These are oculars specially constructed for use 

 with the apochromatic objectives. They compensate for aberrations outside the 

 axis which could not be so readily eliminated in the objective itself. Oculars of 

 this kind, magnifying but once or twice, are made for use with high powers, for 

 the sake of the large field in finding objects ; they are called searching oculars ; 

 those ordinarily used for observation are in contradistinction called working ocu- 

 lars. Part of the compensating oculars are positive and part negative. 



§ 24. Projection Oculars. — These are oculars especially designed for projecting 

 a microscopic image on the screen for class demonstrations, or for photographing 



so that the image is erect as with the simple microscope. Such oculars are most 

 common on dissecting microscopes. Goniometer Ocular; Fr. Oculaire a gonio- 

 metre ; Ger. 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. e., 10 to 20 fold. Huygenian Ocu- 

 lar, Huygens' O., Campani's O. ; Fr. Oculaire d'Huygens, o. de Campaiii; Ger. 

 Huygeus'sches Okular, Campaniches Okular, see \ 22. Kellner's Ocular, see 

 orthoscopic ocular. Low Ocular, also called shallow ocular. An ocular which mag- 

 nifies the real image only moderately, i. e., 2 to S fold. Micrometer or micromet- 

 ric Ocular ; Fr. Oculaire micrometrique or a micrometre; Ger. Mikrometer-Oku- 

 lar, see \ 25. Microscopic Ocular ; Fr. Oculaire microscopique ; Ger. Mikroskop- 

 isches Okular. An ocular for the microscope instead of one for a telescope. 

 Negative Ocular, see § 21. Orthoscopic Oculars ; also called Kelluer's Ocular ; Fr. 

 Oculaire orthoscopique ; Ger. Kellner'sches oder Orthoskopisches Okular. An 

 ocular with an eye-lens like one of the combinations of an objective (PL II, Fig. 

 12, 13) 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. Periscopic 

 Ocular; Fr. Oculaire periscopique ; Ger. Periskopisches Okular. A positive ocu- 

 lar 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 \ 21. Projection Ocu- 

 lar ; Ger. Project io //s-Okular, see \ 24. Ramsden's Ocular ; Fr. Oculaire de Rams- 

 den ; Ger. Ramsden'sches Okular. A positive ocular devised by Ramsden. It 

 consists of two plano-convex lenses placed close together with the convex sur- 

 faces facing each other. Only the central part of the field is clear. Searching 

 Ocular ; Ger. Sucher-Okular, see \ 23. Shallow Ocular, see low ocular. Solid 

 Ocular, holosteric O. ; Fr. Oculaire holostere ; Ger. Holosterisches Okular, Voll- 

 glass-Okular. A negative e\e-piece devised by Tolles. It consists of a solid piece 

 of glass with a moderate curvature at one end for a field-leus, and the other end 

 with a much greater curvature for an eye-lens. For a diaphragm, a groove is cut 

 at the 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 Microspectroscope, Ch. IV. Working 

 Ocular ; Ger. Arbeits-Okular, see \ 23. 



