8 MICROSCOPE AND ACCESSORIES. 



of aperture is ^° = 45°. The sine of 45 is 0.707 whence N.A. = « or 1.52 X sin 

 u or o 707 = 1.074. 



By comparing these numerical apertures : Dry 0.799, water 0.972, homogene- 

 ous immersion 1.074 the same idea of the real light efficiency and image power of 

 the different objectives is obtained, as in the graphic representations shown in 

 PI. V., Fig. 42, 43> 44- 



THE OCULAR. 



§ 19. A Microscopic Ocular or Eye-Piece consists of one or more converging 

 lenses or lens systems, the combined action of which is, like that of a simple mi- 

 croscope, to magnify the real image formed by the objective. 



Depending upon the relation and action of the different lenses forming oculars, 

 they are divided into two great groups, negative and positive. 



\ 20. Negative Oculars, are those in which the real, inverted image is formed 

 within the ocular, the lower or field-lens serving to collect the image-forming rays 

 somewhat so that the real image is smaller than as if the field-lens were absent 

 (PI. I, Fig. 6). As the field-lens of the ocular aids in the formation of the real 

 image it is considered by some to form a part of the objective rather than of the 

 ocular. The upper or eye-lens of the ocular magnifies the real image. 



\ 21. Positive Oculars are those in which the real, inverted image of the object- 

 ive is formed outside the ocular, and the entire system of ocular lenses magnifies 

 the real image like a simple microscope (PI. I, Fig. 5). 



Positive and negative oculars may be readily distinguished, as a positive ocular 

 may be used as a simple microscope, while a negative ocular cannot be so used 

 when its field glass is in the natural position toward the object. By turning the 

 eye-lens toward the object and looking into the field-lens an image may be seen, 

 however. 



Special names have also been applied to oculars, depending upon the designer, 

 the construction, or the special use to which the ocular is to be applied. The fol- 

 lowing are used in the anatomical department of Cornell University : — * 



* In works and catalogues concerning the microscope and microscopic apparatus, 

 and in articles upon the microscope in periodicals, various forms of oculars or eye- 

 pieces are so frequently mentioned, without explanation or definition, that it 

 seemed worth while to give a list, with the French and German equivalents, and 

 a brief statement of their character. 



Achromatic Ocular ; Fr. oculaire achromatique ; Ger. achromatisches Okular. 

 Oculars in which chromatic aberration is wholly or nearly eliminated. Aplanatic 

 Ocular ; Fr. Oculaire aplanatique ; Ger. aplanatisches Okular (see \ 13). Bi- 

 nocular, stereoscopic Ocular ; Fr. Oculaire binoculaire stereoscopique ; Ger. stere- 

 oskopisches Doppel-Okular. An ocular consisting of two oculars about as far 

 apart as the two e3^es. These are connected with a single tube which fits a monoc- 

 ular microscope. By an arrangement of prisms the image forming rays are divided, 

 half being sent to each eye. The most satisfactory form was worked out by 

 Tolles and is constructed on true stereotomic principles, both fields being equally 

 illuminated. His ocular is also erecting. CampauPs Ocular (See Huygenian 

 Ocular). Compound Ocular ; Fr. Oculaire compose" ; Ger. zusammengesetztes Ok- 

 ular. An ocular of two or more lenses, e. g., the Huygenian (see Fig. 5 and 6). 

 Deep Ocular, see high ocular. Erecting Ocular ; Fr. Oculaire redresseur ; Ger. 

 bildumkehrendes Okular. An ocular with which an erecting prism is connected 



