22 



MICROSCOPE AND ACCESSORIES 



[CH. I 



396 ; Lewis Wright, Philos. Mag., June, 1898, pp. 480-503 ; Carpen- 

 ter-Dallinger, Chapter II ; Nelson, Jour. Quekett Micr. Club, VI, pp. 

 14-38. 



THE OCULAR 



g 35. 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 

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



FIG. 30. Sectional view of a Huygenian ocular to show 

 the formation of the Eye-Point. 



Axis .Optic axis of the ocular. D. Diaphragm of the 

 ocular. E. L. Eye-Lens. F. L. Field-Lens. 



E. P. Eye-point. As seen in section, it appears some- 

 thing like an hour-glass. When seen as looking into the 

 ocular, i. <?., in transection, it appears as a circle of light. It 

 is at the point where the most rays cross. 



Depending upon the relation and action of the different 

 lenses forming oculars, they are divided into two great 

 groups, negative and. positive. 



36. 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 

 (Fig. 21 ). 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. 



37. Positive Oculars are those in which the real, inverted image of the 

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

 the real image like a simple microscope (Fig. 16). 



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-lens 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. 



In works and catalogs 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 

 seems 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 \ 21). Binocu- 

 lar, stereoscopic Ocular ; Fr. Oculaire binoculaire stereoscopique ; Ger. stereosko- 

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

 the two eyes. These are connected with a single tube which fits a monocular mi- 

 croscope. By an arrangement of prisms the image forming rays are divided, half 



