. Laboratory work in bacteriology. Bacteriology. 130 BACTEEIOLOGY. rays, which leave the object (when in focus) and enter the front lens. This ang-le, known as the angle of aperture, is shown at a, in Fig. 18. This figure shows, in cross-section, the lens system employed in the construction of an oil immersion objective. The rays of light diverging from the object enter the front of the small lens. It is obvious that the more light admitted by an objective the more dis- tinct the image. The angle of aperture, therefore, conditions the resolving power of an objective. In actual practice, the ra
. Laboratory work in bacteriology. Bacteriology. 130 BACTEEIOLOGY. rays, which leave the object (when in focus) and enter the front lens. This ang-le, known as the angle of aperture, is shown at a, in Fig. 18. This figure shows, in cross-section, the lens system employed in the construction of an oil immersion objective. The rays of light diverging from the object enter the front of the small lens. It is obvious that the more light admitted by an objective the more dis- tinct the image. The angle of aperture, therefore, conditions the resolving power of an objective. In actual practice, the rays of light, after they leave the object, pass into the cover-glass (a denser medium) and from this into the air, which is less dense. The result is refraction, and, more or less, loss of light by reflection. If the air is replaced. Fig. i8. Arrange- by a denser medium, there will be less re- ment of lenses in an one-twelfth inch oil fractiou and heuce, it is possible to utilize immersion objective *â ^fe"f a°'erture'^~^" ^^^^ ^^ ^^^ light that would Otherwise be lost. These considerations led to the intro- duction by Amici, of the water immersion objective. In this objective a drop of water was placed between the cover-glass and the front lens. The ray of light passing from the cover-glass into the water suffered less refrac- tion than it would if it passed into air. Since the index of refraction of water is and that of crown glass is , it is evident that there was still room for improve- ment. In 1878, Stephenson suggested that the water in the immersion objectives be replaced by an oil having the same index of refraction as crown glass. Abbe and Zeiss, thereupon, introduced the homogeneous oil immersion objective. The cedar oil, which is placed between the cover-glass and the front lens of the object, has the same index of refraction as crown glass (). Conse- quently, a ray of light after it once enters the cover-glass passes in a straight line dire
Size: 1192px × 2098px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisherannarbormichgwahr
