The microscope : and its application to vegetable anatomy and physiology / by Dr. Hermann Schacht ; edited by Frederick Currey, M. A.

  • Hermann Schacht
Date:
1855
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Licence: Public Domain Mark

Credit: The microscope : and its application to vegetable anatomy and physiology / by Dr. Hermann Schacht ; edited by Frederick Currey, M. A. Source: Wellcome Collection.

    36/230 (page 12)
    CTTAPTER II. THE APPLICATION OF THE PRINCIPLES EXPLAINED IN CHAPTER I. TO THE MICROSCOPE. We will now proceed to examine how the princij)les ex])lained in the })recediiig eha})ter are aj)])licahle to the niicrosco})e. This instrument, as its name implies, is constructed for enabling the eye to obtain distinct perception of objects, which, fi-om their minuteness, woidd be invisible, or only indistinctly visible, to unaided vision. The eye has the power, within certain limits, of ada})ting itself, that is, of altering its refractive power, in such a manner as to obtain distinct vision either of distant or of near objects; but when an object is placed within a j)articu- lar distance from the eye, the pencils of light proceeding from the ol)ject become so divergent, that the eye is unable to bring them to a focus on the retina. This distance varies with dif- ferent individuals, but may be taken at an average to be 10 inches. If, now, a double convex lens be phiced lietween the eye and the object, the pencils of light being refracted by the lens, are made to emerge with a degree of divergence so much reduced, as to enable the eye to bring them to a focus. Fig. 8 shews the course of the i)encils of light, and the manner in which the divergence is reduced ; :md the result is, that to an eye placed in the axis of the lens, the object a h a])pears to be situated at the distance, and to bo of the nuignitude of the ob- ject c d. Microscope.—A siui]>le microscope in its most simi)le form is nothing more than a convergent lens of short focal length interposed between the eye and the object, by which the obsei'ver is enabled to oVitain di.stinct vision at a distance con- siderably less than woidd be practicable with the naked eye. By the use of such a lens the apparent magnitude of any object