. Quarterly journal of microscopical science. ipping into the water that the light may just becomevisible on looking down the tube, The inclination of theaxis of the tube or body to the horizon gives the semi-aper-ture. This was found to be fifty-three degrees and a angular aperture of the pencil capable of entering fromwater into this immersion lens was, therefore, not 80°, but107°. This clearly depends upon the refractive index of thefront glass of the objective, and upon the refractive index ofwater. This verbal description will be rather more per-spicuous by reference to a diagram
. Quarterly journal of microscopical science. ipping into the water that the light may just becomevisible on looking down the tube, The inclination of theaxis of the tube or body to the horizon gives the semi-aper-ture. This was found to be fifty-three degrees and a angular aperture of the pencil capable of entering fromwater into this immersion lens was, therefore, not 80°, but107°. This clearly depends upon the refractive index of thefront glass of the objective, and upon the refractive index ofwater. This verbal description will be rather more per-spicuous by reference to a diagram; l is the lamp ; c, thecondenser, transmitting parallel rays, without deviation,through the flat side of the vessel of water (w) to the im-mersed object-glass screwed into the body of the microscope(m), whose semi-aperture was thus found to measure 53° 30. But it should be stated that immersion lenses differ cou- 112 DR. ROYSTON-PIGOTT. siderably in their working angular aperture, although thisexperiment proves that it may rise to 107°.. Theoretically, if it were possible to construct an object-glass with nearly 180° angular aperture dry, the angle at whichthe ray would enter from air into the substance of the frontglass is limited to about 41° 30 or 42°, or an angular apertureof from 83° to 84°, according to the nature of the glass. IMMERSION OBJECT-GLASSES FOR THE MICROSCOPE. 113 If we suppose a ray of light reveised, and to travel downthe instrument, and after many crooked refractions at last tobe traced within the substance of the front lens having aplane front, the limiting angle for the dry lens into air isabout 41° or 42°. On entering the stratum of air someextreme rays sj^read out nearly horizontally inwards, andconverge to the focal point. But the moment water replacesthe air all is changed ; none of the rays can possibly be in-clined at a greater angle than about 53° to the vertical, thedeviation from glass into water being, in this case, abouteleven degr
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