CHAPTER XTV 

 MIRROR, STAGE, NOSEPIECE, AND DRAWTUBE 



The Mirror. — The concave mirror is doubtless out of 

 date on the standard high-power microscope, and should 

 be abandoned, in the writer's opinion, when a condenser is 

 fitted. It is still useful, however, on the Greenough 

 binocular, and for objectives with apertures below 0.3. 

 An aperture of 0.3 (or slightly more) in the mirror is too 

 much for the ordinary Greenough, with objective apertures 

 about or below 0.1; and hence the apertures of some 

 mirrors require, as has been already stated, to be cut down, 

 to produce the most distinct vision. (The concave mirror 

 gives a larger side to side than back to front aperture.) 

 But a cone of 0.3 is better given by a corrected dry con- 

 denser than by a concave mirror; or by the same water- 

 immersion condenser of aperture 1.25, which is used for the 

 highest powers; if the condenser is, as it should be, of 

 sufficient focal length to give a large enough image of the 

 strongly illuminated part of the ground-glass disc for the 

 10 objective. 



The plane mirror, in the writer's experience, usually 

 gives two fairly strong extra images, besides the normal 

 one; and this is somewhat prejudicial to correct focusing 

 of the condenser, and must cause some glare. According 

 to Dallinger (43) and Coles (46) by rotating the glass, the 

 three images of the edge of the lamp flame may be placed 

 in a back and front line, but this is no advantage with a 

 circular source diaphragm. A right-angled triangular 

 prism, with total reflection, yields only one image, and so 

 reflects more light than the mirror. Such a prism, with 

 two equal sides, is equivalent to a thick glass plate with 

 plane-parallel sides ; and so is not affected with the ordinary 

 chromatic aberrations, and produces only a slight spherical 



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