PRACTICAL EXERCISES WITH THE MICROSCOPE 257 



drawtube, and using a low eyepiece with movable micro- 

 meter scale.) (Beck.) 



23. Use of Concave Corrected Lens as Amplifier. — 

 Take a microscope fitted with a binocular attachment 

 provided with an achromatic concave amplifying lens. 

 Remove the binocular attachment, and focus the micro- 

 scope as a monocular with a high-power objective, using 

 the correct tube length (160 millimeters, usually). Gently 

 remove the eyepiece tube, and screw the binocular attach- 

 ment into place, without disturbing the focus. If every- 

 thing is correct, the microscope should still be near focus. 

 The amplifier thus allows an increase of tube length, from 

 160 to perhaps 230 millimeters, without altering the work- 

 ing distance of the objective, or causing under- or over- 

 correction. (Siedentopf.) 



24. The Star Test for Spherical Aberration. — Get a 

 just opaque silver deposit by one of the well-known methods, 

 on a slide 1.0 millimeter thick, and polish it w4th fine 

 polishing powder until it shows minute holes. Leave 

 one-third uncovered; put a cover-glass, 0.17 millimeter 

 thick on another third with immersion oil between, and 

 cover the last third with an immersed cover about 0.23 

 millimeter thick. Or clean a slide by rubbing it with dry 

 soap and polishing off dry with towelling paper or filter 

 paper. Smear a saturated solution of nigrosin on this 

 slide with the edge of another slide. The film may be 

 scratched by rubbing with a dry cloth, and divided into 

 three parts and covered like the silver film. Focus a 

 corrected condenser on the film of silver or nigrosin through 

 the 0.17-millimeter cover, using yellow-green light for the 

 latter. Cut down the condenser cone to nine-tenths or 

 four-fifths of the aperture of the objective. Observe, with 

 an oil-immersion objective, a minute hole in the film, giving 

 a more or less circular spot of light. If everything is correct, 

 the disc or ring into which the spot of light expands w^hen 

 out of the focus of the microscope will be equally sharp at 

 equal distances within and without the focus, though it may 

 not be equally bright. On observing the part of the slide 



