10 MANUAL OF HISTOLOGY. 



receive illumination from a point opposite to the sun. North 

 light is very excellent. 



If artificial light is to be used, an ordinary kerosene burner 

 will answer sufficiently well, even better than gas. Some of 

 the highest lenses require artificial light. 



Testing the eye-piece. Eye-pieces are usually free from 

 serious defects, but if we are desirous of testing one, the fol- 

 lowing method may be followed : 



Select a combination of lens and eye-piece that gives a per- 

 fectly flat field. Then remove the eye-piece and substitute the 

 one that is to be tested. If now the image is no longer flat, 

 the eye-piece has aberration of form and should be rejected. 



Testing Jiigh lenses. In combinations that magnify about 

 five hundred times, a good test is ihepleurosigma angulatum, 

 one of the diatoms. A lens that will demonstrate three sets of 

 lines by direct light has a proper amount of defining power, 

 and with the other qualifications already mentioned, is suit- 

 able for the finer sorts of microscopical work. This task is 

 easily accomplished by either the No. 10 immersion of Hart- 

 nack or Prazmowski, the ^ of Wale, and also by lenses of other 

 good makers. 



To test the magnifying power of lenses even more accurately, 

 Nobert's test plates may be used. They consist of bands of 

 fine lines from nineteen to thirty in number. 



It has usually been thought that the eighth or ninth of 

 their series is a good test ; the nineteenth band, 1 however, has 

 been defined by a ten immersion Hartnack, and probably by a 

 goodly number of American lenses. (See Appendix.) 



Measuring the angle of a lens. Take an instrument of 

 which the pillar is hinged, and which also revolves on its ver- 

 tical axis. 



Measure off on the table, in front of the instrument, a semi- 

 circle with the pillar as a fixed point. Divide the semicircle 

 into the proper number of degrees, viz., 180. 



Place opposite the instrument, and without the circle, a 

 candle or lamp. Then interpose between the two a screen hav- 

 ing an aperture to admit a small beam of light. Revolve the 

 tube on its axis until the light can no longer be seen ; then 



1 According to Carpenter, the nineteenth band contains 113,595.13580 spaces to 

 the inch. 



