144 THE USE OF THE MICROSCOPE 



on covered and uncovered objects, as in Coles's method of 

 viewing bacteria, blood parasites and spirocha)tes (46). 

 The absence of a drawtube in the binocular can be remedied 

 by the use of high dry or water-immersion objectives 

 furnished with correction collars. Oil-immersion objec- 

 tives (46) frequently require slight correction by the draw- 

 tube for thicker and thinner covers, since the hard crown 

 glass of the cover has a slightly higher refraction than the 

 thickened cedar oil. This may be obviated by using 

 covers of 0.17 millimeter thickness regularly. One dis- 

 advantage of altering the drawtube is that it alters the 

 height of the eye. Of course, it also alters the magnifica- 

 tion. For purposes of comparative measurement of 

 chromosomes, etc., objectives must be used that do not 

 require a correction collar, and do not require alteration 

 of the tube length. That is, the covers must be of standard 

 thickness, which it requires special care to ensure. When 

 any object is drawn or photographed under the water- 

 immersion objective, a note of the position of the correction 

 collar is required so that the magnification can be calcu- 

 lated. Changing the tube length when the Abbe drawing 

 camera is used, doubly alters the magnification, for it 

 increases or decreases the distance of the ocular from the 

 objective, and also from the drawing surface. Thus 

 changes of tube length are best avoided by using covers 

 of the standard thickness. 



Summary. — The concave mirror has not been required 

 by the writer on the standard microscope. Its use seems 

 mainly Umited to the Greenough binocular. The plane 

 mirror should be fixed in a central line below the condenser. 

 The plane of its surface should pass through the optic 

 axis when turned parallel to this axis. To attain this its 

 stem usually requires shortening. It may be advanta- 

 geously replaced by a 25- or 30-millimeter reflecting prism, 

 silvered on the reflecting surface, if necessary. 



The shding bar with clips on the square stage is useful 

 for searching, as is also the Detto sliding bar on the round 

 stage. A screw object traverser is necessary in many 



