230 Specific Examples of Slide Making 



surface of the stage for examination. No more common accident takes 

 place than the placing of the slide upside down on the surface of the 

 stage with the subsequent loss of all the sections. This will be avoided 

 readily if the worker will make it a matter of routine, as he lifts the slide 

 from the mordant, to hold it at an angle between himself and a light 

 source so that the light is reflected from the surface. If the sections are, 

 as they should be, on the upper surface of the slide when it is so placed, 

 they will appear to be double through the reflection from the under 

 surface as well as the upper surface of the glass. A good rule is never to 

 place a slide which has just been taken from a fluid on the stage of the 

 microscope until one has seen the double reflection. 



If a low-power examination of the section shows the nuclei to be stand- 

 ing out clearly, the entire tray should be removed to distilled water be- 

 cause from this time on differentiation is very rapid, and each slide must 

 be controlled separately. If, h owever, the nuclei are not sharply defined 

 and a considerable degree of black or bluish color remains in the back- 

 ground, then t he entire tray may be left in the iron alum for as long as is 

 necessary. When this preliminary differentiation down to the distinction 

 of the nuclei under low-power examination has been completed, it is 

 necessary to continue differentiation while examining the slides at fre- 

 quent intervals under a very high power of the microscope. It is con- 

 venient to have available a water-immersion objective. It is obviously 

 impossible to place immersion oil on a wet slide, and the short working 

 distance of a high-dry objective renders it particularly liable to cloud up 

 from the evaporation and recondensation of the water. Water-immersion 

 objectives are usually of 3 mm equivalent focus, and this gives a suffi- 

 ciently wide field to permit differentiation to be observed, while at the 

 same time it has sufficiently high magnification for satisfactory control. 

 Each slide should be taken separately, returned to the 2.5 per cent iron 

 alum for a few minutes, and then reexamined. The various phases of 

 mitosis and meiosis do not retain the stain to the same degree, and care 

 must be taken that the color is not washed completely out of the other 

 chromosomes as a result of examination only of metaphase figures, in 

 which the color is retained longer than in any other. A great deal of prac- 

 tice is required to gauge accurately the exact moment at which to cease 

 diffe rentiatio n, which may be stopped almost instantly by placing the 

 slides in a slightly alkaline solution. In Europe most tap waters are suffi- 

 ciently alkaline for the purpose and are generally specified, but in the 

 cities of the United States it is often best to add a very small quantity of 

 lithium carbonate or sodium bicarbonate to the water used to stop differ- 

 entiation. Slides may be left in this for any reasonable period of time; 

 the process is complete when the slide turns from a brown to a blue color 



Then the slides are rinsed in distilled water, graded up through the 



