MOUNTING 125 



to that which passes only through the mounting medium should 

 not greatly exceed a quarter of a wave-length of the light used for 

 examination, if phase-contrast is to be applied effectively.^' 



A transparent object is rendered visible by direct^'' ('ordinary') 

 microscopy if it is mounted in a medium of much lower or higher 

 refractive index than its own. This is convenient if nothing more 

 is needed than a low-power view of a whole mount ; but wherever 

 the object comes up against the medium there will necessarily be 

 a Becke line^^^- ^- in the microscopical image, and this is a mis- 

 leading appearance, since it does not represent anything that was 

 present in the object. The resolution of minute detail is not 

 possible in such circumstances. 



Mounting media may be classified as hydrophil and hydrophobe. 

 The hydrophil media are those that contain water or are miscible 

 with water. Sections or whole mounts may be transferred from 

 water to hydrophil media without the necessity to use an intermedi- 

 ary liquid not contained in the medium. It is reasonable, however, 

 to introduce the mounting medium gradually to the tissue. Thus, 

 in mounting in glycerol (p. 128) or Farrants's medium one may 

 pass the section or other object through a mixture of water and 

 glycerol before mounting in the medium itself. The two sub- 

 stances may conveniently be mixed in the proportion in which 

 they occur in Farrants's medium (50 g = 41*7 ml of glycerol with 

 100 ml of distilled water). 



Hydrophobe media require the dehydration of the object, since 

 they are not miscible with water. It is usual to pass tissues through 

 a graded series of ethanols (often 70%, 90%, absolute). Tissues 

 that have been dehydrated by ethanol in the course of embedding 

 have undergone all the shrinkage of which they are capable, and 

 no harm could come from direct passage of sections from water 

 to absolute ethanol. However, the absolute ethanol would soon 

 be diluted if this were done repeatedly, and similarly the 90% 

 ethanol would soon lose its strength if direct passage were made 

 from water to ethanol of this concentration. The closer the 

 grading of the ethanols, the more accurately they will maintain 

 their concentration, but the more trouble will be involved in 



