PREPARATIONS FOR HIGH POWERS. 153 



to enable thin sections to be cut with a modern microtome, the 

 material for cutting must be imbedded. This may, of course, be 

 accomplished by placing the material in gum solution and freezing 

 in the ordinary way, but, as Dr. Beale has pointed out, once allow 

 the tissues to leave glycerine or syrup, and change immediately 

 sets in, and the whole advantage of the process is lost. Besides, 

 the crystals formed by the freezing of pure gum solution more or 

 less break up all delicate structures. If, however, gum and syrup 

 be used as recommended by Mr. Cole, or a small quantity of 

 glycerine be added to plain gum, change is prevented and the 

 crystals are so finely granular as not perceptibly to injure the most 

 fragile tissues. I think the gum and glycerine is to be preferred. 

 Both syrup and glycerine retard freezing. Glycerine quite pre- 

 vents it, unless a small quantity only be used, and as the strength 

 of glycerine varies this must be ascertained by experiment. 



Sections thus made are mounted in glycerine jelly, prepared 

 as follows : — 



Glycerine ... ... 150 grammes. 



Isinglass ... ... 30 „ 



Water ... ... ... q.s. 



Dissolve the isinglass in as little water as possible, using gentle 

 heat, clarify with white of egg, strain through muslin, add the 

 glycerine, and keep the whole for some days at about 60" C. until 

 the greater part of the water has evaporated. In this way a very 

 highly refracting jelly may be made. Preparations mounted in 

 glycerine, as recommended by Dr. Beale, after a year or more 

 become exceedingly translucent, so that minute points of structure, 

 I'articularly intranuclear networks, can no longer be distinctly 

 made out. This is not tliC case with glycerine jelly. As little 

 heat as possible should be used in mounting, or the sections 

 shrink. After the cover-glass is put on, and all superfluous jelly 

 cleaned off, the mount may be ringed with gold size, which is, I 

 think, the most reliable of any cement for this purpose. 



But where very faithful and instant fixing in statu quo is 

 required, I think a slight modification of Flemmings' chromo- 

 aceto-osmic acid is the best fixing agent, although on account of 

 its t'eeble powers of penetration it can only be employed where 



