74 HOW TO WORK 



appreciable impairment of sight, but I would not recommend any 

 one to subject himself to the same risk unless he passed through the 

 same gradual process, using first only low powers, moderate light, 

 and working only for a short time, slowly increasing the magnifying 

 power, the illumination, and the period of study as he felt he was 

 able to stand it 



14O. General Considerations with reference to the Nature of the 

 Medium in which Tissues should be plaeed for Examination. If the 

 structure be dry and very thin, or if it is required only to make out 

 any general points with reference to its outline, or the character of 

 its surface, it may be examined in air. So also many structures sub- 

 jected to examination by low powers, and by reflected light, exhibit 

 the general arrangement of their component parts very satisfactorily 

 when mounted perfectly dry. 



If, however, the texture be delicate and moist, and readily 

 destroyed by careless manipulation, it is generally desirable to examine 

 it in some aqueous fluid when quite fresh. The character of the 

 fluid must differ in different cases. Water answers well in many 

 instances, but the microscopical character of some textures are com- 

 pletely altered by water, or even altogether destroyed by it. Other 

 tissues are so dark and opaque that they are not well displayed in 

 water. Soft and cell-like structures become distended by it, but it 

 does not follow that when this happens it depends upon a " cell," 

 or bladder closed at all points, being distended. It does not prove 

 that the cell has a membranous wall, for a mass of jelly may be made 

 to swell out just like a " cell." If the jelly be made with a dense 

 fluid, the more limpid water will pass in and mix with it. The 

 " cell " thus becomes distended by this flowing in or osmosis, 

 and often to such a degree that it is invisible. To prevent this 

 result, it is necessary to immerse the structure in some fluid approach- 

 ing to that in its substance, or in its interior, in density. A little 

 white sugar may be dissolved in the water. Saliva, the vitreous 

 humour, serum, or white of egg, from their viscidity do not permeate 

 readily, and are advantageous media. But of all substances soluble 

 in water, glycerine is one of the most useful to the microscopist. 

 With glycerine he may obtain a fluid of any density, and of various 

 degrees of refracting power. Moderately strong solutions of glycerine 

 preserve animal and vegetable structures for any length of time. Glyce- 

 rine is to moist tissues what Canada balsam is to textures which are 

 capable of being dried, without their structure being impaired. The 

 most dense, opaque, and ill-defined structures, immersed in glycerine 

 become clear and transparent ; and anatomical peculiarities which 

 were before indistinct, or not observable, become demonstrable 



