ILLUMINATION OF OBJECTS. 



.green, showing faint traces of a certain reticulated skeleton of 

 vegetable fibre. If it be held up before the sun, all light being 

 excluded from the side presented to the eye, it will appear with a 

 much paler tint of green, and the skeleton will become much 

 more visible, the finer parts before invisible being distinctly 

 seen. 



A stained glass-window viewed from the outside appears to 

 have dark and dull colours, and might be taken to be opaque, 

 showing no form or design. Yiewed from the inside, forms of 

 great beauty, and colours of remarkable splendour, are seen. 



When we say, therefore, that objects viewed in a microscope 

 present very different appearances, according as they are illu- 

 minated by a front or a back light, we only state a general fact 

 common to all visible objects. 



No body can be said to be either opaque or transparent in an 

 absolute sense. Bodies considered to be the most opaque, such 

 as the metals, are found to be translucent when reduced to thin 

 leaves. Even gold and platinum, the most dense of the metals, 

 are rendered translucent under the hammer of the gold-beater, 

 while glass, diamond, air, water, and similar bodies, commonly 

 considered to be transparent, are proved to absorb a portion of the 

 light transmitted through them, this absorption increasing with 

 the thickness of the medium. There is in fine no body which 

 will not become opaque if sufiiciently thick, and none that will 

 not become more or less translucent if sufiiciently thin. 



45. Since microscopic objects are generally of extremely 

 minute dimensions, they are all, with some few exceptions, suf- 

 ficiently translucent to be rendered visible by a back light. 



It is well known that many bodies, which are opaque or nearly 

 so, may be rendered translucent by saturating them with certain 

 liquids. Thus, as every one knows, paper, linen, and other 

 porous bodies, which when dry are imperfectly translucent, 

 become much more so when wetted or oiled, or saturated with 

 white wax. 



This general physical fact has special and important application 

 in the preparation of microscopic objects, which are saturated 

 with various liquids, proper for each of them, by which they are 

 rendered translucent. 



When a translucent object is rendered visible by a back light, 

 the intensity of the light must be regulated according to its 

 translucency. The more translucent it is, the less intense must 

 be the light. A strong back light thrown upon a very trans- 

 lucent object drowns it, and renders it altogether invisible. The 

 light must therefore be reduced in intensity by varying the incli- 

 nation of the reflector, the distance of the lamp from it, and by 



43 



