ON THE USE OF THE MICROSCOPE. 



587 



can only direct attention to this in the shape of examples. The objects of micro- 

 scopical examination are either forms or processes. 



A. With regard to the former, we have to consider two kinds of things : 



a. Actual forms, which are so universally distributed that they may interfere with 

 every examination and obscure its results. 



To these belong every thing which in ordinary life we call dust ; hence small 

 fibres of vegetable or animal tissue, or small granules of inorganic substances. 



As most objects, at least all transparent ones, are moistened with water, there 

 belongs to this category the infusoria usually occurring in water, which, without 

 very tedious preparation, by means of boiling and hermetically sealing the water, 

 can never be wholly excluded. These objects should be well known and frequently 

 observed under various magnifying powers and circumstances, so that, being per- 

 fectly conversant with them, we may at once exclude them from our consideration 

 as not belonging to the object of our observation, should they be present with it. 



b. Apparent substantial forms, consisting of substances which are formless. To 



such belong all kinds of gas which may be mechanically separated in fluids ; also 

 mechanical mixtures of two fluids that do not mix with each other ; for instance, 

 bubbles of atmospheric air in water and oil, or drops of oil in water or gum. Air- 

 bubbles especially have caused many microscopical errors, even up to the present day. 

 They always appear under the microscope, when in a fluid, as spherical bodies, with 

 a very black margin and a very small, clear, round centre. On a more accurate 

 examination of them, we may recognise reflected images of objects which happen 

 to be in the vicinity, on the black margin of the side which is turned towards the 

 light, as, for instance, the cross bars of a window-casement, &c. The explanation 

 of this phenomenon is easy. Rays falling parallel from below experience (with the 



72 a b Is the object-glass of the microscope ; c, d, e, f, a layer of water, in which is 

 enclosed g, h, p, an air-bubble. The ray of light (a-) consequently passes directly 

 through the perpendicular axis of the air-bubble ; near to it, also, the next rays, as, 

 for instance, y. The more remote ones (z), on the other hand, impinge obliquely on 

 the tangential plane of g, are thus refracted, and that from the perpendicular, from v g, 

 as they pass from a denser into a thinner medium, from water into air ; they therefore 

 travel the road g h. They are again refracted at h, but of course upwards from v A; 

 they then travel the road h i, and here they once more take a diverging direction, so 

 that none of the rays, which do not pass through the axis of the air-bubble, or close to 

 it, can ever reach the object-glass, and consequently the eye. An air-bubble must 

 therefore appear to be furnished with a broad black lightless margin, and with a bright 

 nucleus. This explanation may be readily applied to other cases of enclosed air. 



