xl INTRODUCTION, 



polarize light; but of course the foramina will not do so. Charring the structure, or 

 colouring it with reagents, if organic, will sometimes afford decisive proof. 



Foramina cannot bo mistaken for elevations on the surface, because they do not become 

 more luminous as the object-glass is raised after their margin has been brought most 

 distinctly into focus ; in fact the reverse occurs : hence they so far agree with depressions ; 

 but they differ from these in their luminous appearance with high powers, and their not 

 being rendered more distinct by oblique light, but the reverse. 



p. When the structure in which they are situated is somewhat thick, and they form 

 rather tubes than foramina, as the axes of these can hardly coincide with the direction of 

 the transmitted light, their orifices will appear dark or black ; hence they might be mis- 

 taken for granules of pigment : immersion or maceration of the structure in oil of turpen- 

 tine, however, will fill them, and cause the dark appearance to vanish, whilst pigment 

 would still be visible. Examination by reflected light will also readily distinguish the 

 one case from the other. Also where this tubular structure is present, perpendicular sec- 

 tions will exhibit f urrows, which may be recognized as directed above. In distinguishing 

 foramina, the higher the power employed the less is the difficulty. 



q. It has sometimes to be decided whether certain dark lines visible at the surface of 

 objects, represent ridges or grooves, or whether they are illusory shadows arising from the 

 passage of light through a structure furnished with depressions, granules of pigment, &c. 

 This must be done by examining the object when illuminated by reflected light, or a hollow 

 cone of oblique rays, such as is obtained on using the achromatic condenser with the cen- 

 tral stop j when thus illuminated, the lined appearance will vanish, and the true structure 

 will become visible. 



r. It often happens that objects, especially highly refractive bodies, appear surrounded 

 or covered by a number of black lines, rings or annular lines, arising from diffraction, and 

 it becomes an important question whether these lines represent cell-walls, rows of dots, 

 &c. When they arise from diffraction, they vary in number according to the obliquity of 

 the incident light and the angular aperture of the object-glass ; and when the condenser 

 is used, they vary according to its adjustment, and at a particular adjustment they will 

 sometimes disappear entirely. Hence in these cases the condenser should always be used, 

 and the results obtained controlled by the effects of immersion in highly-refractive liquids, 

 and the means mentioned below. 



s. A very ingenious method has been proposed and adopted successfully by Weiiham, 

 for exhibiting the form of certain very minute markings upon objects. A negative photo- 

 graphic impression of the object is first taken on collodion in the ordinary way, with the 

 highest power of the microscope that can be used. After this has been properly fixed, it is 

 placed in the sliding frame of an ordinary camera, and the frame-end of the latter adjusted 

 into an opening cut in the shutter of a perfectly dark room, Parallel rays of sun-light are 

 then thrown through the picture by means of a flat piece of looking-glass fixed outside the 

 shutter in such a manner as to catch and reflect the rays through the camera. A screen 

 standing in the room, opposite the lens of the camera, will now receive an image, exactly 

 as from a magic lantern, and the size of the image will be proportionate to the distance. 

 On this screen is placed a sheet of photogenic paper intended to receive the magnified 

 picture. A portion of the valve of a Plearosiyma magnified in this manner is represented 

 in PI. 15. fig. 41. 



4. Internal Structure, We must be understood here as referring to the general structure 



