MICROSCOPIC ANALYSIS. INTERNAL STRUCTURE. xxxv 



represent ridges or grooves, or whether they are iUusory shadows arising from the passage 

 of Hght through a structure furnished with depressions, granules of pigment, &c. This must 

 be done by examining the object when ilkiminated by reflected hght, or a hollow cone of 

 oblique rays, such as is obtained on using the achromatic condenser with the central stop; 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 hues represent cell-walls, &c. When they 

 arise from diffraction, they vary in number according to the obliquity of the incident hght 

 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 eflfects of immersion in highly refractive Uquids, and the means 

 mentioned below. 



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

 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 sunlight 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. 

 For further particulars see Photography. 



A portion of a valve magnified in this manner is represented at PI. 11. fig. 41. 



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

 of an object, i. e. whether it is solid or cellular, &c. ; and where an object is composed of an 

 aggregation of similar parts, our remarks must be applied to these intUvidually. 



The first question arising is whether a transparent object is solid or semisolid and homo- 

 geneous, or whether it represents a cell, i. e. has an outer membrane or cell-wall and con- 

 tents of a different nature. When objects possess an outer coat, its two margins are some- 

 times easily distinguishable on examination by transmitted light, especially when its thick- 

 ness is considerable. But when the outer coat is thin, these are difficult to distinguish ; re- 

 course must then be had to other means than simple inspection; and these will vary accord- 

 ing to the nature of the object, and especially the softness of its cell-wall. Sometimes 

 crushing it may show clearly that the contents consist of a liquid with numerous molecules 

 and granules, and that the cell-wall is thin and membranous, for the subsequent addition of 

 water may separate and render both distinct. The most valuable test-method however is 

 the production of endosmosis or exosmosis. If we take a cell with a soft and thin wall, and 

 add distilled water to it, it will imbibe a certain quantity of it and become distended, and 

 often the contents will become distuactly separated and visible within ; whilst if a saturated 

 solution of some salt, as chloride of calcium, be added, it will become wrinkled and col- 

 lapsed. On treating a sohd or homogeneous body with water, it remains unaltered, or per- 

 haps swells slightly ; but on treating it with the solution of chloride of calcium, no wrinkling 

 or contraction occm-s, and its appearance is unchanged. If the outer coat be firm and re- 

 sisting, the chloride vaW not cause it to contract and wrinkle. 



If there be two coats, the outer being firmer than the inner, the latter will be wrinkled and 



