236 THEORY OF MICROSCOPIC OBSERVATION. 



every attempt to discover the structure of finely organized objects 

 as, for instance, diatom- valves by the mere observation of their 

 microscopic images, must be characterized as wholly mistaken. 



{/ NOTE. In the above the deviation of the rays caused by fine structures 

 was regarded in the usual way as diffraction, as Abbe also assumes. Never- 

 theless, it is worthy of mention that a membrane consisting of a substance 

 throughout homogeneous that is, equally transparent in every part if it also 

 has fine striated thickenings, will exhibit the same interference images as a 

 real grating, although in such a membrane refraction only and not interference 

 of the rays takes place. The like holds good for the striations which appear in 

 adjacent aqueous particles of various size. Every part projecting outwards, or 

 optically denser, acts as a cylindrical lens ; it forms a real image of the source 

 of light, which acts as a self-luminous line in front of the objective. Such 

 objects are principally met with in diatoms, scales of butterflies, cell-mem- 

 branes, &c. 



.2. REFLEXION OF LIGHT BY SMALL SPHERES, GRANULES, FINE 

 THREADS, &c., AND THE INTERFERENCE PHENOMENA THEREBY 

 PRODUCED. 



If a & (Fig. 129) is a small mercury-globule, say 20 mic. in dia- 

 meter, by medium focal adjustment, if we disregard the reflexion 

 of the objective, it will appear as a black disc encircled by a distinct 

 bright ring the so-called " diffraction ring." This is due to the 

 rays which are reflected at the margins of the sphere; these 

 margins may therefore be regarded as luminous points. Expressed 

 more accurately, the bright line is the virtual image of the 

 diaphragm, which is formed by the zone of the sphere passing 

 through the points a and b. An object crossing the incident cone 

 of light (a window-frame, pencil, &c.) is represented in this reflected 

 image as a dark line, which forms a shorter or longer arc on two 

 opposite sides of the circumference. 



It is evident that if the Microscope is focused to the reflecting 

 margin of the mercury -globule, it will appear in the objective-image 

 as a bright border, since all rays, which contribute to the formation 

 of the image, emerge from points of the plane of adjustment. 

 The result is different if the Microscope is focused to a lower or 

 higher level. The same phenomena will therefore appear, with 

 regard to the luminous edge of the sphere, as those produced by the 

 approximation or withdrawal of a source of light of small extent. 

 But in the case we are considering there is added the special 



