ILLUMINATING APPARATUS 31 



aperture condition would not be fulfilled until the iris dia- 

 phragm was open to the desired amount to fill the back lens of 

 the object glass; but the object itself always has some, and oftert 

 a very great, power of scattering light. Even when the condenser 

 diaphragm is cut down to a minute aperture, such an object as 

 a diatom or a podura scale scatters so much light that on look- Resolution 

 ing at the back of the object glass it is almost as bright over its s,fatSred 

 whole surface as at the spot where the direct light passes through, ^y object. 

 and the image is formed by the scattered quite as much as by the 

 direct light. 



The markings of the podura scale form a good illustration 

 of this point. The scales of this small insect appear to have 

 markings somewhat like small quills. If the aperture of the con- 

 denser is reduced so as to send direct light through only a com- 

 paratively small fraction of the object glass, the best image of 

 these quills is formed somewhat as Fig. 20 (A). If the diaphragm 

 be opened beyond a certain amount the clearness of this image 

 is reduced. If a diaphragm is inserted at the back of the object 

 glass to cut off all the scattered light, and only let 

 the direct light from the condenser through, the image 

 will be absolutely fuzzy and indistinct, somewhat as 

 Fig. 20 (B), shomng that in the delineation of this 

 object the light scattered by the object is doing the 

 work of resolution, and that it is not done so well if 

 the full cone of light is passed through the object by the 

 condenser. The same applies to a lesser extent to 

 some diatom structure, though in this case the finest 

 structure is always best shown by a large cone of light from 

 the condenser. 



The tubercle bacillus embedded in tissue is extremely difficult 

 to see unless a large cone of light is used, and in the early days of 

 its discovery its existence in the tissues themselves was doubted 

 by some of those who were not in the habit of using a condenser 

 to its best advantage. Some delicate semi-transparent objects 

 are quite invisible when a large cone of light is used, but can 

 be seen with a pinhole aperture in the condenser. It should be 

 understood, however, that appearances created by the use 

 of very small apertures may be incorrect representations of the 

 correct structure. In the case of the podura scale and the 

 structure of diatoms, the nature of the actual structure is not 

 definitely known, and it is not certain that the images obtained 

 of these apparently well-marked structures are even approximately 

 correct. Observers have frequently claimed the discovery of 

 delicate envelopes around bacilli and micrococci which a skilful 

 microscopist can at once refer to the result of incorrect illumina- 

 tion. It is generally best to use the largest aperture in the con- 

 denser that does not produce indistinctness, for the image is 

 more likely to be a correct representation ; and it is well to try 



