210 LECTURES TO SCIENCE TEACHERS. 



we are approaching, if we have not already passed, the 

 limit that is allowed to us by the physical characters of 

 light itself. When we have to deal with objects from 



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the length of the waves of light, and must contend with 

 difficulties due to the fact that we are trying to examine objects 

 which are small in proportion to the waves of light. It is a 

 question, if it be not already certain, that even if the optical 

 contraction by transmitted light were in every respect perfect, 

 we could never distinctly see the true outline o"f objects less 

 than half a wave-length of light, though their mere presence 

 might be recognised. It may seem strange to say it, but the 

 fact appears to be that the physical constitution of light itself 

 is too coarse to enable us to see all that we could desire, and 

 could see if light were of a more refined character. A great 

 cause of this difficulty is what are called interference fringes. 

 If you have an object which ought to look like one dark 

 line, and examine it with a high power, you find dark and 

 coloured lines on each side, and in certain cases these are 

 almost as distinct as the object itself. There appears to be 

 little doubt that certain structures have been described by 

 microscopists which look as distinct as if they were real 

 markings, though due to nothing more than interference 

 fringes. Such an inaccurate interpretation is entirely due 

 to the fact that light, so to speak, breaks down when we 

 try to examine objects which are small as compared with 

 the waves of light. It would occupy a very long time 

 to enter into the full particulars, but I may say that one 

 of the principal means of overcoming this difficulty is by 

 increasing the aperture of the lenses. The width of these 

 interference fringes depends to a considerable extent on 

 the angle of divergence of the light used in illumination, 

 and passes into the object-glass, and by increasing the 

 aperture so as to bring into play more and more oblique 

 rays, the size of these fringes is lessened, and thus you 

 improve the performance of the instrument. By theory 

 the defining power ought to vary as the chord of the angle 

 of aperture, and microscopists working practically with- 

 out any regard to the undulatory theory of light had come 

 exactly to the same conclusion. With dry lenses you do not 

 get so good result as with the immersion lenses ; since with 

 the latter the interference fringe would be only three-fourths 



