THE METHOD OF DARK-GROUND 



ILLUMINATION IN BOTANICAL 



RESEARCH 



By S. REGINALD PRICE, B.A. 



Late University Frank Smart Student in Botany, Cambridge 



To the microscopist the method of dark-ground illumination, 

 and its recent extended applications, are so well known as to 

 need no general description, but to many of those who use the 

 microscope as an instrument of research the method is more or 

 less strange. Hence a short description of the general principles 

 may not be out of place, as an introduction to a brief review of 

 botanical work which has been done of late by its use. 



It is a well-known fact that small particles when illuminated 

 strongly from the side appear to the observer as though they 

 were self-luminous — diffraction images are produced and ob- 

 served by the eye. By means of these diffraction images, 

 particles which are too small for observation with the unaided 

 eye may be made visible, much in the same way as the stars, 

 although point sources of light, are visible by their diffraction 

 images. 



Every one must have observed in an early morning walk in 

 the woods, how fine spiders' webs, illuminated by lateral shafts 

 of sunlight through the trees, appear as incandescent silver lines, 

 even when so far from the eye as to be quite invisible under 

 ordinary conditions. 



Prof. Buller 1 has also shown that spores of certain fungi, 

 measuring only 10 /x or even less, can be rendered apparent to 

 the unaided eye by means of an intense beam of light projected 

 through a spore cloud, in a direction perpendicular to the 

 observer's line of vision. It is obvious that both the spider's 

 web at a considerable distance from the eye, and the fungus 

 spore in any case, are outside the possibility of unaided vision, 



1 Buller, Prof. A. H. R., Researches o?t Fungi, vii. p. 94. (Longmans & Co., 

 1909.) 



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