Section III, 1917 [49] Trans. R.S.C. 



The Ultramicroscope. 

 By E. F. Burton, Ph.D., F.R.S.C. 



(Read May Meeting, 1917.) 



The purpose of this paper is to point out some contributions of 

 British scientists in the development of microscopy during the past 

 fifty years. The high state of efficiency of the present form of com- 

 pound microscope is probably owing more to the amateur workers of 

 England than to any other one class. 



Naturally the aim of the microscopist has always been to obtain 

 an instrument which will portray to the observer the minutest detail 

 of the object. Early in the last century these workers had reached 

 practically the limit of perfection in the development of objectives 

 and eye-pieces. Under the leadership of Wenham, they then turned 

 their attention to methods of illumination in order to improve the 

 vision. 



Two general methods of so-called oblique illumination were 

 evolved. In the first, the object was illuminated by light sent in 

 from the side and made visible by the light which it scattered in all 

 directions; part of this scattered light is directed up the microscope 

 tube. In the second method the ordinary substage mirror is used to 

 reflect light up along the axis of the microscope, but the central 

 portion of the beam is cut out by a stop and the outer portions of 

 the beam reflected by some device so as to illuminate the object 

 obliquely from all sides; the object is, again, rendered visible by the 

 scattered light. 



Both of these methods of so-called oblique, or dark-background, 

 illumination have been reinvented as forms of ultramicroscope. The 

 ultramicroscope is essentially an ordinary microscope with some form 

 of lateral or oblique illumination used to render luminous small particles 

 in suspension in solids, liquids or gases. No indication of the size is 

 given directly by the ultramicroscope, but the presence of particles 

 is made known by the light scattered from the particles. 



This particular method of illumination was first adopted by 

 Zsigmondy and Siedentopf at the Zeiss works; it is practically an 

 application to microscopic vision of the very familieir fact that if one 

 views obliquely or at right angles a beam of sunlight passing into a 

 darkened room, one sees floating in the beam particles which are in- 



Sec. Ill, Sig. 4 



