Rendering Visible of Ultra-Microscopic Particles, &c. 575 



particles visible, therefore, by this diffracted light the illumina- 

 tion must be arranged in such a way that none of the illumi- 

 nating rays are permitted directly to enter the eye — in other 

 words, all light, except that which is diffracted by the little par- 

 ticles, must be scrupulously excluded. Ordinary dark-ground 

 illumination would seem to be suitable for this purpose ; but it is 

 important to note that with the usual dark-ground arrangements, 

 when used with arc or sunlight, innumerable reflections occur at 

 "the several lens surfaces of the condensers, and there are besides 

 many inconvenient reflections in the preparation itself (as will 

 be explained more fully further on), so that this kind of illumina- 

 tion will not be suitable for the purpose in question. 



If, however, matters are arranged in such a way that the axis 

 -of the illuminating cone is at right angles to the axis of the cone 

 diffracted upwards into the Microscope, and if the cones are of such 

 a dimension that no part of the one overlies any part of the other, 

 then all reflections in the condenser are made harmless, and no 

 stray light can now enter the objective. This method is therefore 

 a further evolution of the so-called dark-ground illumination, and 

 permits us to use the brightest sources of light. 



Another illustration may be mentioned to make this clear. It 

 is well known that small particles of dust floating in the air 

 become visible as soon as a beam of sunlight is allowed to enter 

 through a hole in a dark room, provided the observer's eye be 

 approximately at right angles to the beam. 



If now the illumination over a small area is increased by 

 focussing a sunbeam by means of a condenser, and if the particles 

 in this area are observed by a Microscope, then we have the 

 principle of this simple method. 



Optical images of ultra-microscopic particles are polarised 

 diffraction discs, in other respects they are subject to the same 

 condition as images of stars in telescopes. 



It is not difficult to explain why this device enables particles 

 in gold ruby glasses to become visible, while ordinary methods do 

 not. Let me remind you that a high power objective only re- 

 produces a sharp image of an exceedingly thin layer of an object. 

 Now, with ordinary methods of illumination a great number of 

 layers above and below a focussed layer receive light, and number- 

 less particles lying in all these layers diffract light up into the 

 objective. As these particles are out of focus they appear in the 

 image-plane as discs of diffused light. As these diffusion discs 

 overlap one another in the image plane they form a veil of light 

 sufficiently powerful to completely eclipse the small diffraction discs 

 representing the particles in the layers actually in focus. 



It is therefore of vital importance to illuminate only those 

 particles ivhich are to be made visible, and the method of doing this 

 is by focussing the arc light upon a small spectroscopic slit, the 



2 p 2 



