D O C U M E N T 1 7 5 A P R I L 1 9 2 8 2 9 7 beyond present day technical achievements. There is, however, no Institution in this country which has the resources and personnel necessary to have the delicate ma- chinery constructed, and the considerable amount of experimental work necessary carried out. And it has occurred to me that if it should appear to you, as it does to me, to promise a power of microscopic resolution, as much as ten times that attainable by the best instruments of today it might possibly be considered a proper matter to be undertaken by the Institute of Physics in Berlin. Perhaps I am too sanguine in think- ing that it may so far interest you, especially as I can only offer it in a rather abstract form, and my description may not be very lucid, although the idea itself is exceed- ingly simple. I have touched on all the main difficulties except one, that of preparing [biolog- ical] sections in suitable mediums with perfectly clean and truly plane surfaces such as would be necessary for the instrument. I have, I think, solved this difficulty, but did not wish to make the enclosed outline too long by going into this detail,[7] Yours Very Truly, E. H. Synge If a small colloidal particle, e.g. of gold, be deposited upon a quartz slide placed above a Zeiss[8] cardioid condenser[9] of N.A [10] 1.05, then, under strong illumi- nation, all rays of light from the condenser which reach the surface of the slide will be totally reflected by the surface, except those which strike the surface at the base of the particle.[11] These will be scattered in all directions, and if the objective of a microscope is suitably arranged above the slide a proportion of the rays so scattered will come to a focus in the eye of the observer, or upon a photographic plate, or photo-electrical cell suitably placed.[12] It will be noticed that no liquid medium is supposed to have been placed upon the slide, as is usual in ultra microscopy. The slide is assumed to be perfectly clean and dry, and in that condition a colloidal par- ticle of gold is deposited on it. If, now, we take a quartz cover-glass of the kind used in ultra-microscope cham- bers (about .7 mm thick) and fix on one side of it a very thin stained biological sec- tion, the exposed side of this section being supposed to be a truly plane surface, and if we bring this cover glass, with the section attached to it, immediately above – but not touching – the colloidal particle already mentioned, then the amounts of light received from the particle by the eye (or photo-electric cell) will depend upon the relative opacity of the different parts of the section. To fix our ideas, we will suppose the colloidal particle to be very small—about c.m. in diameter, and that the illumination is very powerful, so that when the unobstructed light from this particle, after passing through the lenses of the micro- scope, falls upon the photo-electric cell, the current so produced can be amplified 10–6