Rendering Visible of Ultra- Microscopic Particles, &c. 577 



the ultra-microscopic condition) can we differentiate it under the 

 Microscope as a rod, a thread, or an elliptical disc. 



Diffraction discs of various particles show according to their 

 size and formation great differences in brightness and colour. 



I may further mention that we were able to demonstrate 

 ■small particles in gold, silver and copper because the refractive 

 indices of these metals were essentially different from the medium 

 in which they were imbedded. As regards the oxides of organic 

 bodies, such as are contained in colloidal solutions of silicic acid 

 (Si0 2 ), oxide of alumina (A1 2 3 ), and albumen, our method is not 

 as yet applicable, doubtless because the refractive indices of these 

 bodies do not differ sufficiently from those of the medium in which 

 they are contained. 



No doubt the question will present itself to your minds, 

 whether this method of illumination can be applied with advan- 

 tage to the investigation of cellular tissues, &c. Up to the present, 

 so far as time has permitted for experiment in tins direction, I 

 must own that the result is a negative one ; this, however, by no 

 means precludes the possibility of something being done in the 

 future. But experiments with ultra-microscopic bacteria have been 

 more promising, and although at present I cannot say for certain 

 that such ultra-microscopic bacteria have actually been viewed, I 

 think I may say that my experiments point to the perfect feasibility 

 ■of making them visible, so that bacteriologists may actually discover 

 germs which have been suspected to exist. 



I will therefore give you a short description of the special 

 device which I have designed for this purpose, and which, whilst 

 differing in application from the method previously described, 

 •carries out the principle of dark -ground illumination in another 

 manner. 



Bacteria are made visible solely by the light they diffract : and 

 they appear as luminous discs on a dark ground because the direct 

 illuminating rays are stopped out. 



In the arrangement for this purpose the axis of the illuminat- 

 ing cone of light, and that of the rays diffracted by the object, are 

 in a straight line, and not at right angles to each other, as in the 

 •other methods. Preparations of bacteria can therefore be mounted 

 in the usual way. The direct illuminating rays are stopped out by 

 .a method, suggested by Abbe, viz. by grinding fiat and blackening 

 •.a small central portion of the curved surface of the front lens of 

 the objective. The portion ground away is exactly calculated to 

 suit the aperture of the illuminating objective. The technical 

 •execution of this method requires very great precision, but special 

 advantages are secured thereby. In the first place, reflections can 

 'no longer occur between the lenses ; secondly, the tedious centring 

 for dark-ground illumination is obviated ; thirdly, a stop made like 



