14 Mr. John Tf. Gordon [Feb. 17, 



in that case the diffused light from ilhiminated particles lying above 

 and below the focal plane would swamp the light emanating from the 

 focal plane itself, and would substitute a luminous haze for a definite 

 image, obscuring the focused picture precisely as a fog wraps and 

 obscures the objects of a landscape. To provide against this difficulty 

 Dr. Siedentopf employs a beam of hght which, while broad enough to 

 fill the entire field of the instrument, is very shallow in the direction 

 of the line of sight. He takes as his source of light a narrow sht like 

 the sUt of a spectroscope, arranges it in a horizontal position, and 

 illuminating this slit by an electric arc, he forms by means of 

 a suitable and suitably mounted lens an image of the slit in the very 

 middle of the field of the microscope. The beam so transmitted is, 

 of course, extremely shallow at the focal point where it is a reduced 

 image of the external slit. Thus the passage of the beam makes an 

 optical section of the specimen so thin in the middle of the field that 

 the gold particles included in it stand out as individual objects against 

 the dark background of the unilluminated glass before and behind it 

 in the line of sight. Under these conditions it is possible to exhibit 

 particles, however small, provided only that they are sufficiently bright 

 to be visible, and with a source of light so powerful as that which 

 Dr. Siedentopf 's apparatus enables us to employ that limit includes 

 bodies which are extremely minute. The disseminated particles of 

 gold in ruby glass, to take the example akeady cited, can thus be 

 seen with such precision that their distances apart can be directly 

 measured.* 



The particles of gold themselves, which appear as mere shining 

 points, are so excessively small as to be beyond the imaging power of 

 any microscope as yet constructed, but in aggregation they form a 

 picture which is perfectly defined, so that it can be photographed or 

 drawn and the space relations of its constituent parts accurately 

 determined. In this, the latest development of the art of lighting 

 the stage, we seem to have pressed the artifice of dark field illumination 

 to the full limit of its theoretical capacity. 



Passing now to the second of the topics with which we have to 

 deal this evening, we are confronted by a question which may be 

 fonnulated thus : Assuming that we have an object suitably mounted 

 and suitably lighted on the stage of the microscope, how is the best 

 representation of that object in the form of an image to be secured ? 



This question suggests at once the highly technical subject of 

 lens coiTection and the structure of object glasses. That, however, 

 is much too technical to be attempted on this occasion, l)ut it is 

 not possible to allude to it at the present time, even for the pur- 

 pose of dismissing the subject, without recalling in this connection 

 the name of the late Professor Abbe, whose untimely death a few 



* A specimen of Dr. Siedentopf's apparatus exhibiting the particles of gold 

 in ruby glass was shown by the firm of Carl Zeiss, of Jena, in the Library of 

 the Royal Institution at the close of the lecture. 



