Microscopical Researches in Hiyh Power Definition. 263 



moving freely in the fluid, I propose to describe in another portion of 

 the present research. Suffice it to say, that they fairly and surprisingly 

 imitate the starry lustre of a midnight sky — the Milky Way and the 

 various phenomena of the closest double stars.* They display too under 

 the finest attainable amplification the most accurate diffraction rings I 

 have ever witnessed, and as such give admirable indications of quality 

 and correction in the lenses employed. In this case floating mercurial 

 molecules,t uncompressed, reflect astonishingly minute focal images. 



From these observations the conclusion may safely be drawn that 

 real objects of unsuspected minuteness may be microscopically dis- 

 played, as well as minute miniature images, such as I have elsewhere 

 described. % 



There is only one objection to this result. It may, perhaps, be said 

 that bright diffraction diminishes the apparent diameter of these black 

 points. But they may be observed almost touching each other. The 

 great difference in diffraction in the case of black particles seen on a 

 flat illuminated disk of mercury, and black particles seen by trans- 

 mitted light, so soon as the objects are illuminated from beneath, is 

 most striking. Besides this, continued observations of another kind 

 confirm the conclusion arrived at. The question of the minimum 

 visible regarded as visual angle is of equal importance in telescopic as 

 in microscopic researches, and is well illustrated by the appearance 

 of minute stars in large telescopes of great excellence. There the 

 actual angular aperture is infinitesimally small, as the rays enter the 

 instrument in a state of parallelism. As, however, an object may be 

 supposed to move up towards the instrument, the angular aperture 

 gradually assumes a tangible quantity. For instance, a fly's foot 

 viewed with Lord Rosse's gigantic reflector of 6 feet diameter, formed 

 an angular aperture of 5° 44", the object being 60 feet distant. At 

 a certain point, as the focus is shortened, a telescope becomes a long 

 focus microscope. The instruments are, indeed, convertible. A very 

 long focussed microscope is in reality a shortened telescope. At this 

 point the laws of vision apply identically, and the angular aperture is 

 the same for each. The question, then, of vision in the microscope, 

 when only small aperture is engaged as in the telescope, may aptly be 

 illustrated by minute astronomical observations. The eye, too, engages 

 a near object at a small angular aperture : the pupil is then the base, 

 and the object the apex of an isosceles triangle measuring the angular 

 aperture, as much perhaps as a couple of degrees in short sight, but 

 infinitesimally small as in the telescope when directed to a star. The 



* Great pains were taken to secure brilliant illumination in a dark room. 



f These yery minute mercurial globules move freely 5 as though floating. 



| " Phil. Trans.," 1870 ; " Month. Jour. Eoy. Mic. Soc," " Testing Objectives 

 by Miniatures of Mercurial Globules," vol. i, 1875 j "Limits of Yision," vol. i and 

 vol. ii, 1876. 



