916 Transactions of the Society. 



XXVI. — On a New Method of Testing an Ohject-glass used as a 

 Simultaneous Condensing Illuminator of hrilliantly reflecting 

 Ohjects such as minute Particles of Quicksilver. By G. W. 

 KoYSTON-PiGOTT, M.A., M.D. Cantab., F.K.S., &o. 



(Bead ISth November, 1880.) 



The recent advances made in object-glass illumination in America, 

 induce me to describe some results obtained nearly two years ago. 

 These results surprised me very much at the time, and led me to 

 believe an infallible test had at length been discovered. Some of 

 the phenomena are truly remarkable, and were at first extremely 

 puzzling. They appeared to present a new order of diffraction 

 rings of exquisite precision and beauty of arrangement. 



The apparatus employed consisted of some excellent |ths and 

 T^^ths. A Smith illuminator, consisting of a disk of glass placed at 

 45° in the optical tube, illuminated the objects by horizontal rays. 

 The object-glass then condensed the flame upon the stage. 



Mercurial globules, forced by a piston through a leather bag 

 contained in a glass syringe, were formed extremely clean ; these 

 were then continuously smashed with a steel spring ; examined ; 

 selected ; and secured under a glass cover. 



Viewed with dry lenses, extraordinary forms appeared. Minute, 

 flat, circular mirrors (mirrorlets), spherical globules adhering to the 

 upper glass, and particles adhering to the lower glass slide. The 

 latter varied through many sizes, and each presented brilliant 

 rings in the sharpest focal plane of remarkable appearance, totally 

 difierent from anything seen before. 



Diffraction rings for a corrected glass are almost entirely either 

 outside or inside the focal plane. These were hi it. 



The diameter measured by micrometer (reading to ^pg-^^j^^ths) was 

 nearly in every case nine-tenths of the diameter of the globule. In 

 many an exact image of the flame presented edgeways was 

 accurately depicted. 



On the plane side of the circular little mirrorlets could be 

 seen occasionally black points, clustering more or less, of a minute- 

 ness surpassing all previous observation : dealing an astounding 

 blow against the microscopic dogma of a hundred thousandth of an 

 inch being the limit of vision, " light being too coarse a thing to 

 show anything less than half a wave-length." But more of this 

 anon. 



Another order of phenomena is differently produced. In 

 order to preserve the mercurial particles from premature tarnishing, 

 drops of liquid were introduced ; my astonishment was great to find 

 that the brilliant rings were now diminished to one-ninth their 



