240 Transactions of the 



between the focal length and angular aperture of objectives for their 

 best performance. Thus the best performance is not gained by 

 giving an ^th or xV^h as much aperture as works well with xpths, 

 ■^Vths, and Aths ; and glasses with excessive angles may be found to 

 exaggerate the size of refractive spherules to a considerable extent. 

 When spherules do not absolutely touch, the best glasses magnify 

 the interspaces more than the diameters of the spheres. 



The best way of commencing the study of these films is by 

 viewing them with dark-ground illumination, say with a fine ^ inch 

 of moderate angle (or if of large angle reduced by a stop) and a 

 good achromatic condenser of considerable angle and largest central 

 stop. Some films composed of a plurality of bead layers will then 

 look like fine specimens of point lace, aud opaque white. The 

 bacteria-hke and fungoid simulating formations that appear dingy 

 brown, with transparent illumination come out Hke porcelain beads. 

 Formations as in Fig. 2 look somewhat like portions of the moon's 

 surface, and clouds of detached beads give quite astronomical efiects, 

 like nebulae, or portions of the Milky Way. 



When a high power is employed on thin films having beads 

 on more than one plane, the colours long since recognized by 

 Mr. Wenham and others as indicating resolution by the best 

 glasses will be observed. Some of these effects are extremely beau- 

 tiful and polychromatic. Upon these appearances in the slide 

 Dr. Pigott remarks in a letter to the writer, " At the best correc- 

 tion, when the focal point of some beads is brightest and whitest, 

 several coloured beads appear, all on the same plane being of the 

 same colour. The colours vary from red, orange red, yellow, to 

 blue on difi'erent planes. If the glass is over-corrected by opening 

 the screw-collar, so as to throw the diffraction rings above, then the 

 colours disappear, and the beading appears of a duU black; but 

 when under-corrected in colour, as the phrase is, the blackness dis- 

 appears. It is only when the glass is over-corrected spherically that 

 the colours vanish." 



With Mr. Wenham's apparatus for dark-ground illumination 

 with high powers these beautiful efi'ects may be obtained. Care- 

 fully used, this apparatus gives a valuable Ught-ground illumination 

 with such a glass as Nobert's ^Vth, resolving the thin films beauti- 

 fully and making the beads look very small in proportion to the 

 power. With a fine ith and C or D eye-piece the illumination becomes 

 dark ground in character, and by focussing just above isolated beads 

 numerous brilHant chromatic diffraction rings may be obtained, each 

 pair of coloured rings being separated by a black ring. 



It would be interesting to inquire how far a regulated deposit in 

 minute beads is an approximation towards a crystalline formation, as 

 it is in form an approximation towards organic structure. Modern 

 physicists recognize gradations instead of abrupt transitions between 



