ZOOLOGY AND BOTANY, MICHOSCOPY, ETC. 673 



size be mounted in parallel planes, and dipped together into a soap 

 solution, their planes being kept perpendicular to its surface, a suitable 

 series of films results, through which light can be passed and caught on 

 a sheet of paper, showing the desired phenomena very beautifully. 

 Since each film, under the action of gravity, is a very thin wedge, the 

 colours are in horizontal bands, appearing first at the top (where the 

 wedge is thinnest) and moving slowly down across the field as the films 

 evaporate, to be succeeded by other bands of lower orders. Indeed, 

 good films will often hold until two-thirds of the field is coloured with 

 the yellowish-brown of the first order. If the paper be replaced by a 

 good lens and the colours projected on a large scale upon a suitable 

 screen they can be strikingly demonstrated to a class. In practice the 

 important thing seems to be uniformity in size and alignment in the 

 set of rings. The author makes them of 5 • 5 cm. in diameter, of gal- 

 vanised iron wire (d = 1*25 mm.), the ends being twisted together into 

 a sort of handle. Such rings can be temporarily strung on three rods 

 notched at appropriate intervals to insure parallelism in the planes of 

 the rings, while the handles are being clamped between two pieces of 

 soft wood. The rings should be at least a centimetre apart to avoid 

 cylindrical and irregular films, and from fifteen to thirty are sufficient. 

 Before the films have become thin enough to show colours, certain 

 other interesting phenomena of a circulatory nature are noticeable and 

 can be studied. 



Wide Illuminating' Cones.* — " Villagio " expresses his gratification 

 as to the improved results he has obtained by the use of wide-angled 

 condensers and apertures. He was particularly pleased with the appear- 

 ance of A. pellucida mounted in realgar, the objective oil-immersion 

 being 1 • 35 N.A., and the condenser worked up to rather over 1 ■ N.A. 

 immersed. The lines were exquisitely sharp with widest axial cone, and 

 on removing the eye-piece the two spectral beams were seen partially 

 eclipsed by the edge of the back lens. On closing the diaphragm it 

 was instructive to note that these beams diverged until they disappeared, 

 this happening at about *8 to "9 N.A. On using the eye-piece with 

 this cone it was, of course, found that the lines were invisible. The 

 writer has also found the same arrangement of lenses and illumination 

 excellent on sections of well-stained material, beautifully clear images 

 being obtained. Similarly satisfactory results were obtained with living 

 bacteria. 



J.Ilheinberg,t however, in discussing Villagio's communication, points 

 out that arguments in the controversy of wide-angled versus narrow- 

 angled cones are apt to overlook the nature of the object to which such 

 cones may be applied, and that great caution should therefore be em- 

 ployed before any hard-and-fast rules be" adopted. After discussing the 

 effects produced by various kinds of illumination he concludes by point- 

 ing out that " whilst wide axial cones of illumination may cast a haze 

 over, or completely obliterate the appearance of structure, they cannot, 

 save in very exceptional circumstances, create an appearance of false 

 structure, whereas as soon as we proceed to narrow cones, or use oblique, 



* English Mechanic, lxxvi. (1903) p. 463. t Tom. cit., pp. 524-5. 



