ZOOLOGY AN]) BOTANY, MICROSCOPY, ETC. 545 



but quite another to write such a description of the procedure as will 

 enable another person to carry it out. From a perusal of the work it 

 is obvious that his descriptive power is quite equal to his practical know- 

 ledge. After a short description of such Microscope stands as are most 

 suitable for the work, the author enters in fuller detail into the optical 

 equipment — objectives, oculars, condensers, and collecting-lenses. The 

 various types of illuminant and illumination are fully described — a sub- 

 ject as to the paramount importance of which no photomicrographer 

 needs to be reminded. Upon the illumination success primarily depends, 

 and the seventh and eighth chapters, by their insistence on the great 

 advantage to the amateur or beginner of a good deal of preliminary 

 experimenting, are intended to give him a firm grounding in the art 

 of illuminating an object. For such preliminary work nothing can be 

 more instructive than the observation of the image projected on an 

 opaque screen. Colour-filters for securing contrast, or for more perfectly 

 rendering colour differences in monochrome, are next considered, and are 

 followed by plates and their development. Chapter XL deals with photo- 

 micrography by ultra-violet light — a method with great possibilities, but 

 at present only suitable for practised experts — with stereoscopic micro- 

 photographs, and with the production of coloured lantern-slides. Lastly, 

 a series of progressive examples, ranging from botanical, bacteriological, 

 and pathological subjects, to diatoms, foraminifera and metallic sections, 

 and each chosen to show some special point in the structure or lighting, 

 are illustrated by ten collotype plates. 



(5) Microscopical Optics and Manipulation. 



Ultramicroscopical Study of Solutions of Iodine.* — The fact that 

 iodine gives, according to the nature of the solvent, solutions either 

 violet or brownish, has attracted much attention from chemists. J. Amann 

 has, in addition to chemical investigations, now attacked the subject by 

 ultramicroscopical methods. He finds that there is a marked ultra- 

 microscopic difference between the violet and the brown solutions. 

 Although the former only rarely contain ultramicroscopic micellte, yet 

 the brown solutions furnish an ultramicroscopic micellar-phase, more or 

 less abundant. The author fullv discusses the significance of his observa- 

 tions, which seem to confirm the theories of polymerization of iodine. 



Elliptic Interference with Reflecting 6rating.| — C. Barus de- 

 scribes a method for obtaining elliptic interference. In a previous 

 experiment (fig. 66) L was a source of light, M a glass-plate grating, 

 GmGn plane-mirrors, each reflecting a spectrum from M. It was found 

 that elliptical interference was produced whenever the rays returned 

 after passing M by transmission and reflection were made to overlap in 

 the spectrum. The author's present method is the converse of this, since 

 the gratings and the opaque mirrors now change places. Parallel rays 

 from L strike the plate of glass M, and the component rays reach identical 

 reflecting gratings G m and G„ placed symmetrically with respect to M 

 at an angle i to the E and L directions. The undivided rays pass off 



* Bull, de la Soc. Vaudoise des Sci. Naturelles. xlvii. (1911) pp. 1-50. 

 t Proc. Amer. Phil. Soc. Philadelphia, i. (1911) pp. 125-39 (5 figs.). 



