ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 735 



Dark-field Illumination and Ultramicroscopy.* — The first two 

 out of the three sections of this article are devoted by H. Siedentopf 

 to methods which are substiintially those noticed in our abstract imme- 

 diately above. In his third section he discusses the examination of 

 colloidal solutions, serum, and drinking water. He points out that the 

 difficulties caused by the tendency of the ultramicrons to collect on the 

 underside of the cover-slip, or on the upper side of the object-slide, 

 include not only diffraction but those due to currents set up by varia- 

 tions in the concentration. All these difficulties are avoided if an 

 arrangement be adopted similar to that used by Siedentopf and 

 Zsigmondy in their original ultramicroscopic experiment, whereby the 

 directions of illumination and observation are mutually perpendicular. 

 The best test object for this method is a deep red colloidal gold solu- 

 tion, whose parts appear green on a bright ground, with a strong water- 

 immersion objective (Zeiss' D*) and strong ocular (Zeiss' compensation 

 ocular 12 or 18). This method is the only one suitable for the 

 examination of ultramicrons in solid bodies, e.g. glasses and crystals. 

 Direct observation of such objects would require the preparation of very 

 thin, highly polished sections, and it is found that the light-eifects from 

 the polishing errors drown out the other effects. The author states 

 that he has never succeeded in resolving the gold particles of ruby gold 

 glass by direct observation with dark-field illumination. 



Measurements of some Modern Micrometers.! — From a preliminary 

 study of some modern micrometers, M. D. Ewell arrives at the conclu- 

 sion that no advance in precision has been made in the last twenty-five 

 years. The measurements of the different scales are given. 



Microscope Lamp. J — " Anttires " remarks that for ordinary investiga- 

 tion, when daylight is past, an electric lamp is more clean and convenient 

 than any other ; but for " critical " observation its usual form is not 

 successful. His consists of a metal cylinder 2 J in. diameter and 6 in. 

 long ; the aperture is at the top, under an inclined cover, so that the in- 

 candescent lamp is not visible, and it is painted a dead white inside, giving 

 a " white cloud " effect. The cylinder can be inclined at any angle, and 

 is adjustable for height from the base by a racked pillar and pinion. 

 In the back of this cylinder, about the middle, he has now made a 

 longitudinal slit, h in. long and y^^ in. wide (rather narrower might be 

 better), which can be brought, by rotating the lamp-socket, opposite to 

 one of the straight parallel filaments of a tubular Edison-Swan lamp of 

 2^ c.p. and 100 volts, so that h in. of filament glows through the slit. 

 When the lamp is used in this way the ordinary aperture of the 

 cylinder is closed by a bent card, and the illumined slit is brought by 

 movements of the lamp-stand into line with the axis of a substage 

 condenser. The effect is thus similar to that of a small paraffin 

 flame placed edgewise, but more constant, and, he thinks, quite as 

 successful. 



* Zeitschr. wiss. Mikrosk., xxiv. (1907) pp. 1.3-20. 

 t Proc. Amer. Phil. Soc, xlvi. (1907) pp. 187-90. 

 i English Mechanic, Ixxxvi. (1907) p. 42. 



