366 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



and of uniform intrinsic luminosity. The yellow and green rays are 

 fine enough to give interferences observable to a difference in step of 

 22 cm. — i.e. in the neighbourhood of the number of order 400,000. 

 The old form of mercury arc (Perot-Fabry model) gives almost the 

 same result immediately after lighting. But, after a few minutes' action, 

 probably in consequence of vapour-heating, the rays widen out, and the 

 limit of interference falls almost to half of its primitive value. In the 

 Cooper-Hewitt, on the contrary, the first state is indefinitely maintained. 

 The yellow rays give especially clean interferences, and the phenomena 

 of disparition, or of successive doubling when the two rays are simulta- 

 neously employed, are observable to very great differences of step. The 

 fact that the lamp is constructed for industrial purposes is found to be 

 an advantage. 



Davis, D. J. A. — A Method of Microscopic Observation by means of Lateral Illumi- 

 nation. Trans. Chicago Pathol. Soc, vi. (1904) p. 90. 

 Geeil — Beleuchtungsapparate mit Nernstchem Gliihlicht. 



Anat. Anzeig., Erganzungsh., xxv. (1904) p. 178. 



Kala'hne, A. — TIeber das Woodsche Lichtfilter fur ultraviolette Strahlen. 



Phys. Zeitschr. v. (1904) p. 415. 



Plugeb, A. — Die Quecksilberlampe als ultraviolette Lichtquelle. 



Tom. cit., p. 414. 



Anon. — Vorrichtung zum Wechseln der Bilder im Projektionsapparat. 



Deutsche Median. ZeiL, 1905, p. 127. 



(5) MicroscopicalfOptics and Manipulation. 



Ultramicroscopy of Oleosole.* — J. Schneider and J. Just have been 

 investigating the appearances presented, when viewed by ultramicroscopic 

 methods, by finely divided particles of gold and other noble metals in a 

 viscous, yet completely homogeneous, transparent, and combustible fluid. 

 For this purpose fats and ethereal oils were treated with chlorides of the 

 metals, and the products of reaction ultramicroscopically studied. The 

 authors summarise their conclusions as follows : — 



1. The ultramicroscope is adapted to the determination of the identity 

 and purity of oils and to the testing of oil-mixtures. 



2. The oils are tested with the ultramicroscope after they have been 

 surrounded with oleosole of the noble metals by means of their reactions 

 with compounds of the latter. 



3. In the ultramicroscopic study observation was made of the points 

 and groups sparsely aggregated in the whole cone ; the determination of 

 the diameter of the interference rings produced by any slight alteration 

 of the fine-adjustment ; the especial examination of ringless or ringed 

 points and groups, with and without the analyser. 



4. In the examinations made of commercial oils the various forms 

 assumed by the fat, as distinct from the oil, under the ultramicroscope 

 were a distinct hindrance to the observation of the metal particles ; the 

 use of ultramicroscopes with heating arrangements were necessary for 

 further work. 



5. The effect of the reaction is affected by many circumstances, 



* Zeitschr. wiss. Mikrosk., xxii. (1905) pp. 481-530. 



