608 SUMMARY OF CURRENT RESEARCHES RELATING TO 



yellow screen, are used ; and when monochromatic light is desirable, a 

 suitable light-filter is utilised. I have used ' zenith ' and ' imperial 

 sovereign ' plates for high-power work not needing an orthochromatic 

 plate, and so reduced the exposures materially." 



Glasenapp, M. — Die Bedeutung der Spitzertypie fur die Reproduktion von Mikro- 

 photographien. (An appreciation of the advantages of Spitzertype for the re- 

 production of photomicrographs.) 



Zeitschr. wiss. Mikrosk., xxiii. (1906) pp. 174-82 (8 figs.). 



(5) Microscopical Optics and Manipulation. 



Ultramicroscopical Examination of Plant-cells.* — N. Gaidukov 

 made an examination of the crushed-out contents of a Vaucheria cell. 

 The object was placed between a glass slide and a cover-slip in the usual 

 way, but great pains were taken to secure purity of the media and 

 cleanliness of the glass. The apparatus was arranged according to 

 Siedentopf's method. The result was a clear distinction between the 

 corpuscles of chlorophyll and of plasma, the diffraction-disks of the 

 former being usually red and green, those of the latter being white and 

 blue. It was possible to watch the formation of a colloidal solution of 

 chlorophyll with oil, for the drops of the former were attracted by, and 

 seemed to disappear in, the oil-drops. Observations were also made on 

 certain other plant-cells. 



Dispersion in Electric Double Refraction.^ — H. L. Blackwell, 

 after an historical sketch of the labours of previous experimenters on 

 this subject, describes the new results obtained by himself. Tempera- 

 ture and field-strength being kept constant throughout an experiment, 

 the observed double refraction, or separation (8), is proportional to the 

 difference between the ordinary and electrically-produced extraordinary 

 indices of the liquid (fx e -fx M ). This quantity is the separation in 

 centimetres introduced between the mutually perpendicular components 

 of a wave of plane-polarised light in traversing 1 cm. of the substance 

 in question — in this case carbon bisulphide — at a certain temperature in 

 a field of certain strength. The double refraction of naturally uniaxial 

 crystals is usually similarly expressed as the difference between the two 

 indices. The factor necessary to reduce this value to absolute index of 

 refraction was found to be about 7 - 04xl0~ 7 . Observations were 

 terminated in the violet by the absorption of the liquid. At wave- 

 length 4180 the spectrum was still very bright ; at 4144 very faint. The 

 experiments do not confirm Kerr's law, and it may be that the approach 

 to an absorption band is the controlling factor rather than the change 

 of wave-length. 



Arrangement for Simultaneously Obtaining Minimum Deviation 

 with Several Prisms.! — P- Lambert, having had occasion to mount a 

 spectroscope composed of three prisms and of two half -prisms in such 

 a manner that the luminous ray after once traversing the system should 

 return upon its path after incidence at a mirror, was led to study the 



* Ber. Deutsch. Bot. Ges., xxiv. (1906) pp. 107-12. 



+ Proc. Amer. Acad. Arts and Sci., xli. (1906) pp. 647-67 (1 pi. and several figs.). 



X Comptes Eendus, cxlii. (1906) pp. 1509-11 (3 figs.). 



