ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 733 



then only portions of them can be simultaneously seen. In this latter 

 case, however, comparison may be easily made if one of the stages be 

 adapted for swinging out of the tield. The method, which is naturally 

 only adapted to transparent preparations, succeeds best with low-power 

 objectives, for the difference in the magnification of the two objects 

 might be inconvenient with strong lenses. The oculars employed should 

 be weak and not too intensive. If the Abbe condenser is used, its iris 

 should be as much as possible closed. The author recommends the 

 arrangement not only for subjective observation, l)ut also for photo- 

 micrography. 



Simple Method of Adjusting the Nicols in a Mineralogical 

 Microscope.* — In order to provide a cheap and trustworthy means of 

 accurately determining whether the nicols of a mineralogical Microscope 

 are in a perfectly crossed position, E. Sonnnerfeldt proposes the use of 

 a twin crystal of gypsum. This crystal would be applied to the slit, and, 

 owing to its extreme fissibility, it would be easy to obtain it in the 

 required degree of thinness. Such a crystal plate must be so applied 

 that in the rotation of the object stage, a position is found in which 

 both members of the twins appear equally bright. The twin limit then 

 accurately coincides with a thread of the thread-cross when the 

 Microscope is properly adjusted^otherwise the adjustment must be 

 corrected until this condition is attained. If the preparation is then 

 rotated 45° either way in its plane, a second position of equal brightness 

 in the twins is found. In this way those positions can be noted, or tested, 

 which are usually indicated on the tube of an expensive mineralogical 

 Microscope. 



Siedentopf's Paraboloid Condenser : a New Method for Dark- 

 field Illumination.! — This apparatus of H. Siedentopf's is especially 

 adapted for securing visibility of living bacteria (especially of Spirochceta 

 pallida), and for their instantaneous photography. From the optical 

 standpoint Sjnrochmta pallida is characterised less by its spiral form than 

 by its extreme thinness, which usually lies below the resolution-limits 

 of microscope objectives. While, no doubt, large specimens can be seen 

 with bright-field illumination, the observer will experience greater diifi- 

 culties and obtain less satisfactory results than with dark-ground effects. 

 But for dark ground the objective must have deeper penetrating power 

 in consequence of the greater contrasts ; while, on the other hand, owing 

 to the naturally increased resolution of high aperture with oblique 

 light, moderately strong systems of 7-4 mm. focus suffice, one effect 

 being to produce a larger and more extensive view-field. In contrast 

 with these advantages, dark-ground illumination has the disadvantage 

 of increasing the difficulties due to any deficiency of cleanness in the 

 preparation, or to dust on the cover-glass. Ultramicroscopic methods 

 are not required for revealing living bacteria. A very simple and 

 successful dark-ground illumination of another kind is obtained by 

 inserting a diaphragm of 24 mm. diameter under the immersion con- 

 denser of 1*4 N.A., the object-slide l)eing connected with the contlenser 



* Zeitschr. wiss. Mikrosk., xxiv. (1907) pp. 24-5. 

 t Tom. cit., pp. 104-8 (1 tig.). 



