ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 281 



the angle. " Finders " are also engraved. The milled-heads on the 

 edge are for centering the rotation on the optic axis. A simple and 

 effective plan has been adopted of applying the iris-diaphragm, hemi- 

 spherical immersion-condenser, or Wenham's semi-disk illuminator 

 beneath the stage, where they are held by a small projecting peg and 

 a spring latchet. 



Altmann's "Abend-Condenser." * — Dr. E. Altmann has designed 

 a condenser ("Evening Condenser"), which consists of a convex 

 hemispherical lens of short focus with a disk of ground glass over 

 it, and one of light-blue beneath it. The lens and disks fit into a 

 tube similar to that used for the ordinary (German) cylinder- 

 diaphragms. 



Heating Apparatus."]" — Thoulet describes a new method of heating 

 objects upon the stage. He has constructed a small " stove " or 

 chamber, to rest upon the stage, and to contain the object and the 

 thermometer. It consists of a glass tube fitting into a copper cylinder 

 which rests upon a disk of copper, furnished with lateral prolonga- 

 tions, which can be heated by a gas jet. The whole is insulated by 

 resting upon a disk of cork. The temperature of the chamber can be 

 raised by heating the prolongations of copper, and lowered by intro- 

 ducing a current of fresh air through a small tube fixed in the side. 

 Very exact measurements can be taken with this simple apparatus, 

 which is well adapted for determining the temperature of the disap- 

 pearance of bubbles in liquid inclusions, for studying the formation 

 of crystals at various temperatures, or for other micro-chemical in- 

 vestigations. 



Abbe's Test-plate. — Dr. C. Zeiss has now issued directions for 

 using this test-plate, which, notwithstanding that the subject was fully 

 dealt with at p. 120, may, we think, be usefully reproduced here (with 

 a few verbal alterations in the original text) : — 



" This test-plate is intended for the examination of objectives with 

 reference to their corrections for spherical and chromatic aberration, 

 and for estimating the thickness of the cover-glass for which the 

 spherical aberration is best corrected. 



The test-plate consists of a series of cover-glasses, ranging in 

 thickness from 0*09 mm. to 0'24 mm., silvered on the under surface, 

 and cemented side by side on a slide, the thickness of each being 

 marked on the silver film. Groups of parallel lines are cut through 

 the films, and these are so coarsely ruled, that they are easily resolved 

 by the lowest powers, yet from the extreme thinness of the silver they 

 also form a very delicate test for objectives of even the highest power 

 and widest aperture. 



To examine an objective of large aperture, the disks must be 

 focussed in succession, observing in each case the quality of the image 

 in the centre of the field, and the variation produced by using alter- 

 nately central and very oblique illumination. When the objective is 

 perfectly corrected for spherical aberration for the particular thickness 



* Arch. f. Anat. u. Physiol. (Anat. Abtheil.) 1881, pp. 219-24. 



t Bull. Soc. Mineral. France. Cf. Amer. Natural,, xvii. (1883) p. 76. 



