ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 287 



objectives possess over those of Zeiss and other makers is that they do 

 not require a specially constructed and corrected eye-piece, but give 

 equally good results with any well-constructed Huyghenian ocular." 



Cheap Objectives.* — Is there not a little something wanting in the 

 following recommendation of an objective which we quote from a learned 

 contemporary ? " The oil-immersion objective is remarkable for its 

 " powers of definition ; it has been tester! against many of Leitz's, which 

 " hitherto have been the cheapest obtainable, and Las been found superior 

 " to them. This is high praise, as the price of the two is the same." 



Giffoed, J. W. — Apochromatic Objectives. 



Journ. of Microscopy, I. (1888) pp. 9-11. 



(3) Illuminating' and other Apparatus. 



Geissler's Culture Tubes. f — Dr. O. Brefeld's researches on Bacillus 

 suhtilis were undertaken with the apparatus of G. F. Geissler, shown in 

 fig. 45. 



Fig 45. 



A 



A glass tube of nearly capillary diameter widens in the centre in the 

 form shown at A, the upper and lower sides approaching each other so 

 closely, that there is only a very small space between them. A drop of 

 liquid drawn through the tube remains, by capillary attraction, in the 

 centre without drying up, and can thus be easily subjected to examina- 

 tion by the strongest objective. 



Other forms have dissimilar-tubes as shown at B or C ; the centre in 

 the latter is open beneath and fastened upon a glass plate, whilst another 

 smaller aperture above is intended to take the glass cover with the 

 object in a hanging drop. 



Gas and Moist Chambers. — It is often necessary to ascertain the 

 influence of various gases upon the objects under examination, and for 

 this purpose various devices have been made use of, known as " gas 

 chambers." J The different forms of " culture cells " are readily con- 

 vertible into gas chambers, and a great variety of suggestions have been 



* Brit. Med. Journ., 1887, No. 1391, p. 470. 



t Bericht ii. d. "Wiss. Instrumente a. d. Berliner Gewerbeausstelhmg ira Jahre 

 1879 (Lowenherz), 1880, pp. 304-5 (1 fiv.). 



X C. Robin describes Poiseuille's " Porte-objet pneumatique" of 1832, as the first 

 known " gas chamber." This was, however, a copper box (with two apertures closed 

 with glass) in connection with an air-pump to experiment upon the effects on living 

 organisms of condensing or rarefying the air. Cf. ' Traite du Microscope,' 1S77, 

 p. 159. 



x 2 



