SCIENCE- GOSSIP. 



311 



Abbe condenser (1 N.A.) with iris diaphragm and 

 swinging arm, with blue and groimd-giass discs, 

 35s. ; set of patch-stops (for dark-ground illumina- 

 tion) and spectrum green glass, in box, 12s. ; 

 double nose-piece, 9s. ; objectives. Series A, 2" 

 (N.A. -06), 15s.; 1" (N.A. -13) or |" (N.A. -15), 

 each 12s.; i" (N.A. -68) or 1" (N.A. -71) at 21s. 

 and 30s. respectively. The choice of objectives 

 depends on your work ; 1" and |-" are suitable for 

 botanical work, and 1" or |" and |" for human his- 

 tology, etc. The 2" is a very useful lens, but it, 

 as well as the patch-stops and nose-piece, can be 

 added at any time. If I can help you further I 

 shall be pleased to do so. 



W. C. B (Doncaster).— It is difficult to get good 

 dark-ground illumination with an objective of so 

 large a N.A. as -65 whilst utilising the whole of 

 the aperture. The stop originally made for you 

 was probably the best that could be done under 

 such conditions. What Messrs. Zeiss have done 

 is to give you a smaller stop, and to reduce the 

 aperture of the objective by a diaphragm placed 

 behind it so that it will work with this stop. A 

 less expensive objective of this smaller aper- 

 ture would probably give you almost equal results 

 with the same stop. You can get dark-ground 

 illuminations with your '95 objective in the same 

 way, i.e. by reducing its aperture by means of 

 a similar diaphragm,- but not otherwise. The 

 best way of all is to have two or three stops of 

 varying sizes, and to place a Davis shutter behind 

 the objective. This is simply an iris diaphragm, 

 by means of which the aperture of the objective 

 can be regulated so as to give the best results. 

 There is, of course, a slight loss in definition ; but 

 dark-ground illumination does not give a critical 

 image under any circumstances, however useful it 

 may be at times. There is a corresponding gain , 

 in depth of focus, or what is generally known as 

 penetration, which is sometimes very useful in 

 examining or photographing objects of a con- 

 siderable thickness, whether with dark-ground, 

 reflected, or transmitted light. The minute par- 

 ticles in the blood are the " elementary particles" 

 or "blood-tablets," but their use is not known. I 

 answered this question on a former occasion 

 (S.-Ct., vol. vi., p. 313). You can refer to Schafer's 

 "Essentials of Histology," but you will not find 

 much information on this particular point any- 

 where . 



K. G. B. (Highgate). — Spirits of wine is a 

 solvent for so many cements that I cannot quite 

 confidently recommend one that will permanently 

 attach aquatic insect larvae to a card in a pre- 

 servative medium of this sort. It is a severe test. 

 The most likely is Clarke's Spirit-proof Cement, to 

 be obtained from Mr. T. Bolton, 25 Balsall Heath 

 Eoad, Birmingham. 



EXTEACTS FROM POSTAL MICROSCOPICAL 

 SOCIETY'S NOTEBOOKS. 



[Beyond necessary editorial revision these notes 

 are printed as written by the various members. — 

 Ed. Microscopy, S.-G.] 



Section of Young Stem of Palm (T.S.). — The 

 fibro-vascular bundles in this section are of the 

 type known as the " closed collateral " — i.e. they 

 contain no cambium, and are therefore not capable 

 of further development. The true " wood " or 

 "xylem" is remarkably weak, the great strength 



of the plant being derived from the highly- 

 developed " sheaths." The elements of the sheath 

 are strongly thickened, and its duties strictly 

 mechanical. The outermost zone here contains no 

 wood ; it belongs to the ground tissue, and serves, 

 as a protective covering, but the true secondary 

 bark of the exogenous tree is of totally dift'erent 

 origin. The fibro-vascular bundles of endogens, 

 with the exceptions to be mentioned later, all show 

 the same general structure ; and the elements may 

 be easily recognised from the accompanying draw- 



pyoem. 



sieOe-tobes 

 pitied duct. 



bundle 



ar)d spiral 



cjrouQC 

 tissue. 



Fig. 1. Section of Sugar-cane. 



ing (fig. 1) of a transverse section from the sugar- 

 cane, which shows the typical arrangement of the- 

 closed collateral fibro-vascular bundle. The excep- 

 tions to this general arrangement are the familiar 



oripi'3° 



Ier5tl2els 



cell/ , 



tx»pl)idtt 



lQ^Je^ 



■flUo-Oasculi 

 bundltsai 

 Jrour)d ti^s 

 iVf. result o- 

 conjbturr) 

 cv.c+i\3it\J. 



pHnr)Or<j oi 

 oriQirjal 

 tibfO'Dasculav- 

 burjcllcs 



Fig. 2. Stem of Dracaena terminalis. 



Dracaeneae and Alo'ineae (i.e. the so-called arbor- 

 escent Liliaceae) and the Dioscoreae. 



Stem of Dracaena terminalis (T.S.). (Fig. 2). — 

 This is interesting from many points of view. Com- 

 mencing to observe at the centre of the section, we 

 perceive the ground tissue to be composed of rounded 

 cells, in which tissue are placed the wood-bundles. 

 As we move outwardly we perceive that the bundles- 



