"4 



KXOWLEUGE .'v: SCIENTIFIC NEWS. 



[Mav, 1905. 



Conducted by F. Shili.ington Scales, f.r.m.s. 



Fibrous Constituents 

 of Paper. 



(Concluded from Page 93.) 



Esparto fibres are generally finer and much shorter 

 than those of straw. They are smooth and cylindrical, 

 and free from knots. The walls are thick, and the 

 central canal accordingly very small and uniform. 

 The ends of the fibres are generally rounded. The 

 serrated epidermal cells found in straw are also found 

 in esparto, and can scarcely be distinguished from 

 these, but the large, thin-walled parenchyma cells are 

 absent. The esparto leaf, however, bears on its inner 

 surface a number of little hairs or teeth, some of which 

 are nearly always found in papers made from esparto, 

 and which are quite a trustworthy characteristic. 



Chemical wf)od-pulp shows flat, ribbon-like fibres, 

 not unlike cotton, and even at times twisted like the 

 latter, but with unbroken ends. It would take up too 

 much space were I to endeavour to discriminate be- 

 tween the various kinds of wood, such as pine, birch, 

 poplar, etc., but they all show distinct woody charac- 

 teristics. The pits in pine wood are quite unmis- 

 takable, as are the obliquely-placed slit-like pores of 

 birch and poplar. 



Mechanical wood has a strongly-marked woody ap- 

 pearance, but the fibres are not properlv separated, and 

 the fragmented nature of the material, due to the way 

 the fibres have been torn and wrenched across instead 

 of separated, is quite unmistakable. 



It only remains to add that fibres stained with 

 chlor-zinc iodine are, unfortunately, not permanent. 

 Permanent preparations can be stained with benzo- 

 brown with a trace of soda to deepen the colour, 

 washed slightly, and then stained with benzo-azurin 

 without soda, and gently warmed, and will form beauti- 

 ful and instructive mounts, though the differentiation 

 will not be that of chlor-zinc iodine. 



The mounting medium may be water or glycerine and 

 water, but for permanent mounts glycerine is not con- 

 venient to use owing to the fact that it will not harden or 

 dry. To get over this difficulty, glycerine jelly may be 

 used with advantage, the fibres being first carefully 

 soaked in water from which the air has been expelled by 

 previous boiling. Even then there is often much trouble 

 with minute air bubbles entangled in the fibres. I 

 have found it quite satisfactory to proceed as follows: 

 The fibres, whether stained or not, after soaking in 

 boiled water, are arranged in the centre of the slide, 

 which is placed upon a brass mounting table. Sufficient 

 glycerine jelly is then added, and, after melting, the 

 cover-glass is placed in position and held lightly in place 

 with the point of a dissecting needle. The glycerine 

 jelly IS now heated until it just begins to boil, when the 

 lamp is quickly removed. This disentangles and carries 

 away from beneath the cover-glass any air bubbles. 



After the glycerine jelly has set it should be cleaned up 

 by dipping the slide in water and wiping it carefully with 

 a rag, and then the cover-glass is surrounded with two 

 coats of gold size. Farrant's solution is also useful, as 

 it is a glycerine mounting medium which hardens at the 

 edges. Canada Balsam is less suitable than glycerine 

 media for mounting fibres. They may also be mounted 

 in water with a little added carbolic acid, enclosed in a 

 thin cell of gold size. 



High-power Microscopy. 



Mr. |. W. (iurdon, IMC.M.S., who has contri- 

 buted several interesting papers on Microscopical 

 Optics to the Royal Microscopical Society, which 

 have, however, led to some controversy, recently 

 gave an address at the Royal Institution, in which 

 many of his views were summarized in a more 

 popular way and without the mathematical argu- 

 ments which are necessary to an adequate discussion of 

 such a subject. A resume will doubtless interest many of 

 the readers of "Knowledge iS: Scientiiic News." 

 Mr. Gordon observed that in the exhibition of a micro- 

 scopic object under high magnifying power there are 

 three stages in which difficulties have to be met and sur- 

 mounted — (i) In the preparation of the object for exhibi- 

 tion under suitable conditions of illumination ; (2) in the 

 representation of the object by means of an image ; (3) 

 in the transmission of the image so found in the instru- 

 ment to the eye of the observer. Professor Wright 

 classified the preparation of objects into colour pictures 

 by means of stains and outline pictures. The method of 

 staining having manifest limitations, Mr. Gordon pro- 

 ceeded to refer to the use of cross-lighting or "dark- 

 ground illumination" in order to show outlines, with 

 especial reference to Dr. Siedentopf's application of this 

 principle to the exhibition of so-called " ultra-micro- 

 scopical particles." In ruby glass, for instance, the 

 colour is due to minute particles of gold difTused through 

 the glass, so small as to be beyond the powers of the 

 microscope as ordinarily used. l!y special methods of 

 illumination, however, at right angles with the optical 

 axis of the microscope, and by limiting the plane of such 

 illumination, the particles come into view as diffraction 

 discs. Mr. Gordon then dealt with some experiments of 

 his own, originally suggested by a paper of Lord Ray- 

 leigh's, but which were still incomplete, which consisted 

 especially of a method of lif^hting up the object by means 

 of diffracted light, the principle being explained by a 

 diffraction slit formed by the edges of two knives stuck 

 in a board so that their edges overlapped towards the 

 points, but were about an eighth of an inch a()art near 

 the handles. It was with such a piece of apparatus thai 

 Sir Isaac Newton worked when he made his first precise 

 recorded observations on the subject of difl'racted light. 

 Mr. Gordon referred to the observation of Ilclmholtz, as 

 far back as 1H74, that the limit of useful power in a high- 

 power objective is reached when the lens of the objective 

 is of such focal length that its diameter is rather less 

 than the diameter of the pupil of the eye, and that beyond 

 that point there was no advantage in increasing the 

 magnifying power of the objective, but that further mag- 

 nification was best obtained by increasing the power of 

 the eyepiece. Hut this method had also drawbacks owing 

 to the smallness of the emergent pencil of light ; such, 

 for instance, as the greater prominence of dust ujwn the 

 lens or of (loatinj; particles in the eye. Mr. Gordon con- 

 sidered that this was responsible for the limitation of 

 magnifying powers at present in use by microscopists to 

 ijof) or 2000 diameters, whilst most good work was done 



