194 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1904. 



Conducted by F. Shii.lington Scales, f.r.m.s. 



Gelatin Plates as Light Filters. 



A recent number of the "Journal of the Ro3'al Microscopical 

 Society" t;ives an abstract of a method described b}' a German 

 writer for maUing gelatin plates which ma}^ serve as substitutes 

 for glass light filters for microscopical and photomicrographical 

 purposes. A solution of the best gelatin, such as is used for 

 making dry plates, is made in the usual way, the proportion to 

 the water being as i to 200. To the filtered solution 3 cubic 

 centimetres of 1 to 50 aqueous solution of alum are added. 

 The films are made by pouring the gelatin on a glass plate 

 placed on a levelling stand. \Vhen quite dry the gelatin is 

 overlaid with a film of collodion stained with some aniline dye. 

 Red plates may be made as follows: Dissolve (i) 2 grni. 

 aurantia in 40 com. of absolute alcohol, (2) 5 grm. rose Bengal 

 in 20 com. methyl alcohol. Then mi.\ 20 com. of (i) with 

 10 com. of (2), and add go com. of 4 per cent, collodion. 

 Yellow plates can be made by adding 20 c.cm. of a saturated 

 alcoholic solution of aurantia to So c.cm. 4 per cent, collodion. 

 The gelatin plates may be doubled so as to strengthen the film, 

 or one may be placed on either side of the coloured layer. 

 The advantage of using coloured screens when endeavouring 

 to photograph ot)jects stained in such a way as to give little 

 contrast on a photographic plate is obvious to all, and expert 

 photomicrographers depend largely upon such means of dif- 

 ferentiating. In an early number of this magazine I hope to 

 give a brief and elementary account of the ordinary procedure 

 in photomicrography, mainly for those who are photographers 

 and who wish to utilise their knowledge in order to photo- 

 graph microscopic sections, but find their results unsatis- 

 factory, through want of knowledge of a few elementary rules 

 of procedure. 



Dry and Immersion Objectives. 



Several inquiries have been sent to me as to the relative 

 advantages of dry and immersion objectives, and though the 

 subject is adequately treated in the various microscopical 

 text-books, a few words upon the matter may be of service. 

 Let us assume that an extraordinarily wide-angled dry lens 

 can embrace an angle of 170° from an object placed un- 

 covered on the slide, though, of course, so large an angle as 

 this is really barely possible. Then a cover-glass placed over 

 the objective will produce a certain amount of refraction, 

 according to the well-known law that rays of light from a 

 medium (in this case glass) entering another less dense (in 

 this case air) are refracted away from the perpendicular. 

 By this refraction a large portion of the extreme rays, which 

 ought to enter the objective, will be refracted, some being 

 even totally reflected, and so fail to enter it. If a denser 

 medium were to take the place of the air, this refraction 

 would be minimised, and if it were as dense as the cover- 

 glass, it would be practically non-existent. This resolves 

 into the fact that an oil-immersion objective of 82", a 

 water-immersion of 97', and a dry lens of 170', all admit ap- 

 proximately the same amount of light. Therefore a water- 

 immersion, and still more an oil-immersion can be made to 

 admit light of an angle much greater than the widest angled 

 dry lens. There is therefore a great gain of light, and with 

 the increase of aperture there is a corresponding gain of 

 resolving power. There is yet a further gain of working dis- 

 tance. There is still a further gain, sometimes overlooked, 

 due to the fact that the intensity of the rays are less as they 

 become more oblique, but that they increase in intensity 

 according to the density of the medium, in a ratio, in fact, 

 that is measured by the squares of their refractive indices. 



linough has been said to show the advantage of the immer- 

 sion system of objectives. It follows naturally that the term 

 angular aperture no longer expresses the value of an objective, 

 and thus a new system of rating has sprung into existence, due 

 to Professor Abbe, which takes into consideration the refrac- 

 tive index of the medium, whether air, water, or oil, as well as 

 the angular aperture. The formula is ;; si;; m, where n is the 

 index of refraction of the medium in front of the objective, 

 and ti the sine of half the diameter of the emergent pencil of 

 light at the back of the objective. This is the " Numerical 

 Aperture," or N.A. Finally, there is yet another advantage. 

 Any variation in the thickness of the cover-glass in a dry lens 

 upsets the corrections of the objective, and must be corrected 

 by a collar which adjusts the position of the individual lenses 

 which make up the objective, or by an adjustment of tube 

 length. Water has a refractive index nearer to the cover- 

 glass than air, and therefore the necessary corrections are 

 much less serious ; but cedar oil has almost exactly the same 

 refractive index as crown glass, and so there are practically 

 no corrections required. Of course, immersion-lenses are 

 always high powers, and equally, of course, it is not quite as 

 convenient, and is now and again impracticable, to use such 

 lenses. It is scarcely necessary to add that a dry lens 

 cannot be used as an immersion lens, nor an immersion lens 

 in any other medium than that for which it is constructed. 

 Immersion condensers are made in order to reduce aberra- 

 tions, and to enable a cone of light to be passed which is pro- 

 portional to the wide apertures of immersion objectives, and 

 with an oil immersion condenser, an oil immersion objective, 

 and an object mounted in Canada Balsam, we have a con- 

 denser, a connecting medium, a slide, a moimting medium, a 

 cover-glass, again a connecting medium, and finally an objec- 

 tive, which arc. to all mtents and purposes, one homogeneous 

 whole. 



Wa.tson's "Fa.cility" Object-Changer. 



Messrs. W. Watson and Sons have sent for inspection a 

 new object-changer of novel construction. It is square in 

 shape and screws on to the end of the draw-tube in the ordi- 

 nary way. On the under side are a pair of opposite jaws, a 

 quarter of an inch wide, which open by means of the handle 

 shown in the illustration, and when released eng.age the threads 

 of the objective screw and carry it up to the shoulder where it 



is firmly and squarely held in place. It is made of Magnalium 

 and so is very light ; it is only half an inch thick, and it pro- 

 vides a rapid and easy method of changing objectives. Most 

 workers have found that they possess one or more objectives 

 whose screws are not cut strictly to the proper gauge, and to 

 obviate this Messrs. Watson provide rings of absolute gauge to 

 fit such objectives, and ensure accurate gripping in the jaws 

 of the object-changer. These rings do not interfere with the 

 fit of the objective in its box. 



Roya-l Microscopica.1 Society. 



June 15th.— Dr. Dukinfield H. Scott, F.K.S., President, in 

 the chair. Mr. T. H. Powell exhibited PUurosif^ina aii^iilatiim 

 under a ,i„-inch, 1-35 N.A. apochromatic homogeneous immer- 

 sion objective made by him. Professor Hartog exhibited a 

 slide prepared and lent to him by Professor Vejdovsky showing 

 the first segmentation spindle and centrospheres in the embryo 

 of Khynchelinis. This was so large as to be visible with a 

 pocket lens and was distinctly shown under a J, -inch objective 

 and " B " ocular. Mr. Beck exhibited a portable microscope 

 designed by Mr. Arthur Hollick. It was daily used for the 

 examination of botanical subjects, but was equally useful for 

 other purposes. The mirror was so mounted that it could be 

 used above the stage for illuminating opaque objects, swing- 

 ing on a centre that was at the level of the object. An ingeni- 

 ously contrived rotating cell, made of cardboard, forming a 

 convenient revolving object holder, and a simple method of 



