July. 1913. 



KNOWLEDGE. 



273 



apertural range extending from 0-85 to 1-33, are allowed to 

 participate in the illumination. The objectives employed for 

 observation should in these cases have apertures which are 

 less than N.A. 0-85, such as the Leitz objectives, Nos. 2 to 5, 

 having respectively focal lengths ranging from 24 to 5-4 

 millimetres, as well as the apochromatic lenses of 8 millimetres 

 and 16 millimetres focus. To use high-power objectives, 

 .having apertures exceeding N.A. 0-85 in conjunction with the 

 central stop, it becomes necessary to reduce their aperture to 

 less than N.A. 0-85 by a stop situated behind the back lens. 

 To modify the range of the peripheral zone three loose 

 discs, two of which are shewn in Figure 274, bearing the 

 following numbers, 1-0, 1-1, 1-2, are provided with the 

 stop. Each disc has a central hole fitting a small pin at 

 the centre of the immovable disc inscribed 0-85. When 

 one of these discs is pressed down upon this pin the cone of 

 illumination becomes reduced, so that an aperture of 1-0, 

 i-1, and 1-2 is respectively employed; these discs are 

 used when observing with objectives having an aperture 

 of 1'0. Oil-immersion lenses have their aperture reduced 

 to 1-0 for observation with dark-ground illumination by 

 unscrewing from the body of the objective the nickel- 

 plated part of the mount (see Figure 275), which con- 

 tains the optical system, and screwing it to an adapter fitted 

 with a conical stop, which is so arranged that it cuts off all 

 rays which would pass if the numerical aperture of the lens 

 were greater than 1-0. In the case of dry lenses it is gene- 

 rally desirable, to obtain an intensely dark background, to 

 employ one of the larger stops ; in the majority of cases the 

 one marked 1 ■ is sufficient for this purpose. For observation 

 with dark-ground illumination it is always an advantage to use 

 objectives of high degree of chromatic correction — for instance, 

 fluorite lenses or apochromatic lenses — since under the 

 conditions of dark-ground illumination the residual colour 

 defects of the lenses, known as the secondary spectrum, are of 

 much greater importance than is the case in ordinary observa- 

 tion with transmitted light. The best source of light to use 

 with dark-ground illumination is the Liliput Arc Lamp, which 

 has been specially devised for this purpose. This lamp burns 

 carbons set at right angles to one another and consumes five 

 amperes: it is so arranged that it may be connected with the 

 ordinary domestic electric supply by means of a plug contact. 

 The light emitted by this lamp is rendered parallel by a con- 

 densing lens attached to the lamp casing, and is reflected by 

 the plane mirror of the microscope into the condenser, the 

 latter being adapted for parallel light. The lamp, being hinged, 

 moves in a vertical plane, and should be so set that the plane 

 mirror may be completely filled with light. The lamp should 

 be placed at such a distance from the microscope that the 

 observer may be able to manipulate it without inconvenience, 

 as shown in Figure 272. When using dry lenses a ground-glass 

 screen may be interposed between the lamp and the mirror; 

 the contrast between a dark-ground and a brightly illuminated 

 object is thereby intensified. When using low-power lenses 

 having apertures not exceeding 0-40 the condenser may be 

 used dry ; with lenses of higher power the condenser should 

 be used in immersion contact with the object slide ; it will 

 generally be sufficient to employ water as the optical medium. 

 The adjustment of the dark field is not always an easy matter 

 with high-power lenses ; it may, however, be greatly simplified 

 by the following procedure : — 



The object should be viewed first with an objective of 

 medium power, say No. 3 Leitz Objective, in conjunction with 

 a high-power eyepiece. Incidentally it may here be noted 

 that high-power oculars are particularly useful in observations 

 under dark-ground illumination. Focus the lens in the plane 

 of the object, the object slide being at this stage connected to 

 the condenser by a drop of oil. By the requisite movement 

 of the substage mirror direct the light upon the object. 

 Under good conditions of adjustment the circle of bright 

 light should appear in the middle of the field of view, and it 

 should be sharp and free from colour. The condenser should 

 be raised or lowered until the spot of light becomes as sharp 

 as it is possible to make it. Having prepared the adjustment 

 in this way a very small correction in the position of the 

 mirror and condenser will be needed to secure a very perfect 



dark ground. These manipulations are sufficient to obtain 

 the necessary adjustments, and there is no need to have 

 recourse to centring screws. 



The transition from dark background illumination to 

 ordinary observation with transmitted light, and vice versa, 

 can only be made in the case of a refracting and not in that of 

 the usual reflecting condenser ; for this purpose a useful 

 special arrangement has been devised. 



The ordinary conical stop with which lenses of high aper- 

 ture are fitted to render them available for observation with 

 dark-ground illumination is replaced by a small iris-diaphragm 

 fitted to the body of the lens mount. When the lens is used 

 for observation with an ordinary bright field this diaphragm 

 should be opened to its full extent, and it should be partly 

 closed for observation with dark-ground illumination. In 

 order to pass rapidly from one mode of illumination to the 

 other the iris-diaphragm carrier below the condenser is 

 provided with a central stop attached to a slide by means of 

 which it may readily be pushed in or withdrawn. 



It may be useful to point out the distinctive features of a 

 refracting condenser. In the case of a reflecting condenser 

 the course of the rays is solely determined by a series of 

 reflections. In refracting condensers the illuminating pencils 

 are brought to a focus by refractions at the surfaces of the 

 constituent lenses. In addition to these refractions, however, 

 the rays undergo certain reflections which may impair the 

 distinctness of the image as seen in the dark-ground field. It 

 seems almost hopeless to construct an aplanatic and 

 achromatic condenser which needs correction by the use of 

 so many kinds of glass and refracting surfaces, and in such a 

 way that every surface must be excluded from which reflected 

 rays after successive refraction and reflection shall yet leave 

 the condenser at such an angle as to enter the plane of the 

 object. These indirect rays which cannot altogether be 

 avoided, however faint they may be, impair the blackness of 

 the background which is never the same as that obtainable 

 with a reflecting condenser, and under these circumstances 

 the clearness of the image may suffer. 



The condenser described above, on the other hand, has the 

 advantage of being of the nature of a universal illuminator, 

 and in this respect it surpasses all existing reflecting con- 

 densers. Primarily, a highly refined illuminator adapted for 

 the various purposes of visual microscopy with transmitted 

 light and photo- micrography, its performance as a dark- 

 ground illuminator entitles it to a prominent position among 

 the condensers devised for this purpose. Far from being 

 restricted in its use to high-power objectives, it illuminates a 

 sufficiently large field to render it suitable for use with low- 

 power objectives whose focal lengths may be as much as 

 twenty millimetres. The only optical device of the nature of a 

 reflecting condenser which can be regarded as a dark-ground 

 illuminator is our old friend the concave mirror : this can be 

 converted into a makeshift for a dark-ground condenser by 

 covering up the central portion of the mirror. Owing to its 

 small apertural angle the mirror can be used in this way only 

 in conjunction with lenses of a very low power. 



C. Metz, Wetzlar. 



THE BRITISH SPECIES OF PISIDIUM — Those 

 who study British land and freshwater shells — and they are 

 many — owe a deep debt of gratitude to Mr. B. B. Woodward 

 for determining what species of Pisidium are British and 

 for clearing up all the doubts and difficulties which have 

 long surrounded these small freshwater bivalves. After 

 examining many thousands of specimens under the microscope, 

 and devoting his leisure hours during nine winters to the 

 subject, he has come to the conclusion there are fifteen 

 British forms worthy of specific rank, of which all but one 

 (P. astartoides) are still living in this country. Photographs 

 of these by the kindness of Mr. Woodward we are permitted 

 to reproduce on page 272 (see Figures 279 to 293). 



The determination of the species is based to a large extent 

 upon the hinge characters of the shell and the form and 

 position of the various hinge " teeth." 



P. casertanum (see Figure 281) is a species which haslong 

 been known on the Continent under this name or that of 



