270 



KNOWLEDGE. 



July, 1913. 



the unusual haziness that overspread most of the northern 

 hemisphere during June, 1912. This appears to have been 

 the result of the eruption of Katmai Volcano in Alaska. This 

 volcano became suddenly active on June 6th, and violent 

 explosions were frequent during the three days that followed. 

 The eruptions continued with greatly diminished energy until 

 the end of October, and perhaps until the end of the year. 

 Professor H. H. Kimball, of Mount Weather Observatory, 

 U.S.A., who has been investigating the subject, has found that 

 in connection with the atmospheric haziness noted by many 

 observers, but more especially by those engaged in astronomical 

 photography, there was a decrease in atmospheric transparency 

 of from ten to twenty per cent. The solar radiation intensities 

 measured at the Mount Weather Observatory, after June 9th, 

 1912, were below the average, and the percentage of polarisa- 

 tion of skylight on cloudless days was abnormally low. These 

 low values seem to be due in some measure to volcanic dust 

 in the upper atmosphere. 



INTERNATIONAL METEOROLOGICAL COM- 

 MITTEE. — A meeting of this Committee, which consists of 

 representatives of the principal meteorological services in 

 various parts of the world, was held in Rome from April 7th to 

 12th. Dr. W. N. Shaw, F.R.S., the Director of the Meteoro- 

 logical Office, London, is the President, and Dr. G. Hellmann, 

 the Director of the Royal Prussian Meteorological Institute, 

 Berlin, is the Secretary of the Committee. Among the 

 subjects which came up for consideration were Weather 

 Telegraphy, Weather and Agriculture, Investigation of the 

 Upper Air, Meteorological Units, Sunshine Recorders, and 

 Storm-warning Signals. 



MICROSCOPY. 



By F.R.M.S. 



AN APLANATIC AND ACHROMATIC CONDENSER. 

 — An attempt to modify the or- 

 dinary refracting substage conden- 

 ser used for observation with 

 transmitted light, so as to render 

 it available for use as a dark- 

 ground illuminator, has led to the 

 construction of an Aplanatic and 

 Achromatic Condenser. The or- 

 dinary condenser of N.A. 1.40, 

 which consists of three lenses, 

 though corrected for neither spher- 

 ical nor chromatical aberration, 

 is nevertheless available as a 

 means of producing dark-ground illumination, but its deficient 

 chromatic correction gives rise to colour effects which inter- 

 fere with the quality of the resulting image : this defect can be 

 remedied by achromatising the condenser. This quality alone 

 would not, however, have placed the condenser on a level with 

 the reflecting condensers, seeing that these, in addition to 

 being naturally achromatic, are also excellently corrected with 

 respect to spherical aberration. To compete with these the 

 refracting condenser must needs be aplanatic as well as 

 achromatic. 



To meet this requirement the condenser constructed in 

 1907-8 by Leitz has recently been further improved in the 

 matter of spherical correction. In the majority of cases 

 objects which are observed with dark-ground illumination are 

 examined in an aqueous medium, having a refractive index 

 barely exceeding that of water, i.e., 1-33. Every ray whose 

 aperture is numerically greater than the refractive index 

 of the medium containing the objects is totally reflected at the 

 surface of the medium, as can be easily shown by a simple 

 calculation, and the rays so reflected do not contribute to the 

 illumination of the object. In order that the whole of the 

 light which a condenser is capable of receiving when opened 

 to its full aperture may be brought to bear upon the prepara- 



Figure 271. 

 The Condenser. 



tion, the condenser must have an aperture of 1.33, that being 

 the refractive index of water. 



The new condenser has lenses of the same diameter as the 

 older type with N.A. 1 -40 ; however, when used with aqueous 

 media, it produces a more brilliant illumination than the 

 latter. In the most recent form of the reflecting condenser, 

 the so-called Concentric Reflecting Condenser, we have also a 

 practical application of the above reasoning." 1 



As will be seen from Figure 271, the condenser con- 

 sists of two doublets on either side of a meniscus and a 

 hemispherical front lens. In its general construction it 

 resembles an oil-immersion lens and, as a matter of fact, it is 

 computed on precisely similar lines. 



Although the condenser has been corrected as an 

 immersion condenser its qualities will be impaired only very 

 little when being used dry if the layer of air between 

 condenser and slide is kept as thin as possible. In this case, 

 however, the aperture is reduced to N.A. 1-0, since all rays 

 of higher aperture are totally reflected. 



In addition to being both achromatic and aplanatic, this 

 condenser also satisfies the sine condition. 



The focal lengths are 14-5 in immersion contact and 9-6 

 millimetres dry, and the effective aperture of the back lens is 26 

 millimetres. The light-transmitting power of the condenser is 

 at least not inferior to that of a condenser constructed of single 

 lenses of similar diameter ; the loss of light due to absorption 

 in its passage through the six lenses of the new condenser is 

 fully compensated for by the manner in which the rays 

 appertaining to the different zones are brought to a focus. 

 This quality, coupled with the other conditions which are 

 satisfied by the formula of the condenser, furnishes an 

 efficient means of projecting a sharp, even, and colourless 

 image of the source of light in the plane of the object ; in the 

 methods of photomicrography this is of great importance. 



Continuous use and practical experience must show whether 

 the refinements in the correction of the aplanatic condenser 

 and its large aperture will secure tangible results in ordinary 

 microscopic observations with transmitted light, both direct 

 and oblique, especially in the case of objects which are diffi- 

 cult to resolve. Under the conditions of dark-ground 

 illumination the excellent correction of the aplanatic condenser 

 shows itself in an unmistakable manner, and every effort has 

 been made to provide the condenser with qualities that will 

 satisfy all the most advanced requirements of modern observa- 

 tion by this method of illumination. The extent to which 

 the spherical aberration has been successfully corrected is 

 clearly shown by the photograph reproduced in Figure 276 of 

 the path, within fluorescent uranium glass, of the intersecting 

 pencils of rays, which method was first used by the firm of 

 Ernst Leitz in 1910. It will be seen that the pencils 

 intersect within a very small area, and this furnishes an 

 estimate of the resulting brightness of the illuminated field. 

 By reason of the excellent correction of spherical and chro- 

 matic defects, and since also the sine condition is satisfied, 

 the objects can be illuminated on a dark ground in such a 

 manner that the image is seen free from colour, without 

 distortion, and without disturbing flares. 



The requisite conditions for observation against a dark 

 background are established by the use of a central stop (see 

 Figure 273) which rests on the carrier of the iris-diaphragm. 

 The illumination is derived from the peripheral zones only 

 of the pencil of light : these pencils are indicated by dotted 

 lines in the annexed diagram, which shows the path of the 

 rays through the aplanatic condenser and an oil-immersion 

 lens. The image as seen in the microscope (see Figure 277), 

 derives its existence exclusively from refracted and diffracted 

 light emitted by the object. 



The following points respecting the central stop may be 

 noted. The disc of the smallest central stop, which is 

 attached to the outer ring by three radial arms, has a 

 diameter of 16 millimetres, and bears the number 0-85, 

 whilst the ring is inscribed 1-33. All parallel rays entering 

 the condenser within the annular zone, corresponding to an 



• Cf. F. Jentzsch, Ueber Dtmkelfeldbeleuchtung, Phys. Zcitschr., XI, 1910. pages 993-1000; Verhandl. dcr D. Physik. 



Gesellschaft, XII, 1910, pages 975-91. 



