320 SUMMARY OF CUKEENT RESEARCHES RELATING TO 



The opening in the stage measures 2 in. square, to allow for the 

 examination of large specimens. Holes are provided to receive the peg 

 of the mirror when the latter is required for use in super-illumination. 

 A sliding bar fits at the top end of the containing case in a special 

 groove for the purpose, and can be removed when not required ; this 

 is a new feature, and is of great service when extremely low power 

 objectives are used in photomicrographical work, especially the 4-in. 

 The stage can be removed from the limb of the Microscope, and fitted 

 into this bar and screwed into position (fig. 39). 



Methods of Worlring* — The postage stamps can be examined visually 

 by placing them on the ordinary 3x1 glass slips, and in photomicro- 

 graphy by using a very thin square cover-glass placed over the specimen 

 on the glass slip. For surface features reflected light is used, both for 

 visual and photographic purposes, but for watermarks transmitted light 

 is used, and the specimen is photographed with the gum side turned 

 towards the sensitive plate. An eyepiece x 5 is the one recommended 

 for this kind of work. Objectives of all kinds can be fitted to the 

 instrument, and for surface features the 2-in. and 3-in. parachromatic 

 form are recommended ; for watermarks, cancellations, overprints, the 

 4-in. parachromatic is used. The Philatelic Microscope has been fitted 

 with the standard threads of the R.M.S. gauge. 



John Cuthbert's Reflecting Microscope : Correction. — The figures 

 illustrating this instrument are transposed. For fig. 5, p. 99, read fig. 6 ; 

 and for fig. 6, p. 100, read fig. 5. 



Old Microscope by Andrew Pritchard : Correction. — On p. 101, 

 line 1, for turret read Turrell. 



(3) Illuminating: and other Apparatus. 



Leitz' Aplanatic and Achromatic Condenser : also its use as 

 an Apertometer.f — C. Metz describes this auxiliary, which is intended 

 to be applicable either to bright-ground or dark-ground illumination. 

 As fig. 40 shows, the condenser consists of a pair of double lenses cemented 

 together and separated by a meniscus, with a hemisphere as a front lens. 

 In its construction it resembles an oil-immersion objective. It is, in 

 fact, an immersion system, but it can be used as a dry condenser, in 

 which case, however, the numerical aperture does not exceed 1 ■ 0, because 

 rays of higher aperture are totally reflected at the air-layer separating 

 condenser and object. In addition to possessing spherical and achro- 

 matic correction, the condenser also satisfies the sine condition. The 

 focal lengths of the two systems are respectively 14* 5 mm. and 9 * 6 mm., 

 and the available lens aperture is 26 mm. The brightness is in no wise 

 inferior to that of a simple lens condenser of equal lens diameter. The 

 loss of light suffered by absorption at the six lenses is made good by 

 the intense concentration of the light-rays belonging to the various 

 zones. The image attained in the object-plane is sharp and colour-clean. 



The manifold requirements which modern dark-ground illumination 



* Watson's Catalogue, 1912-13, pp. 20-1. 



t Zeitschr. wiss. Mikrosk., xxix. (1913) pp. 553-62 (7 figs.). 



