528 SUMMARY OF CURRENT RESEARCHES RELATING TO 



small ; a preparation only rose 4° C. in degree after some hours' ex- 

 posure to the lamp rays. Its application to photomicrography has given 

 excellent results. As regards micro-projection, the 4-ampere lamp is 

 probably the best for medium distances, but for short distances the 

 2-ampere lamp is better. 



E. Wychgram* discusses the merits of the Ewon-lamps, and practically 

 agrees with M. Wolff. 



New Low-power Condenser.t — The difficulty attending the con- 

 struction of a low-power condenser has hitherto been the fact that 

 Microscope-stands do not afford the necessary optical length. Messrs. 

 C. Baker have, however, from the designs of E. M. Nelson, obviated 

 the difficulty by constructing the condenser upon the telephoto principle. 

 Their apparatus has a focal length of 4 in. and requires only 1 in. 

 of working distance. With this condenser the image of the flat of the 

 flame bears the same relation to a 4-in. objective with the large field 

 of a Powell and Lealand No. lA eye-piece, as the image with one of the 

 ordinary universal condensers, with the top off, does to a ^-in. ; and 

 this is precisely what was wanted. The condenser has a low aperture 

 of N.A. 0*14, but large enough for the objectives for which it is 

 intended to be used. 



C4) Photomicrogrraphy. 



Stereography. t — Under the above title F. E. v. Wellheim discusses 

 the principles and methods of stereomicrophotography. His treatise is 

 divided into two parts, the first of which deals with stereomicrographs 

 taken transparently with various kinds of light (ordinary, polarized, 

 dark-ground). Tlie second part discusses the stereography by reflected 

 light of small objects in natural size or reduced. In addition to 

 elucidating the principles, the author summarizes many details of 

 practical importance, and gives several useful numerical tables. 



(5) Microscopical Optics and Manipulation. 



Method of obtaining Microscopical Bench-marks exactly 

 Circular in Micrometric Observations : Application to the. Study of 

 Trunnions in Equatorial Telescopes.§ — E. Esclangon has found that 

 drops of mercury vapour condensed on a glass slide make excellent 

 bench-marks for micrometric work. They have the advantage of being 

 clearly defined and are always perfectly circular. They can be obtained 

 of any size, even down to a fraction of a micron. He suggests as a 

 useful illustration of this property its application to the determination 

 of the movement of the truimion axis of an equatorial telescope. A 

 divergence from perfect circularity of the trunnion introduces errors 

 into the telescopic observations which are by no means negligible. The 



* Zeitschr. wiss. Mikrosk., xxix. (1913) pp. 336-9 (1 fig.). 

 t Journ. Quekett Micr. Club, xii. (1913) pp. 95-6. 

 X Zeitschr. wiss. Mikrosk., xxx. (1913) pp. 1-28 (5 figs.). 



§ Proces-verbaux des Stances, Soc. Sci. Phys. et Nat. de Bordeaux, 1910-11, 

 pp. 9-13. 



