756 SUMMABY OF CURRENT RESEARCHES RELATING TO 



through the inclined coverslip into the objective. By means of a Zeiss 

 vertical illuminator, high magnifications and oil immersions could be 

 applied. It was found that palladium dust gave the best results. 



The examination of organic preparations involved greater experi- 

 mental difficulties. The light from an electric arc projection lantern 

 was passed through a diaphragm and focused by a lens through another 

 diaphragm and a ground glass screen on to a Zeiss vertical illuminator, 

 which reflected it down through the Microscope tube and the objec- 

 tive on to the mirror, which again reflected it upwards through the pre- 

 paration on to the Nicol eye-piece. An arrangement was also made 

 so that the mirror might reflect directly upwards, thereby enabling a 

 comparison observation with transmitted light to be made. Braun 

 succeeded in accurately identifying by this means the composition of a 

 substance previously unknown to him. The arrangement of apparatus 

 for examination by reflected polarised light is more difficult and 

 elaborate than in the case of transmitted polarised light, but the results 

 give a useful criterion for detecting how far the images are due to any 

 double refraction possessed by the substance itself. The author 

 describes several of his methods for obtaining polarised light. 



Microscopical Determination of the Position of a Reflecting 

 Surface during Optical Contact.* — K. Prytz-Kopenhagen, when the 

 surface is a plane reflecting solid, sets on the plane a suitable object (e.g. 

 a grating on a glass plate) appropriately illuminated. This is then 

 viewed through a Microscope whose axis is perpendicular to the plane. 

 The position of sharp definition will be the position of optical contact. 

 In the case of a reflecting liquid, its surface is, of course, plane, and the 

 Microscope is arranged as before. But into the body of the Microscope 

 near the eye-piece focus is introduced a horizontal solid glass rod, whose 

 outer end is opposite a light source, and the inner (i.e. inside the tube) 

 is bent vertically in the axis of the Microscope. The end of this vertical 

 portion is accurately plane and horizontal, and bears two fine diamond 

 scratches ^ mm. apart. These scratches project an image through the 

 objective towards the reflecting surface, and when adjusted the image 

 will be in the reflecting surface, and will be the conjugate point of the 

 glass rod end. In this position the image on the reflecting plane may 

 now be regarded as origin. Just above the objective is a prism of very 

 obtuse angle, the edge being uppermost. The effect of the prism is to 

 throw the ray proceeding from the origin on to the reflecting surface 

 slightly out of the microscopic axis, so that it reaches the eye-piece 

 without being blocked out by the glass rod. Thus, the position of clear 

 definition of the scratches will again be the position of optical contact. 

 Descriptions are given of the application of the method to the measure- 

 ment of Newton's rings and of other physical quantities. 



Brass. A. — Grundgesetzeder Optik. 



[Deals largelv with interference.] 



Central-Zeit. f. Opt. u. Mech., xxvi. (1905) Nos. 15-20. 



* Central-Zeit. f. Opt u. Mech., pp. 242-4 (3 figs.). 



