654 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Recent Progress in Ultramicroscopy.* — Under this title H. Sieden- 

 topf describes the cardioid condenser and many of its results. He also 

 discusses various technical difficulties in the application of the apparatus. 

 These have already been noticed in our pages. He draws attention, 

 however, to a new and convenient form of special chamber, f which is 

 illustrated in fig. 92. A small circular quartz plate of about 2 cm. 

 diameter and 1 ■ 2 mm. thick is provided on its upper face with a cir- 

 cular groove. The area within this groove forms a sort of plinth and 

 is polished down about 1-2 /* deeper than the peripheral ring. If now 

 a dust-free cover-glass of uniform thickness is laid on this external ring 

 there is a formation of Newton's colours, but an air-layer of 1-2 /x in 

 thickness is superposed above the plinth. If a small drop of any fluid 

 had been previously introduced by means of a platinum loop, the liquid 

 would have spread itself out and any excess would have found its way 

 into the circular groove. In this way the operator can easily obtain a 

 fluid layer of 1-2 p thick. It is found that ordinary cover-glasses of 



V////////////////////////) 



^//>»zzmzto/MP 



Fig. 92. 



0*16 mm., as generally used, are unsuitable. They reveal curvature 

 and exhibit a tendency to cleave to the plinth, with the effect of squeez- 

 ing out the fluid. It is necessary to use stouter cover-glasses of f mm. 

 thickness. 



Interferometer, with Inverse Superposed Luminous Rays, giving 

 in White Polarized Light a Narrow Central Fringe of Sensible Tint 

 and Narrow Coloured Fringes at White Intervals.^ — Gr. Sagnac 

 describes (fig. 93) an interferential apparatus, by means of which he 

 combines rays differing by small velocities of propagation. 



Two isosceles triangular prisms, P x , P 2 , cut out of the same piece of 

 glass (ji = 1'514 for the radiation X s = 0*56 /a), with angles a identical 

 to a few seconds, are placed with their faces, I V, very close together so 

 that a rhombic prism is formed. The layer of air, I V, acts as a trans- 

 parent silvered surface. Hence the light issuing from the collimator 

 C divides at the air-layer into transmitted vibrations (relative amplitude 

 T) and reflected vibrations (relative amplitude R), which are propa- 

 gated in opposite directions along the same ti'iangular circuit I M x M 2 . 

 The lens L receives together the vibration T, a second time transmitted 

 by the air-layer (amplitude T 2 ) and the vibrations R, a second time 

 reflected by the same layer (amplitude R 2 ). A polarizer, not represented 



* Ver. Deutsch. Phys. Gesell., xii. (1910) pp. 1-42 (many figs.). 



t Tom. cit., p. 13. % Comptes Rendus. cl. (1910) pp. 1676-9 (1 fig.). 



