98 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



oue side. The author attains his purpose by the arrangements shown 

 in tigs. 5, 6, 7. Thus, in fig. . r >, by means of the two reflecting 

 spherical surfaces, each ray undergoes four reflexions, two before, and 

 two after, reaching the particles contained in the upper spherical cavity. 

 It will be noticed that the rays quit the condenser on the same side of 

 the apparatus as they enter it. The pattern in fig. 5 is built up of 

 two constituent pieces of glass ; that in fig. G is composed of three 

 constituents. The substance under examination in the upper cavity 

 may be a gas, a vapour, or a fluid. For many fluid examinations, 

 especially when only a small quantity of a strongly absorbing substance 

 is available, the form shown in fig. 7 is better adapted. In this form 



Fig. 7. 



the hollow space is so arranged that boundary surface breaks the rays 

 and takes part in forming the ray-combination. In addition to great 

 brilliance, these designs, at any rate Nos. 5 and 6 , offer the great 

 advantage of freedom from colour-error, as their action depends exclu- 

 sively on reflexion. Even with No. 7 colour-error need not arise if the 

 aqueous solution be so chosen as to preserve the principle of " homo- 

 geneous dispersion." 



If tobacco smoke be blown into this condenser, a very great number 

 of bright particles is seen in active molecular movement. If an 

 electric spark be introduced, the tiny particles of metal torn off from 

 the electrodes can be seen whizzing about. 



Jentsch's Concentric Condenser.* — F. Jentsch, after some general 

 remarks about mirror-condensers in general, discusses in detail the 

 principles of one based on the properties of two concentric circles. 

 These will be understood from fig. 8, where AP 1 P 2 B is a ray originating 

 at A and arriving at B after a convex reflexion at P t , and a concave 



• Verh. Deutsch. Phys. Gesell., xii. pp. 975-91 (8 figs.). 



