234 SUMMARY OF CURRENT RESEARCHES RELATING TO 



image-field free from astigmatism. Although the firm of Swift and Son 

 manufactured the Smyth lens, it does not appear to have found a favonr- 

 alile reception, and it remained forgotten until the firm of 0. Zeiss dis- 

 interred it and applied it to the method of " independent " correction. 



In the case of the biotar lens, the application of a divergent correc- 

 tion lens in the neighbourhood of the objective was unsuitable, as the 

 focus would thereby have been prolonged and the aperture proportions 

 reduced ; whereas, at the beginning of the analysis the need of an 

 aperture as great as possible was emphasized. Hence the application of 

 the old Smyth negative lens, close up to the image spot, exactly answered 

 all requirements. 



The author's diagrams and numerical quotations fully illustrate the 

 effect of the Smyth lens. 



Fluid Condensers of Large Aperture.* — For use with the biotar lens 

 A. Kohler recommends the use of condensers of hyperboloid shape. Such 

 a condenser consists of two thin glass lamina, forming a trough which 

 can be filled with water or other suitable fluid. If the shape of the 

 whole is plano-convex, the convex facing the light source and having a 

 hyperbolic curve, the refracted rays are parallel to the axis and are free 

 from spherical aberration. Two such shapes with their plane surfaces 

 parallel or in contact have the further property of forming an aplanatic 

 lens. It is also a distinct advantage, besides economy, that the tele- 

 centric beam and the absorption of the heat-rays are attained by only 

 two refractions at air, whilst the same effect with ordinary condensers 

 requires six refractions at air, viz. two lenses with four surfaces in a 

 duplex lens and two surfaces in the water chamber. 



Arrang-ement for Circularly Polarized Light.f — A. E. Oxley points 

 out that certain inconveniences arise when one -tries to rotate the Fresnel 

 rhomb about its long axis. He has therefore devised, 

 on Fresnel's principle, two arrangements which are suit- 

 able for white light, and in which the emergent rays are 

 not displaced with regard to the incident rays. 



The first arrangement (fig. o2) consists of two glass 

 parallelepipeds applied to one another, the incident rays 

 being totally reflected at A, B, C, and D. If, then, the 

 polarization plane of the incident linear polarized light 

 makes an angle of 45° with the plane of the quadruple 

 inner reflexions, then the emergent light is circularly 

 polarized when the angle of the parallelopiped is suitably 

 chosen. Assuming that the refractive index of the 



glass used is ^j-- = 1-5035, a phase retardation of - 

 D^ ^ 2 



takes place between the two components vibrating either in the reflexion 



plane or perpendicularly to it, when the angle of the parallelopiped has 



the value 74° 38-2', or 42° 34-8'. The larger of these values should 



be chosen, because in that case the dependence of the phase-difference 



* Zeitschr. f. Instrumentenk , sxxi. (1911) pp. 270-6 (9 figs.), 

 t Chem. News, No. 102 (1910) p. 189. See also Zeitschr. f. Instrumentenk., 

 xxxi. (1911) p. 355. 



/ 



